pH calculations

This question is about monobasic acids and buffer solutions.

Nitric acid, HNO₃, is a strong monobasic acid.

Calculate the pH of 0.150 mol dm^–3 HNO₃.

Give your answer to 2 decimal places.

Nitrous acid, HNO₂, is a weak monobasic acid.

Predict whether the pH of 0.150 mol dm^–3 HNO₂ is higher, lower or the same as the pH of 0.150 mol dm^–3 nitric acid, HNO₃.

Explain your answer.

Prediction........

Explanation........

When heated, aqueous HNO₂ disproportionates. The equation is shown below.

• What is meant by disproportionation?

• Use oxidation numbers to show that disproportionation has taken place.

* A student plans to prepare a buffer solution with a pH of 4.85 using propanoic acid, CH₃CH₂COOH, and sodium propanoate, CH₃CH₂COONa.

K_a for CH₃CH₂COOH = 1.32 × 10^–5 mol dm^–3.

The student mixes 125 cm³ of 0.180 mol dm^–3 CH₃CH₂COOH(aq) with a calculated mass of CH₃CH₂COONa(s). The student assumes that the volume of the solution does not change.

• Calculate the mass of CH₃CH₂COONa needed to make 125 cm³ of a buffer solution with a pH of 4.85.

• Explain why this solution resists changes in pH when a small amount of alkali is added.

This question is about different compounds that each contain O and H atoms covalently bonded together.

The alcohol 2,3‑dimethylhexan‑1‑ol has the molecular formula C₈H₁₈O.

Draw the skeletal formula of 2,3‑dimethylhexan‑1‑ol.

What is the number of H atoms in 8.45 g of 2,3‑dimethylhexan‑1‑ol?

Give your answer to 3 significant figures and in standard form.

Why does paracetamol not react with Na₂CO₃(aq)?

The student uses the reaction of nuromol tablets with Na₂CO₃(aq) to determine the mass of ibuprofen and the mass of paracetomol in one nuromol tablet.

The student’s method is outlined below.

• Weigh four nuromol tablets.

• Add the tablets to an excess of aqueous sodium carbonate.

• Collect the gas produced and measure its volume.

Results Mass of four nuromol tablets = 2.80 g Volume of gas collected at RTP = 48.0 cm³

Assume that ibuprofen is the only compound in nuromol tablets that reacts with Na₂CO₃(aq).

Determine the mass of ibuprofen and the mass of paracetomol, in mg, in one nuromol tablet.

Barium hydroxide, Ba(OH)₂, is an alkali. A student is supplied with a solution of Ba(OH)₂ with a concentration of 46.80 g dm^–3 at 25 °C.

The student dilutes 20.0 cm³ of this solution of Ba(OH)₂ with water and the solution is made up to 100.0 cm³ at 25 °C.

Calculate the pH of the diluted solution of Ba(OH)₂ at 25 °C.

Give your answer to 2 decimal places.

Which solution can be added to CH₃COOH(aq) to make a buffer solution?

A CH₃COONa(aq)

B HCOOH(aq)

C HC l (aq)

D NaC l (aq)

Your answer

This question is about acids and bases.

Chloroethanoic acid, Cl CH₂COOH, is a weak monobasic acid.

Write the expression for the acid dissociation constant, K_a, of Cl CH₂COOH.

The expression for the acid dissociation constant, K_a, of Cl CH₂COOH can be simplified to:

State one approximation that allows the expression from (a)(i) to be simplified to Expression 19.1.

A student carries out an experiment to determine the pK_a value of a solution of Cl CH₂COOH.

• The concentration of Cl CH₂COOH is 0.090 mol dm^–3.

• The pH of Cl CH₂COOH is 1.95.

Use Expression 19.1 to calculate the pK_a value of Cl CH₂COOH.

Give your answer to 2 decimal places.

Draw a best-fit curve on the graph and calculate the concentration of the CH₃COOH solution.

CH₃COOH concentration =........ mol dm^–3

The end point of the titration can also be found by observing the colour change of an indicator.

The pH ranges of some indicators are shown in the table.

Identify the indicator in the table that would be suitable to observe the end point of the titration between CH₃COOH and Ba(OH)₂.

This question is about transition elements.

Suggest the formulae of A, B and C.

Complex ion A:........

Solid B:........

Complex ion C:........

Cobalt (III) forms an octahedral complex ion D, which contains both ammonia and chloride ligands.

Complex ion D has a molar mass of 197.9 g mol^–1.

Determine the formula and charge of complex ion D.

Red blood cells contain haemoglobin which transports oxygen around the body.

For efficient transportation of oxygen, healthy human blood must be maintained at a pH value between 7.35 and 7.45.

Human blood acts as a buffer due to the presence of carbonic acid, H₂CO₃, and hydrogencarbonate, HCO₃^–, ions as shown below.

• Explain, using ligand substitution, how haemoglobin transports oxygen around the body.

• Determine whether a sample of blood with a [HCO₃^–]: [H₂CO₃] ratio of 8.5:1 is healthy.

This question is about acids, bases and salts.

What is the pH of 1.00 dm³ of 0.400 mol dm^–3 of NaOH(aq) at 298 K?

Water is added to 10.0 cm³ of 0.750 mol dm^–3 HCl (aq) to produce 100 cm³ of diluted HCl (aq).

What is the pH of the diluted HC l (aq)?

Give your answer to 2 decimal places.

A solution has concentrations of 0.300 mol dm^–3 CH₃COOH(aq) and 0.100 mol dm^–3 CH₃COONa(aq).

K_a for CH₃COOH = 1.75 10^–5 mol dm^–3 at 298 K.

What is the pH of the solution at 298 K?

Give your answer to 2 decimal places.

A student is provided with hydrated copper(II) nitrate, Cu(NO₃)₂•3H₂O.

The student needs to prepare a standard solution of Cu(NO₃)₂•3H₂O with a concentration of 0.200 mol dm^–3. The student has access to usual laboratory apparatus and equipment.

Describe how the student would prepare 100.0 cm³ of this solution, giving quantities, apparatus and method.

This question is about acids and bases.

Table 20.1 shows the ionic product, K_w, of water at 25 °C and 40 °C.

Table 20.1

Temperature /°C	Kw / mol2 dm–6
25	1.00 10−14
40	2.92 10−14

Calculate the pH of water at 40 °C.

Give your answer to 2 decimal places.

Table 20.1 shows different K_w values at 25 °C and at 40 °C. A student suggests that water is neutral at these temperatures.

Explain why this student is correct.

A student reacts strontium metal with water to make a 250.0 cm³ solution of aqueous strontium hydroxide, Sr(OH)₂. The solution contains 0.145 g of strontium hydroxide.

• Write an equation for the reaction of strontium with water.

• Calculate the pH of this 250.0 cm³ solution of strontium hydroxide at 40 °C. You should refer back to Table 20.1 at the start of (a).

Give your answer to 2 decimal places.

Equation........

Calculation

A student reacts 1.00 g of strontium carbonate, SrCO₃, with an excess of dilute nitric acid, HNO₃. A gas is produced.

Construct the equation for this reaction.

The student then reacts 1.00 g of calcium carbonate, CaCO₃, with an excess of dilute nitric acid, HNO₃.

Explain why the student’s two reactions produce different volumes of gas.

A student reacts an excess of magnesium with 25.0 cm³ of 0.500 mol dm^–3 hydrochloric acid, HC l.

The student also reacts an excess of magnesium with 25.0 cm³ of 0.500 mol dm^–3 ethanoic acid, CH₃COOH.

Construct an ionic equation for the reaction of magnesium with an acid.

Explain why these two reactions of magnesium produce the same volume of gas but at different rates.

Butanoic acid, CH₃CH₂CH₂COOH, is a weak monobasic acid.

Explain what is meant by the term monobasic acid.

A buffer solution is prepared by dissolving 3.39 g of potassium hydroxide in 250 cm³ of 0.376 mol dm^–3 butanoic acid.

This buffer solution has a pH of 5.07 at 25 °C.

Calculate the acid dissociation constant, K_a, of butanoic acid at 25 °C.

Assume that the volume of the solution remains constant at 250 cm³ when the potassium hydroxide is dissolved.

K_a =........ mol dm^–3

A buffer solution has a pH of 4.50.

When a small volume of water is added to this buffer solution, the pH does not change.

Explain why the pH does not change.

These questions are from different areas of chemistry.

This question is about two salts of rubidium (atomic number 37): RbClO₃ and RbClO₄.

The oxidation number of chlorine is different in the two rubidium salts, RbClO₃ and RbClO₄.

What is the name of RbClO₄?

A student carries out an experiment to determine the enthalpy change of solution of RbClO₃ using the method below.

• A 2.00 g sample of solid RbClO₃ is added to water in a well-insulated container. The initial temperature is 23.0 °C.

• The mixture is stirred until all the RbClO₃ has dissolved. The final temperature is 21.5 °C. The final solution has a mass of 102 g.

Determine the enthalpy change of solution, _solH, of RbClO₃ in kJ mol^–1.

Assume that the specific heat capacity of the solution is the same as that of pure water.

_solH (RbClO₃) =........ kJ mol^–1

A student investigates the rate of a reaction that is 1st order with respect to hydrochloric acid, HC l (aq).

• The student carries out a reaction using 0.680 mol dm^–3 HCl(aq). The initial rate is

• The student dilutes a different sample of 0.680 mol dm^–3 HCl(aq) with water. The pH of this diluted acid is 1.50.

• The student repeats the reaction using the same volume of this diluted acid.

Determine the initial rate using this diluted acid.

initial rate =........ mol dm^–3 s^–1

This question is about acids and buffer solutions.

Succinic acid, HOOC(CH₂)₂COOH, is a weak dibasic acid that is used in tablet form in health supplements.

A student plans to determine the mass of succinic acid in one tablet of a succinic acid health supplement.

The student carries out a titration with potassium hydroxide.

The end point occurs when both acidic protons in succinic acid have been replaced as shown in Equation 19.1.

The student uses the following method.

Stage 1 The student crushes four tablets of the health supplement and dissolves the powdered tablets in distilled water.

Stage 2 The student makes up the solution from Stage 1 to 250.0 cm³ in a volumetric flask.

Stage 3 The student titrates 10.0 cm³ portions of the solution obtained in Stage 2 with 0.0600 mol dm^–3 potassium hydroxide, using phenolphthalein as the indicator.

The student carries out a trial titration, followed by three further titrations. The results are shown below.

Complete the table and calculate the mean titre that the student should use for analysing the results.

Use the student’s results and Equation 19.1 to calculate the mass, in mg, of succinic acid in one tablet of the health supplement.

Give you answer to 3 significant figures.

Glycolic acid, HOCH₂COOH, (pK_a = 3.83) is a weak monobasic acid used in some skincare products.

A buffer solution is prepared by adding 60.0 cm³ of 0.750 mol dm^–3 glycolic acid to 40.0 cm³ of 0.625 mol dm^–3 potassium hydroxide, KOH.

Explain why a buffer solution is formed.

Calculate the pH of the buffer solution that has been prepared.

Give your answer to 2 decimal places.

A small amount of aqueous ammonia, NH₃(aq), is added to the buffer solution.

Explain, in terms of equilibrium, how the buffer solution would respond to the added NH₃(aq).

This question is about the reactions of Group 2 metals and their compounds.

A student adds magnesium to dilute hydrochloric acid in one test tube. The student adds calcium to dilute hydrochloric acid in a second test tube.

A redox reaction takes place in each test tube.

Suggest two observations from the student’s experiment that would show that calcium is more reactive than magnesium.

Write half-equations for the reaction of magnesium with hydrochloric acid.

Oxidation half-equation:........

Reduction half-equation:........

A sample of barium oxide is added to distilled water at 25 °C. A colourless solution forms containing barium hydroxide, Ba(OH)₂.

The solution is made up to 250.0 cm³ with distilled water. The pH of this solution is 13.12.

Determine the mass of barium oxide that was used.

Give your answer to 3 significant figures.

10 cm³ of dilute sulfuric acid is added to 10 cm³ of the colourless solution of Ba(OH)₂

Write an ionic equation, including state symbols, for the reaction.

Limestone and huntite are two calcium minerals.

A typical sample of limestone contains 95.0% by mass of calcium carbonate, CaCO₃

Fertiliser Z, Ca₅NH₄(NO₃)₁₁•10H₂O (M_r = 1080.5 g mol^–1) can be made from limestone.

Calculate the mass, in g, of limestone needed to make 1.50 kg of fertiliser Z.

Give your answer to 3 significant figures.

Huntite is a carbonate mineral with the chemical formula Mg₃Ca(CO₃)₄.

Huntite reacts with dilute hydrochloric acid to produce bubbles of a gas and a colourless solution.

Construct the equation for the reaction. Include state symbols.

A student carries out an investigation on vitamin C, C₆H₈O₆.

Vitamin C is extremely soluble in water. This means that vitamin C is removed rapidly from the body. ‘Vitamin C ester’ is available in tablet form as a less soluble source of vitamin C which stays in the body for longer.

Suggest why vitamin C is extremely soluble in water.

A ‘vitamin C ester’ tablet contains an ester with the molecular formula C₂₂H₃₈O₇.

This ester can be prepared by reacting vitamin C with a long chain carboxylic acid, C_xH_yCOOH, in the presence of an acid catalyst.

Vitamin C and the long chain carboxylic acid react in a 1:1 molar ratio.

Determine x and y in the formula of this carboxylic acid.

Vitamin C, C₆H₈O₆, is a weak acid (K_a = 7.94 x 10^–5 (mol dm^–3)), which is often referred to as ascorbic acid.

The label on a carton of orange juice lists the mass of vitamin C, in mg, in a typical serving of 150 cm³.

The student carries out an investigation to check the vitamin C content in the orange juice.

Vitamin C can be oxidised by iodine:

The student dilutes 150 cm³ of the orange juice with water to 250.0 cm³ in a volumetric flask.

The student then titrates 25.0 cm³ volume of this solution with 9.60 × 10^–4 mol dm^–3 iodine solution, I₂(aq).

The mean titre of I₂(aq) is 22.50 cm³.

Determine the mass, in mg, of vitamin C in a 150 cm³ serving of the orange juice.

An aqueous solution of ethanoic acid, CH₃COOH, has a concentration of 0.50 mol dm^–3. pK_a for CH₃COOH = 4.76 at 25 °C.

What is the pH of the ethanoic acid solution at 25 °C?

A 2.53

B 2.68

C 4.91

D 5.06

Your answer

This question is about acids, bases and buffers.

Sodium hydroxide, NaOH, is a strong base.

Calculate the pH of 0.140 mol dm^–3 NaOH(aq) at 298 K.

Give your answer to 2 decimal places.

Sulfuric acid reacts with sodium hydroxide as shown in Equation 22.1. Equation 22.1 H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l) H₁ This is a neutralisation reaction.

A student carries out an experiment to determine the enthalpy change H₁ and uses this value to deduce the enthalpy change of neutralisation, _neutH.

The student measures out two solutions:

• 25.0 cm³ of 1.60 mol dm^–3 H₂SO₄(aq)

• 55.0 cm³ of 1.50 mol dm^–3 NaOH(aq) (an excess).

The temperature of each solution is the same.

The student mixes the two solutions. The temperature increases by 13.0 °C.

Show that NaOH is in excess.

Calculate the enthalpy change, H₁, for Equation 22.1, and deduce the value for the enthalpy change of neutralisation, ∆_neutH, in kJ mol^–1.

Assume that the densities of all solutions and the specific heat capacity, c, of the reaction mixture are the same as for water.

enthalpy change, H₁ =........ kJ mol^–1

enthalpy change of neutralisation, _neutH =........ kJ mol^–1

The student repeats the experiment using 50.0 cm³ of 1.60 mol dm^–3 H₂SO₄ and 110.0 cm³ of 1.50 mol dm^–3 NaOH.

Predict the increase in temperature.

Explain your reasoning.

* Nitrous acid, HNO₂, is a weak Brønsted–Lowry acid with a pK_a value of 3.34 at room temperature.

HNO₂ can be prepared by reacting N₂O₃ with water. HNO₂ is the only product.

A chemist makes up a buffer solution by the following method.

Step 1 The chemist weighs a sample of N₂O₃. Water is then added to form 100 cm³ of 0.500 mol dm^–3 HNO₂(aq).

Step 2 The chemist adds 100 cm³ of 0.150 mol dm^–3 NaOH(aq) to the 100 cm³ solution of 0.500 mol dm^–3 solution of HNO₂(aq). The resulting solution is made up to 1.00 dm³.

Explain why a buffer solution forms in Step 2. Determine the pH of this buffer solution and the mass of N₂O₃ that was used in Step 1.

Healthy human blood needs to be maintained at a pH of 7.40 for the body to function normally.

Red blood cells contain haemoglobin.

Explain using ligand substitutions:

• how haemoglobin transports oxygen around the body

• why carbon monoxide is toxic.

This question is about weak acids.

The K_a values of three weak acids are shown in Table 20.1.

Calculate the pH of 0.0800 mol dm⁻³ C₂H₅COOH(aq).

Give your answer to 2 decimal places.

Show by calculation that 20.0 cm³ of NaOH(aq) is required to reach the end point.

Calculate the pH of the final solution.

Give your answer to 2 decimal places.

On the axes below, sketch a pH curve for the pH changes during the addition of 45.0 cm³ of 0.100 mol dm⁻³ NaOH(aq) to 25.0 cm³ of 0.0800 mol dm⁻³ C₂H₅COOH(aq).

The student considers using the four indicators in Table 20.2 for the titration.

Explain which indicator would be most suitable for the titration.

The student repeats the experiment starting with 25.0 cm³ of 0.0800 mol dm⁻³ HCN(aq) and adding a total of 45.0 cm³ of 0.100 mol dm⁻³ NaOH(aq).

Predict one similarity and one difference between the pH curve with C₂H₅COOH(aq) and the pH curve with HCN(aq). Use the information in Table 20.1, and your answer to (b)(iii).

Similarity........

Difference........

The student calculates the pH of 0.0800 mol dm⁻³ HIO₃(aq). The student assumes that the equilibrium concentration of HIO₃(aq) is the same as the initial concentration of HIO₃(aq).

The student measures the pH, and finds that the measured pH value is different from the calculated pH value.

Explain why the measured pH is different from the calculated pH.

These short questions are from different areas of chemistry.

Explain why a CF₄ molecule has polar bonds but does not have an overall dipole.

Explain why a small proportion of molecules in water have a relative molecular mass of 20.

What is the partial pressure of O₂ (in Pa) in a gas mixture containing 21% O₂ by volume and with a total pressure of 1.0 × 10⁵ Pa?

What mass of carbon dioxide (in g) is formed by the complete combustion of 42.0 m³ (measured at RTP) of propane?

What is the number of oxygen atoms in 4.26 g of P₂O₅?

Dettol ® is a disinfectant containing the antiseptic chloroxylenol, shown below.

Chloroxylenol is a weak Brønsted–Lowry acid.

What is the systematic name of chloroxylenol?

Predict the number of peaks in a¹³C NMR spectrum of chloroxylenol.

Name the functional group responsible for the acidity of chloroxylenol and describe a simple test which would confirm the presence of this group.

Functional group........

Test........

A student measures the pH of the contents in a bottle of Dettol ® as 5.14.

The label on the bottle shows the percentage of chloroxylenol in Dettol ® as 4.80% i.e. 100 cm³ of Dettol^® contains 4.80 g of chloroxylenol.

Assume the following:

• Chloroxylenol is the only acidic component in Dettol ®.

• Chloroxylenol is a weak monobasic acid.

• The density of Dettol ® is 1.00 g cm −3.

Write the equation, using molecular formulae, for the acid dissociation of chloroxylenol.

Calculate the acid dissociation constant, K_a, for chloroxylenol.

K_a =........ mol dm⁻³

Dettol^® contains other chemicals including -terpineol, shown below.

-Terpineol is a chiral compound.

Show with an asterisk, (*), the chiral centre(s) in the structure of -terpineol.

-Terpineol meets the requirements for E / Z isomerism. However, only one E / Z isomer of -terpineol exists.

Explain

• why -terpineol meets the requirements for E / Z isomerism

• whether -terpineol is an E- or Z- isomer

• why only one E / Z isomer of -terpineol exists.

-Terpineol contains two functional groups.

For each functional group, choose a reagent that reacts with that group only. Draw the structures for the organic products of the reactions.

Show structures for organic compounds.

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

A buffer solution is prepared by mixing 200 cm³ of 2.00 mol dm^–3 propanoic acid, CH₃CH₂COOH, with 600 cm³ of 1.00 mol dm^–3 sodium propanoate, CH₃CH₂COONa.

K_a for CH₃CH₂COOH = 1.32 × 10^–5 mol dm^–3

What is the pH of the buffer solution?

A 4.58

B 4.70

C 5.06

D 5.18

Your answer

This question is about acids and bases found in the home.

Ethanoic acid, CH₃COOH, is the acid present in vinegar.

A student carries out an experiment to determine the pK_a value of CH₃COOH.

• The concentration of CH₃COOH in the vinegar is 0.870 mol dm^–3.

• The pH of the vinegar is 2.41.

Write the expression for the acid dissociation constant, K_a, of CH₃COOH.

Calculate the pK_a value of CH₃COOH.

Give your answer to two decimal places.

Determine the percentage dissociation of ethanoic acid in the vinegar.

Give your answer to three significant figures.

Many solid drain cleaners are based on sodium hydroxide, NaOH.

• A student dissolves 1.26 g of a drain cleaner in water and makes up the solution to 100.0 cm³.

• The student measures the pH of this solution as 13.48.

Determine the percentage, by mass, of NaOH in the drain cleaner.

Give your answer to three significant figures.

Sodium carbonate, Na₂CO₃, is a base used in washing soda.

Na₂CO₃ contains the carbonate ion, CO₃^2–, shown below.

Draw the ‘ dot-and-cross ’ diagram for the carbonate ion.

Show outer electrons only and use different symbols for electrons from C and O, and any ‘extra’ electrons.