Moles & amount of substance

This question is about gas volumes.

Figure 9 shows some of the apparatus needed to determine an experimental value for the M_r of an unknown volatile liquid Y.

A student uses this method to determine an experimental value for the M_r of Y.

1. Record the volume of air in a gas syringe.

2. Collect a sample of Y in a hypodermic syringe.

3. Record the mass of the hypodermic syringe and Y.

4. Push the needle of the hypodermic syringe through the rubber cap on the gas syringe.

5. Inject a small amount of Y into the gas syringe.

6. Remove the hypodermic syringe from the gas syringe.

7. Record the mass of the hypodermic syringe and the remaining Y.

8. Record the volume of gas in the gas syringe.

9. Record the temperature in the oven and the atmospheric pressure.

Use the results to determine an experimental value for the M_r of Y.

The gas constant, R = 8.31 J K^–1 mol^–1

Show your working.

The percentage uncertainty in the volume of Y in the gas syringe is 0.95%

The percentage uncertainty in the temperature is 0.12%

The uncertainty in each balance reading is ± 0.0005 g

Assume that the uncertainty in the value for pressure is negligible.

Calculate the percentage uncertainty in the use of the balance in this experiment.

Use your answer and the percentage uncertainties for volume and temperature to calculate a range of values for the M_r of Y.

(If you were unable to calculate an experimental value for the M_r of Y in Question 08.1, assume it is 141. This is not the correct answer.)

A teacher suggests placing a seal over the needle of the hypodermic syringe after collecting the sample of liquid Y, and then replacing the seal after injecting the liquid into the gas syringe.

Give a reason why the teacher’s suggestion will improve the experiment.

A mixture of methane and propane has a volume of 40 cm³

This mixture needs exactly 170 cm³ of oxygen for complete combustion.

The equations for these reactions are

Calculate the volume

• of methane in the original mixture

• of propane in the original mixture.

All volumes are measured at the same temperature and pressure.

50.0 cm³ of 0.015 mol dm^–3 Fe₂(SO₄)₃(aq) are mixed with 50.0 cm³ of

The mixture is left for several hours until equilibrium is reached.

A sample of the equilibrium mixture is titrated to determine the amount of I^–(aq) in the equilibrium mixture.

Give the expression for the equilibrium constant, K_c, for the equilibrium.

Before the titration, the sample is ‘quenched’ by adding 50 cm³ of ice-cold water.

Suggest why the sample is ‘quenched’ in this way before the titration. Explain your answer.

The amount of I^–(aq) in the equilibrium mixture is found to be 3.00 × 10^–4 mol

Calculate the value of K_c at 40 °C for the equilibrium.

Assume that the total volume of the mixture remains constant at 100 cm³

This question is about atomic structure.

In 1897 JJ Thomson discovered the electron. He suggested that atoms were positively charged spheres with electrons embedded within them.

Figure 1 represents an atom using Thomson’s model.

Suggest the identity of this atom.

Give two differences between the modern model of an atom and the Thomson model of an atom.

Tellurium has a relative atomic mass of 127.6 Iodine has a relative atomic mass of 126.9

Define relative atomic mass.

Suggest one property of tellurium that justifies its position before iodine in the modern Periodic Table.

A sample of tellurium is analysed in a time of flight (TOF) mass spectrometer using electron impact ionisation.

Give an equation, including state symbols, for this ionisation.

In the TOF mass spectrometer an ion of an isotope of tellurium, with mass number y, travels along a 1.25 m flight tube with a kinetic energy of 1.88 x 10^–12 J

The ion takes 3.00 x 10^–7 s to reach the detector.

KE = kinetic energy / J m = mass / kg

Calculate the mass, in g, of 1 mole of these tellurium ions.

Use your answer to suggest the mass number y of the tellurium isotope.

The Avogadro constant, L = 6.022 × 10²³ mol^–1

Tellurium has several other isotopes. Two of these isotopes are¹²⁶Te and¹²⁴Te A different sample of tellurium is analysed using a TOF mass spectrometer.

Which statement about kinetic energy (KE) is correct?

Tick (✓) one box.

This question is about an experiment to determine the solubility of strontium hydroxide in water at 20 °C

Strontium hydroxide is slightly soluble in water. Strontium hydroxide solution reacts in a similar way to calcium hydroxide solution.

• Some solid strontium hydroxide is added to approximately 1 dm³ of distilled water in a stoppered flask.

• The mixture is kept at 20 °C. Every day, the mixture is checked. If no solid is present in the flask, more solid strontium hydroxide is added.

• On the day when no more solid needs to be added, the flask is opened and the mixture is filtered into another flask and stoppered.

• A 25.0 cm³ sample of the filtrate is transferred to a conical flask with a pipette and a few drops of indicator added.

• This sample is titrated with 0.100 mol dm^–3 hydrochloric acid.

• The titration is repeated several times with further samples of the filtrate. The results are shown in Table 1 on page 8.

Suggest why the solution is kept until no more solid needs to be added.

Suggest why it is important to remove the undissolved strontium hydroxide before the titration.

After the filtration, the solution is stored in a stoppered flask.

Suggest a reason for stoppering the flask.

The diagrams in Figure 2 show the part of a pipette with the graduation line.

Which diagram identifies the pipette that is correctly filled?

Tick (✓) one box.

Figure 2

Solubility can be quoted as ‘g of solute per 100 cm³ of solution’.

Table 1 shows the results of the titrations between strontium hydroxide and hydrochloric acid. These can be used to determine the solubility of strontium hydroxide.

Give the equation for the reaction between strontium hydroxide and hydrochloric acid.

Use the results in Table 1 to calculate the mean titre.

Use the mean titre to calculate the solubility of strontium hydroxide, in g per 100 cm³ of solution, at 20 °C

Equation

At 40 °C the ionic product of water, K_w = 2.92 × 10^–14 mol² dm^–6

Give the expression for K_w

Calculate the pH of pure water at 40 °C Give your answer to 2 decimal places.

35.0 cm³ of 0.150 mol dm^–3 aqueous sodium hydroxide are mixed with 20.0 cm³ of a 0.100 mol dm^–3 solution of hydrochloric acid. The temperature of the solution formed is 40 °C

Calculate the pH of the solution formed. Give your answer to 2 decimal places.

This question is about an equilibrium.

A 25.0 cm³ sample of a solution of P is added to a 20.0 cm³ sample of a solution of Q The mixture is allowed to reach equilibrium.

The amounts in the equilibrium mixture are

P = 0.0145 mol Q = 0.0275 mol R = 0.0115 mol S = 0.0345 mol

Calculate the amount, in moles, of P before the reaction with Q.

Use your answer to calculate the concentration, in mol dm^–3, of P in the

Give the expression for the equilibrium constant, K_c

Calculate the value of K_c and deduce its units.

Explain why the amount of S increases when water is added to the equilibrium mixture.

The concentration of dilute hydrochloric acid can be found by titration using a standard solution of barium hydroxide.

Calculate the mass, in g, of solid barium hydroxide (M_r = 171.3) needed to prepare 250 cm³ of 0.100 mol dm^–3 barium hydroxide solution.

The mass of barium hydroxide from Question 06.1 is dissolved in a beaker containing

Describe how this solution is used to make 250 cm³ of the 0.100 mol dm^–3 barium hydroxide solution.

Before the first titration, the 25 cm³ pipette is rinsed with a small volume of the 0.100 mol dm^–3 barium hydroxide solution.

State why it is good practice to rinse the pipette in this way.

Hydrochloric acid is added to the burette using a funnel.

State why it is good practice to remove the funnel from the burette before the titration.

In a different experiment, 0.952 g of solid barium hydroxide is used to make 250 cm³ of standard barium hydroxide solution.

25.0 cm³ of this barium hydroxide solution reacts with exactly

Calculate the concentration of the hydrochloric acid.

The uncertainty in the 25.0 cm³ of solution from the pipette is ±0.05 cm³

The total uncertainty in the 24.50 cm³ of solution from the burette is ±0.15 cm³

Calculate the total percentage error in using the pipette and burette.

This question is about complexes containing the aluminium ion.

Give the electron configuration of the Al³⁺ ion.

When anhydrous aluminium sulfate, Al₂(SO₄)₃, is added to water a solution forms that contains the complex aluminium ion, [Al(H₂O)₆]³⁺

Give the equation for the reaction.

Explain why the solution containing [Al(H₂O)₆]³⁺ is acidic.

State why the concentration of aluminium sulfate solution can not be determined by

An excess of aqueous ammonia is added to a solution containing [Al(H₂O)₆]³⁺

Give an ionic equation for the reaction and state one observation.

Equation

An excess of dilute sulfuric acid is added to the products of the reaction in Question 07.5

Identify the aluminium species produced.

Figure 3 shows the structure of the EDTA^4– ion.

Atoms of two different elements in EDTA^4– can form co-ordinate bonds with an aluminium ion.

On Figure 3, draw circles around the atoms of two different elements that would link to an aluminium ion by a co-ordinate bond.

Hydrated aluminium sulfate, Al₂(SO₄)₃.xH₂O, is soluble in water.

The relative formula mass and value of x can be found from a titration experiment.

Aqueous [Al(H₂O)₆]³⁺ ions react to form a stable complex when treated with an excess

The excess of EDTA^4– ions is determined by titration with ZnSO₄ solution.

• Dissolve 1.036 g of Al₂(SO₄)₃.xH₂O in distilled water and make up to 250 cm³

• Add 25.0 cm³ of this solution to 50.0 cm³ of a solution containing EDTA^4– ions of concentration 0.0100 mol dm^–3

• Determine the excess of EDTA^4– ions by titrating with ZnSO₄ solution in the presence of an indicator.

The excess of EDTA^4– ions requires 18.00 cm³ of 0.0105 mol dm^–3 ZnSO₄ solution to

The equations for the reactions are

Use the information given to calculate the M_r of Al₂(SO₄)₃.xH₂O

Give your answer as an integer.

There are no questions on this page

This is a question about time of flight (TOF) mass spectrometry.

Give the equation, including state symbols, for the formation of Sr⁺ ions from Sr atoms by electron impact.

A sample of strontium is analysed by TOF mass spectrometry. The sample is ionised using electron impact.

The ions are accelerated to have a kinetic energy (KE) of 7.02 × 10^–20 J An ion takes 9.47 × 10^–4 s to travel along a 95.0 cm flight tube.

Use the information given to deduce the mass number of this ion.

The Avogadro constant, L = 6.022 × 10²³ mol⁻¹

Explain how the ions are detected in the TOF mass spectrometer.

State how the relative abundance of the ions is determined.

How ions are detected

How relative abundance is determined

A sample of strontium contains three isotopes,⁸⁶Sr,⁸⁷Sr and⁸⁸Sr

Calculate the percentage abundance of⁸⁷Sr in this sample.

Use your answer to deduce the relative atomic mass (A_r) of the sample. Give your answer to 1 decimal place.

Electrospray ionisation is used instead of electron impact for the ionisation of a protein in a mass spectrometry experiment.

Suggest why.

The molar enthalpy of vaporisation (H_vap) of a liquid is the enthalpy change when one mole of liquid is converted to vapour at the boiling point of the liquid.

A student does an experiment to determine H_vap for water.

The student:

• places a large beaker on a balance

• pours 500 cm³ of water into the beaker

• uses a 2.4 kW heater to raise the temperature of the water to 100 C

• records the mass of the beaker and hot water

• uses the 2.4 kW heater to boil the water for 100 s

• records the mass of the beaker and remaining water.

The loss in mass is 103 g

Calculate H_vap for water.

Table 2 shows some data about three compounds that all contain the same number of electrons.

All three compounds in Table 2 are polar. Ethanol is the most polar and ethylamine is the least polar.

Explain why all three molecules are polar and why ethylamine is the least polar. In your answer refer to the shapes around, and relative electronegativities of, the most electronegative atoms.

Explain the trend in the boiling points of the three compounds. Refer to the intermolecular forces in all three compounds in your answer.

Rhenium has an atomic number of 75

Define the term relative atomic mass.

The relative atomic mass of a sample of rhenium is 186.3

Table 2 shows information about the two isotopes of rhenium in this sample.

Calculate the relative isotopic mass of the other rhenium isotope. Show your working.

State why the isotopes of rhenium have the same chemical properties.

A sample of rhenium is ionised by electron impact in a time of flight (TOF) mass spectrometer.

A¹⁸⁵Re⁺ ion with a kinetic energy of 1.153 × 10⁻¹³ J travels through a 1.450 m flight tube.

The kinetic energy of the ion is given by the equation KE = mv²

where m = mass / kg

KE = kinetic energy / J

Calculate the time, in seconds, for the ion to reach the detector.

The Avogadro constant, L = 6.022 × 10²³mol⁻¹

State how the relative abundance of¹⁸⁵Re⁺ is determined in a TOF mass spectrometer.

This question is about olive oil.

A sample of olive oil is mainly the unsaturated fat Y mixed with a small amount of inert impurity.

The structure of Y in the olive oil is shown. Y has the molecular formula C₅₇H₁₀₀O₆ (M_r = 880).

The amount of Y is found by measuring how much bromine water is decolourised by a sample of oil, using this method.

• Transfer a weighed sample of oil to a 250 cm³ volumetric flask and make up to the mark with an inert organic solvent.

• Titrate 25.0 cm³ samples of the olive oil solution with 0.025 mol dm⁻³ Br₂(aq).

A suitable target titre for the titration is 30.0 cm³ of 0.025 mol dm⁻³ Br₂(aq).

Justify why a much smaller target titre would not be appropriate.

Calculate the amount, in moles, of bromine in the target titre.

Calculate a suitable mass of olive oil to transfer to the volumetric flask using your answer to Question 09.1 and the structure of Y. Assume that the olive oil contains 85% of Y by mass.

(If you were unable to calculate the amount of bromine in the target titre, you should assume it is 6.25×10⁻⁴ mol. This is not the correct amount.)

The olive oil solution can be prepared using this method.

• Place a weighing bottle on a balance and record the mass, in g, to 2 decimal places.

• Add olive oil to the weighing bottle until a suitable mass has been added.

• Record the mass of the weighing bottle and olive oil.

• Pour the olive oil into a 250 cm³ volumetric flask.

• Add organic solvent to the volumetric flask until it is made up to the mark.

• Place a stopper in the flask and invert the flask several times.

Suggest an extra step to ensure that the mass of olive oil in the solution is recorded accurately.

Justify your suggestion.

State the reason for inverting the flask several times.

A sample of the olive oil was dissolved in methanol and placed in a mass spectrometer. The sample was ionised using electrospray ionisation. Each molecule gained a hydrogen ion (H⁺) during ionisation. The spectrum showed a peak for an ion with = 345 formed from an impurity in the olive oil. The ion with = 345 was formed from a compound with the empirical formula C₅H₁₀O

Deduce the molecular formula of this compound.

Show your working.

Molecular formula 12

Complete combustion of 0.0100 mol of an organic acid produced 0.0200 mol of carbon dioxide.

The same amount of the acid required 20 cm³ of 1.00 mol dm^–3 NaOH (aq) for neutralisation.

Which could be the formula of the acid?

A mixture of methanoic acid and sodium methanoate in aqueous solution acts as an acidic buffer solution.

The equation shows the dissociation of methanoic acid.

Calculate the mass, in g, of sodium methanoate (HCOONa) that must be added to 25.0 cm³ of 0.100 mol dm³ methanoic acid to produce a buffer solution with pH = 4.05 at 298 K

For methanoic acid, pK_a = 3.75 at 298 K

Assume that the volume of the solution remains constant.

What is the mole fraction of 1.0 g of a compound of relative molecular mass 100.0 dissolved in 30.0 g of a solvent of relative molecular mass 50.0?

This question is about the isotopes of chromium.

Give the meaning of the term relative atomic mass.

A sample of chromium containing the isotopes⁵⁰Cr,⁵²Cr and⁵³Cr has a relative atomic mass of 52.1

The sample contains 86.1% of the⁵²Cr isotope.

Calculate the percentage abundance of each of the other two isotopes.

State, in terms of the numbers of fundamental particles, one similarity and one difference between atoms of⁵⁰Cr and⁵³Cr

The sample of chromium is analysed in a time of flight (TOF) mass spectrometer.

Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.

A⁵³Cr⁺ ion travels along a flight tube of length 1.25 m

m = mass of the ion / kg

Calculate the time, in s, for the⁵³Cr⁺ ion to travel down the flight tube to reach the detector.

Propanoic acid (C₂H₅COOH) is a weak acid.

The acid dissociation constant (K_a) for propanoic acid is 1.35 × 10^–5 mol dm^–3

State the meaning of the term weak acid.

Give an expression for the acid dissociation constant for propanoic acid.

A student dilutes 25.0 cm³ of 0.500 mol dm^–3 propanoic acid by adding water until the

Calculate the pH of this diluted solution of propanoic acid.

Give your answer to 2 decimal places.

A buffer solution with a pH of 4.50 is made by dissolving x g of sodium propanoate (C₂H₅COONa) in a solution of propanoic acid. The final volume of buffer solution is 500 cm³ and the final concentration of the propanoic acid is 0.250 mol dm^–3

Calculate x in g For propanoic acid, K_a = 1.35 10^–5 mol dm^–3

This question is about citric acid, a hydrated tricarboxylic acid. Its formula can be

A 1.50 g sample of H₃Y.H₂O contains 0.913 g of oxygen by mass. The sample burns completely in air to form 1.89 g of CO₂ and 0.643 g of H₂O

Show that the empirical formula of citric acid is C₃H₅O₄

A 3.00 g sample of H₃Y.H₂O (M_r = 210.0) is heated to constant mass. The anhydrous H₃Y that remains has a mass of 2.74 g

Show, using these data, that the value of = 1

Figure 5 shows the structure of H₃Y

Complete this IUPAC name for H₃Y

State the number of peaks you would expect in the¹³C NMR spectrum for H₃Y

A and B react together to form an equilibrium mixture.

An aqueous solution containing 0.25 mol of A is added to an aqueous solution containing 0.25 mol of B.

When equilibrium is reached, the mixture contains 0.015 mol of C.

Calculate the amount of A and the amount of B, in moles, in the equilibrium mixture.

At a different temperature, another equilibrium mixture contains 0.30 mol of A, 0.25 mol of B and 0.020 mol of C in 350 cm³ of solution.

Calculate the value of the equilibrium constant K_c

When an excess of water is added to chloroethanal, an equilibrium mixture is formed.

An expression for an equilibrium constant (K) for the reaction under these conditions is

Suggest why an expression for K can be written without the concentration of water.

Distilled water is added to 4.71 g of chloroethanal (M_r = 78.5) to make 50.0 cm³ of solution. The mixture is allowed to reach equilibrium.

The value of the equilibrium constant (K) is 37.0

Calculate the equilibrium concentration, in mol dm^–3, of ClCH₂CH(OH)₂

When an excess of water is added to ethanal a similar nucleophilic addition reaction occurs.

Suggest why this reaction is slower than the reaction in Question 09.5.

Explain why complexes formed from transition metal ions are coloured.

The iron content of iron tablets can be determined by colorimetry.

Method:

• Dissolve a tablet in sulfuric acid.

• Oxidise all the iron from the tablet to Fe³⁺(aq).

• Convert the Fe³⁺(aq) into a complex that absorbs light of wavelength 490 nm

• Make the solution up to 250 cm³

• Measure the absorbance of light at 490 nm with a colorimeter.

• Use a calibration graph to find the concentration of the iron(III) complex.

Calculate the energy, in J, gained by each excited electron in the absorption at 490 nm

Energy gained by each electron
J

Describe how a calibration graph is produced and used to find the concentration of the iron(III) complex.

The concentration of iron(III) in the solution is 4.66 x 10^–3 mol dm^–3

Calculate the mass, in mg, of iron in the tablet used to make the 250 cm³ of solution.

Mass of iron in the tablet
mg

Standard electrode potentials are measured by comparison with the standard hydrogen electrode.

State the substances and conditions needed in a standard hydrogen electrode.

It is difficult to ensure consistency with the setup of a standard hydrogen electrode. A Cu²⁺(aq) /Cu(s) electrode ( = +0.34 V) can be used as a secondary standard.

A student does an experiment to measure the standard electrode potential for the TiO²⁺(aq)/Ti(s) electrode using the Cu²⁺(aq)/ Cu(s) electrode as a secondary standard.

A suitable solution containing the acidified TiO²⁺(aq) ion is formed when titanium(IV) oxysulfate (TiOSO₄) is dissolved in 0.50 mol dm³ sulfuric acid to make 50 cm³ of solution.

Describe an experiment the student does to show that the standard electrode potential for the TiO²⁺(aq) /Ti(s) electrode is 0.88 V

The student is provided with:

• the Cu²⁺(aq)/ Cu(s) electrode set up ready to use

• solid titanium(IV) oxysulfate (M_r = 159.9)

• 0.50 mol dm³ sulfuric acid

• a strip of titanium

• laboratory apparatus and chemicals.

Your answer should include details of:

• how to prepare the solution of acidified TiO²⁺(aq)

• how to connect the electrodes

• measurements taken

• how the measurements should be used to calculate the standard electrode potential for the TiO²⁺(aq)/Ti(s) electrode.

Give the half-equation for the electrode reaction in the TiO²⁺(aq)/Ti(s) electrode in acidic conditions.

Table 2 shows some electrode potential data.

Use the data in Table 2 to explain why copper does not react with most acids but does react with nitric acid.

Give an equation for the reaction between copper and nitric acid.

Equation

Time of flight (TOF) mass spectrometry can be used to analyse large molecules such as the pentapeptide, leucine encephalin (P).

P is ionised by electrospray ionisation and its mass spectrum is shown in Figure 2.

Describe the process of electrospray ionisation.

Give an equation to represent the ionisation of P in this process.

Equation

What is the relative molecular mass of P ? Tick (✓) one box.

A molecule Q is ionised by electron impact in a TOF mass spectrometer. The Q⁺ ion has a kinetic energy of 2.09 x 10^–15 J This ion takes 1.23 x 10^–5 s to reach the detector. The length of the flight tube is 1.50 m

Calculate the relative molecular mass of Q.

KE = mv² where m = mass (kg) and v = speed (m s^–1) The Avogadro constant, L = 6.022 x 10²³ mol^–1

Relative molecular mass

This question is about sulfuric acid and its salts.

Draw the displayed formula of a molecule of H₂SO₄

In aqueous solution, sulfuric acid acts as a strong acid. The H₂SO₄ dissociates to form HSO₄ ions and H⁺ ions.

The HSO₄ ions act as a weak acid and dissociate to form SO₄² ions and H⁺ ions.

Give an equation to show each stage in the dissociation of sulfuric acid in aqueous solution.

Include appropriate arrows in your equations.

A student is required to make 250 cm³ of an aqueous solution that contains an accurately measured mass of sodium hydrogensulfate (NaHSO₄).

Describe the method that the student should use to make this solution.

A solution that contains 605 mg of NaHSO₄ in 100 cm³ of solution has a pH of 1.72

Calculate the value of K_a for the hydrogensulfate ion (HSO₄) that is behaving as a weak acid. Give your answer to three significant figures.

Some sodium sulfate is dissolved in a sample of the solution from question 02.4.

Explain why this increases the pH of the solution.

Lead(II) nitrate and potassium iodide react according to the equation

In an experiment, 25.0 cm³ of a 0.100 mol dm^–3 solution of each compound are mixed together.

Which amount, in mol, of lead(II) iodide is formed?

This question is about s-block metals.

Give the full electron configuration for the calcium ion, Ca²⁺

Explain why the second ionisation energy of calcium is lower than the second ionisation energy of potassium.

Identify the s-block metal that has the highest first ionisation energy.

Give the formula of the hydroxide of the element in Group 2, from Mg to Ba, that is least soluble in water.

A student added 6 cm³ of 0.25 mol dm^–3 barium chloride solution to 8 cm³ of 0.15 mol dm^–3 sodium sulfate solution. The student filtered off the precipitate and collected the filtrate.

Give an ionic equation for the formation of the precipitate. Show by calculation which reagent is in excess. Calculate the total volume of the other reagent which should be used by the student so that the filtrate contains only one solute.

Total volume of other reagent

A sample of strontium has a relative atomic mass of 87.7 and consists of three

State why the isotopes of strontium have identical chemical properties. Calculate the percentage abundance of the⁸⁸Sr isotope in this sample.

Why isotopes of strontium have identical chemical properties

A time of flight (TOF) mass spectrum was obtained for a sample of barium that

Identify the ion with the longest time of flight.

A¹³⁷Ba⁺ ion travels through the flight tube of a TOF mass spectrometer with a

The Avogadro constant, L = 6.022 × 10²³ mol^–1

Calculate the length of the flight tube in metres. Give your answer to the appropriate number of significant figures.

This question is about sodium and some of its compounds.

Use your knowledge of structure and bonding to explain why sodium bromide has a melting point that is higher than that of sodium, and higher than that of sodium iodide.

When 250 mg of sodium were added to 500 cm³ of water at 25 °C a gas was produced.

Give an equation for the reaction that occurs. Calculate the volume, in cm³, of the gas formed at 101 kPa

The gas constant, R = 8.31 J K^–1 mol^–1

Calculate the concentration, in mol dm^–3, of sodium ions in the solution produced in the reaction in Question 08.2.

Sodium reacts with ammonia to form the compound NaNH₂ that contains the

Draw the shape of the NH₂^– ion. Include any lone pairs of electrons that influence the shape.

Predict the bond angle. Justify your prediction.

Shape

A student added 627 mg of hydrated sodium carbonate (Na₂CO₃.xH₂O) to 200 cm³ of 0.250 mol dm^–3 hydrochloric acid in a beaker and stirred the mixture. After the reaction was complete, the resulting solution was transferred to a volumetric flask, made up to 250 cm³ with deionised water and mixed thoroughly. Several 25.0 cm³ portions of the resulting solution were titrated with 0.150 mol dm^–3 aqueous sodium hydroxide. The mean titre was 26.60 cm³ of aqueous sodium hydroxide.

Show your working. Give your answer as an integer.