Moles & amount of substance

Magnesium sulfate is an ionic compound.

A student adds magnesium sulfate to water to form a solution.

Which of these will also form a solution when added to water?

Calculate the percentage by mass of oxygen in magnesium sulfate, MgSO₄.

(relative atomic mass: O = 16.0 relative formula mass: MgSO₄ = 120)

Give your answer to 3 significant figures.

percentage by mass =_........

Calculate the number of oxygen atoms in 7.20 g of MgSO₄.

(relative formula mass: MgSO₄ = 120 Avogadro constant = 6.02 × 10²³)

number of oxygen atoms =_........

*(c) Magnesium sulfate solution can be electrolysed using the apparatus shown in Figure 3.

Explain how the products are formed at the electrodes in terms of

• the ions present in the solution

• the oxidation and reduction reactions that take place at the electrodes.

You should include ionic equations as part of your answer.

Group 1 elements are known as the alkali metals.

Which row gives two physical properties of the alkali metals?

The alkali metals react with water.

A teacher adds a small piece of potassium to water.

Give two observations that would be seen.

The reaction of lithium with water is much less vigorous than the reaction of potassium with water.

Explain why, in terms of their electronic configurations.

When sodium reacts with water, a solution of sodium hydroxide is formed.

When the solution is left in air, the hydroxide ions react with carbon dioxide, forming carbonate ions, CO₃^2–, and one other product.

Complete the ionic equation for this reaction.

_........ OH^– +_........ CO₃^2– +_........

The relative atomic mass of caesium is 132.905.

Calculate the average mass of one atom of caesium

(Avogadro constant = 6.02 × 10²³)

mass =_........ g

This question is about carbon and some carbon compounds.

Two forms of pure carbon are graphite and Buckminsterfullerene.

Explain why graphite is used as a lubricant.

Calculate the mass, in kg, of five moles of Buckminsterfullerene, C₆₀.

(relative atomic mass: C = 12)

mass =_........ kg

A molecule in an homologous series has the formula C₉H₂₁N.

What is the formula of the next molecule in this homologous series?

A Bunsen burner burns methane gas, CH₄.

A student investigates the time taken to heat up a beaker of water using this Bunsen burner.

Figure 5 shows the apparatus used.

The student sees that a large amount of carbon forms as a black solid on the underside of the gauze.

The teacher said that the investigation would be improved if no carbon forms.

Explain what the student should do to stop carbon forming on the underside of the gauze.

*(d) Natural processes and human activities change the amount of carbon dioxide in the atmosphere.

Figure 6 shows the concentration of carbon dioxide in the atmosphere measured in one location and the mean global temperature from 1960 to 2000.

Evaluate whether the data in Figure 6 shows that the change in the amount of carbon dioxide is leading to climate change.

Your answer should include

• the natural processes that change the amount of carbon dioxide in the atmosphere

• the human activities that change the amount of carbon dioxide in the atmosphere

• how carbon dioxide contributes to the greenhouse effect and climate change.

An investigation was carried out on the reactivity of four metals, D, E, F and G.

Equal sized pieces of these metals were placed in excess dilute hydrochloric acid and left for three minutes.

Figure 6 shows the observations of the reactions for metals D, E and F.

The order of reactivity for these metals is shown in Figure 7.

most reactive
least reactive

Figure 7

Use the information in Figure 6 and Figure 7 to suggest the observations that would be made for metal G.

The dilute hydrochloric acid used in this reaction is a strong acid.

Explain the meaning of the terms dilute and strong acid.

dilute

strong acid

The formula of lead ethanoate is Pb(CH₃COO)₂.

Calculate the number of atoms that combine together to form 16.25 g of lead ethanoate.

(relative atomic masses: H = 1.00, C = 12.0, O = 16.0, Pb = 207

Avogadro number = 6.02×10²³)

number of atoms =_........

Damp iron wool reacts with oxygen in the air. A student uses the apparatus in Figure 4 to investigate the percentage of oxygen in the atmosphere.

The initial volume of air in the measuring cylinder was 18.0 cm³.

The student left the apparatus overnight.

The volume of gas in the measuring cylinder the next day was 14.5 cm³.

To the nearest whole number, what percentage of the air has reacted with the iron wool?

Describe one improvement the student could make to this method to ensure that all of the oxygen in the measuring cylinder has reacted.

When hydrocarbon fuels are burned, the products are water and carbon dioxide.

Describe what needs to be done to the apparatus in Figure 5 to collect the water and show that carbon dioxide has been produced.

A hydrocarbon, C_xH_y, is burned in excess oxygen, forming 26.4 g of carbon dioxide and 5.4 g of water.

The relative formula mass of C_xH_y is 78.

Calculate the molecular formula of the hydrocarbon C_xH_y.

(relative atomic masses: H = 1.0, C = 12; relative formula masses: H₂O = 18, CO₂ = 44)

molecular formula =_........

A student investigates the mass of copper produced when copper chloride solution in a beaker is electrolysed using inert electrodes.

Where is copper formed during the electrolysis?

The student investigated the change in the mass of copper formed when the current was altered.

The results are shown in Figure 4.

State and explain the trend shown in these results.

Describe how, after the power supply has been switched off, the mass of copper formed can be measured.

In another experiment, 74 mg of copper is formed.

Calculate the number of copper atoms in 74 mg of copper.

(relative atomic mass Cu = 63.5; Avogadro constant = 6.0210²³)

number of atoms =_........

A student carried out an investigation to determine the order of reactivity of four metals, W, X, Y and Z. A piece of metal W was added to a test tube containing excess dilute hydrochloric acid. This was repeated with the other three metals, X, Y and Z. In each case, the size of each piece of metal was the same. The student recorded observations on each reaction for three minutes.

The observations obtained are shown in Figure 6.

Use the information in Figure 6 to place the metals in order of reactivity from the least reactive to the most reactive.

The experiment was repeated using an excess of dilute sulfuric acid in place of the dilute hydrochloric acid.

metal + sulfuric acid metal sulfate + hydrogen

When metal Y reacts with dilute sulfuric acid, bubbles form quickly at first and then the reaction stops. Most of the solid metal remains.

Explain why the reaction between metal Y and excess dilute sulfuric acid stopped even though there was solid metal Y left.

The reactions between metals and dilute ethanoic acid are slower than reactions between metals and dilute hydrochloric acid. This is because ethanoic acid is a weak acid.

Explain the meaning of the term weak acid.

The formula of aluminium sulfate is Al₂(SO₄)₃.

Calculate the total number of atoms that combine to form 5.13 g of aluminium sulfate.

(relative atomic masses: O = 16.0, A l = 27.0, S = 32.0 Avogadro number = 6.0210²³)

number of atoms =_........

Iron is more reactive than lead. Iron reacts with lead nitrate solution to form solid lead. Two possible balanced equations for the reaction are

Equation 1 Fe + Pb(NO₃)₂ Fe(NO₃)₂ + Pb

Equation 2 2Fe + 3Pb(NO₃)₂ 2Fe(NO₃)₃ + 3Pb

In one experiment, it was found that 4.48 g of iron reacted with excess lead nitrate solution to form 24.84 g of lead.

Carry out a calculation, using the information above, to show which equation represents the reaction taking place.

(relative atomic masses: Fe = 56.0, Pb = 207)

This question is about gases.

When sodium is added to water, hydrogen gas is produced.

Which observation shows that a gas has been produced?

Some damp litmus paper is placed in a gas. The litmus paper is bleached.

Which gas bleaches damp litmus paper?

When calcium carbonate is heated it decomposes.

When 5.000 g of calcium carbonate is heated, the mass of solid remaining is 2.800 g.

Calculate the mass of carbon dioxide that has been released.

Give your answer to three significant figures.

mass of carbon dioxide =_........ g

A diagram of an atom of helium is shown in Figure 3.

Explain, using Figure 3, why helium is inert.

Helium is used to fill balloons.

State one property of helium, apart from it being inert, that makes it suitable for filling balloons.

Oxygen gas has the formula O₂.

Calculate the number of oxygen atoms in 3.50 mol of oxygen gas.

(Avogadro constant = 6.0210²³)

number of oxygen atoms =_........

Copper carbonate reacts with dilute nitric acid.

During the reaction the copper carbonate powder completely disappears.

State what can be deduced about the amount of acid used.

During the reaction, the pH of the mixture changed from 2 to 6.

By what factor has the concentration of the hydrogen ions in the mixture changed?

Using different reactants, a solution of copper sulfate was prepared.

Describe what should be done to obtain copper sulfate crystals from this copper sulfate solution.

Hydrated copper sulfate has the formula CuSO₄.5H₂O. The formula tells us that each mole of copper sulfate contains 5 moles of water.

A sample of CuSO₄.5H₂O was heated gently until all the water was removed to form anhydrous copper sulfate, CuSO₄.

The mass of water formed was 4.5 g.

Calculate the mass of hydrated copper sulfate that was heated.

(relative atomic masses: H = 1.0, O = 16.0; relative formula mass: CuSO₄.5H₂O = 249.5)

mass of CuSO₄.5H₂O =_........ g

The order of reactivity of copper, magnesium and zinc can be determined by the displacement reactions between these metals and solutions of their salts.

You are provided with

• samples of the three metals

• solutions of copper sulfate, magnesium sulfate and zinc sulfate.

Describe the experiments that can be done to determine the order of reactivity of these metals by displacement reactions.

Metals can be extracted from ores found in the Earth’s crust.

Explain why aluminium cannot be extracted from its ore by heating with carbon but can be extracted by electrolysis.

Titanium is extracted from its ore in several stages.

In the first stage, titanium chloride is formed as a gas.

The gas is cooled to form liquid titanium chloride containing dissolved impurities.

Suggest how pure titanium chloride could be separated from the impurities.

In another stage, the pure titanium chloride, TiCl₄, is reacted with 500 moles of magnesium, an excess.

Calculate the number of moles in 45 000 grams of titanium chloride.

(relative atomic masses: C l = 35.5, Ti = 48.0)

number of moles titanium chloride =_........

Show that the 500 moles of magnesium added is an excess.

After this reaction, there is a mixture of the solids magnesium, titanium and magnesium chloride.

Titanium does not react with dilute hydrochloric acid.

Suggest a simple method to separate titanium from the mixture.

The first four elements in group 1 are lithium, sodium, potassium and rubidium.

Rubidium reacts with water to form rubidium hydroxide and hydrogen.

Predict what you would see when a small piece of rubidium is placed in a large volume of water.

Why is rubidium more reactive than potassium?

8.5 g of rubidium are reacted completely with water.

The reaction makes a solution of rubidium hydroxide. The volume of this solution is 2.5 dm³.

Calculate the concentration of the rubidium hydroxide solution in g dm^–3.

(relative atomic mass: Rb = 85; relative formula mass: RbOH = 102)

concentration =_........ g dm^–3

An example of an endothermic reaction is the reaction between rubidium hydroxide and ammonium carbonate, (NH₄)₂CO₃.

This reaction forms rubidium carbonate, Rb₂CO₃, ammonia and one other product.

Write the balanced equation for this reaction.

Hydrated copper sulfate, CuSO₄.5H₂O, is a blue solid. Anhydrous copper sulfate, CuSO₄, is a white solid.

Heat energy is needed to convert hydrated copper sulfate to anhydrous copper sulfate. This is a reversible reaction.

Devise an experiment to show that this is a reversible reaction.

Hydrogen reacts with iodine to form hydrogen iodide. Iodine gas is purple and hydrogen iodide gas is colourless.

Hydrogen and iodine are placed in a sealed container. The container is left until equilibrium is reached.

The conditions are changed favouring the forward reaction.

Explain what you would see.

Calculate the number of atoms combined in one mole of copper iodide, CuI₂. (Avogadro constant = 6.02 × 10²³)

number of atoms =_........

Pieces of zinc react with copper sulfate solution. Zinc sulfate solution is colourless.

Describe what you would see when an excess of zinc is added to copper sulfate solution and the mixture left until the reaction is complete.

This reaction is described as a redox reaction.

Explain, in terms of electrons, which particles have been oxidised and which particles have been reduced in this reaction.

The copper sulfate solution used has a concentration of 15.95 g dm⁻³.

Calculate the number of moles of copper sulfate, CuSO₄, in 50.00 cm³ of this solution. (relative atomic masses: O = 16, S = 32, Cu = 63.5)

number of moles of copper sulfate =_........ mol

In another experiment, 0.043 mol of copper sulfate, CuSO₄, is used.

Calculate, to one decimal place, the minimum mass of zinc that must be added to react with all the copper sulfate. (relative atomic mass: Zn = 65)

mass =_........ g

Carbon dioxide is one of the gases in the Earth’s atmosphere. The percentage of carbon dioxide in the Earth’s atmosphere has changed over time.

Which row of the table shows the approximate percentage of carbon dioxide thought to be in the Earth’s early atmosphere and how this percentage changed to form the Earth’s atmosphere today?

The actual percentage of carbon dioxide in the Earth’s atmosphere today varies.

Explain two factors that cause the percentage of carbon dioxide in today’s atmosphere to vary.

factor 1_........

Carbon dioxide is a simple molecular, covalent compound.

It has a low boiling point of –78.5C.

Explain why carbon dioxide has a low boiling point.

Calculate the number of molecules in 0.11 g of carbon dioxide.

Give your answer to two significant figures.

(relative formula mass: CO₂ = 44 Avogadro constant = 6.0210²³)

number of molecules =_........

(a) A student placed a piece of metal P in a test tube containing excess dilute sulfuric acid. The student repeated this with three other metals, Q, R and S. All the pieces of all four metals were the same size.

The student recorded the observations until each metal had reacted with the acid for two minutes. The observations are shown in Figure 6.

Use this information to put the four metals in order of reactivity from the least reactive to the most reactive.

least reactive
most reactive

Acid solutions contain hydrogen ions.

Aluminium reacts with dilute hydrochloric acid to form a solution containing aluminium ions, Al³⁺.

Complete the balanced ionic equation for this reaction.

_........ +_........ H⁺ _........ Al³⁺ +_........

The hydrogen ion concentration in a solution is decreased by a factor of 10.

State how the pH of this solution changes.

Calculate the mass, in g, of a hydrogen atom, using the data below. (relative atomic mass: H = 1.00; Avogadro constant = 6.02 × 10²³)

mass of hydrogen atom =_........ g

(a) The rate of reaction between magnesium ribbon and dilute hydrochloric acid at room temperature is investigated.

The apparatus used is shown in Figure 6.

The volume of hydrogen gas given off was measured at regular intervals during the reaction.

The graph in Figure 7 shows the results of this experiment.

State a change that can be made to the apparatus in Figure 6 to measure the volumes of gas more accurately.

A tangent has been drawn to the line on the graph in Figure 7.

Calculate the rate of reaction at this point.

rate of reaction =_........ cm s

On the graph in Figure 7, draw the line you would expect to obtain if the magnesium ribbon in this experiment was replaced with an equal mass of powdered magnesium. All other conditions are kept the same.

The balanced equation for this reaction is

In another experiment, 0.1 moles of hydrochloric acid, HC l, were reacted with 0.1 g of magnesium ribbon.

Calculate the number of moles of magnesium, Mg, in the 0.1 g sample of magnesium ribbon. (relative atomic mass: Mg = 24)

number of moles =_........

In a further experiment, 0.5 mol of hydrochloric acid, HC l, were mixed with 0.5 mol of magnesium, Mg.

Use the equation to show that, in this experiment, the magnesium is in excess.

*(c) Two substances, A and B, each form a colourless solution. If the solutions are mixed in a beaker, A and B react to form a coloured product. The rate of the reaction between A and B can be investigated by placing the beaker containing the mixture on a cross on a piece of paper and timing how long it takes for enough coloured product to be produced to make the cross invisible when viewed from above, through the solution.

Use the results of these experiments to explain, in terms of the behaviour of particles, the effect of changing temperature and the effect of changing the concentration of A in solution on the rate of this reaction.