Formulae, Equations & Calculations (AQA A Level Chemistry)

A synthetic carbohydrate was isotopically labelled with carbon-13.

Analysis of a 12 g sample of the carbohydrate found it to contain 6.93 g carbon, 0.80 g hydrogen and the remainder was oxygen.

Determine the empirical formulae of the carbohydrate.

The fermentation of glucose, C₆H₁₂O₆, is one method scientists can use to produce ethanol.  

Write a balanced symbol equation for this reaction. 

During the fermentation process in the reaction in part (b), 312 g of ethanol was produced from 1.5 kg of glucose. 

Calculate the percentage yield of this reaction to 1 decimal place. If you were unable to produce an equation for part (b), assume that 1 mole of glucose would produce 4 moles of ethanol for this question (this is not the correct molar ratio). 

Ethanol can also be produced by the hydration of ethene, as shown in the following equation: 

C₂H₄ (g) + H₂O (g)  →  C₂H₅OH (g)

A student predicts that the production of ethanol from the fermentation of glucose will have a higher atom economy than from the hydration of ethene. 

Concrete is one of the most common building materials and also one of the top three producers of carbon dioxide in the world. One of the main components of concrete is cement which is made by the thermal decomposition of limestone with other materials
such as clay in a kiln at temperatures around 1450 C.

Calculate the mass, in kg, of carbon dioxide produced when 1 tonne of limestone is thermally decomposed.

‘Asbestos’ is a generic, commercial term used to describe a number of mineral fibres, which were once commonly found in most buildings. The most common type of asbestos, chrysotile, makes up the vast majority of all asbestos products. Its chemical composition is 28.03% magnesium, 21.60% silicon, 1.16% hydrogen and the rest is oxygen. 

Calculate the empirical formula for chrysotile asbestos.

The M_r of chrysotile is 520.2. 

Use your answer to part (b) to calculate the actual molecular formula of chrysotile.

Another industrial mineral, belite, is used in the manufacture of cement. It’s main component is dicalcium silicate, Ca₂SiO₄, which reacts with water to form Ca₃Si₂O₄(OH)₆ and calcium hydroxide. 

Write an equation for this reaction.

A sample of hydrated copper sulphate, CuSO₄.xH₂O, has a relative formula mass of 285.5. 

Calculate a value for ‘x’. Give your answer to 2 decimal places.

A student completed an experiment to determine the value of ‘x’ in a hydrated salt. They heated a solid sample of the salt, CuSO₄.xH₂O, in a crucible for 60 seconds and recorded the following set of results.

Table 1

Use the data obtained by the student in Table 1 to calculate a value for ‘x’. Give your answer to 2 decimal places.

The actual value for x in CuSO₄.xH₂O is 5.

Copper (II) sulfate undergoes a displacement reaction with magnesium, because magnesium is a more reactive metal than copper.

Strong heating of calcium nitrate will cause a thermal decomposition reaction to occur. During the reaction, calcium oxide, nitrogen dioxide and another gas are formed.

Write a balanced chemical equation, with state symbols, for this reaction. 

A sample of 0.655 g of calcium nitrate was thermally decomposed. 

Calculate the amount, in moles, of the calcium oxide which was produced.
Give your answer to the appropriate number of significant figures.

Calculate the amount, in moles, of gas which was produced during the thermal decomposition of the 0.655 g sample of calcium nitrate.

Give your answer to the appropriate number of significant figures.

In a different experiment, a 1.23 g sample of pure lead nitrate, Pb(NO₃)₂, was heated until it had completely decomposed, in the same way as calcium nitrate in part (a).

Calculate the total volume, in cm³, of nitrogen dioxide gas produced at 50.0 C and 101 kPa.

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

Record your answer to the appropriate number of significant figures.

Calcium nitrate can be produced by the reaction of calcium sulphate with nitric acid as shown in the equation.

CaSO₄ + 2 HNO₃ → H₂SO₄ + Ca(NO₃)₂ 

Excess nitric acid was added to a 3.50 g sample of calcium sulphate forming a 50.0 cm³ solution.

Calculate the concentration of calcium nitrate in this solution.

Give your answer to 3 significant figures.

A student prepared a solution of a nitrate salt by reacting magnesium oxide with a dilute laboratory acid.

Write the equation for this reaction.

Calculate the mass in g of magnesium oxide which the student would need to use, to produce 250 cm³ of a 1.25 x 10^-2 mol dm^-3 solution of the nitrate salt in part (b). 

Give your answer to the appropriate number of significant figures. 

The student then completed a different experiment. They dissolved 3.75 g of calcium chloride in water to form a solution. This was then added to a flask containing an excess of sulfuric acid. The reaction produced a white calcium sulfate precipitate and hydrogen chloride. 

The percentage yield of the reaction was 87.2 %. 

A hydrated salt, R, has the formula Na₂CO₃.xH₂O. 

Describe the chemical tests, including reagents and observations, that could be performed on the hydrated salt, R, to show that it is a sodium carbonate salt.

A sample of the hydrated sodium carbonate salt, R, was heated to a constant mass in a 92.200 g evaporating basin. Mass measurements were taken every five minutes and are shown in Table 1. 

Table 1

Determine the value of x in Na₂CO₃.xH₂O for the hydrated salt, R.

Explain the effect that not heating the salt to a constant mass will have on the value of x in Na₂CO₃.xH₂O.

Another hydrated salt, S, was analysed and found to have the following percentage composition by mass:

16.98% Ca; 11.86% N; 67.77% O; 3.39% H

Determine the empirical formula of the compound, showing clearly that it is a hydrated salt. 

A student completed an investigation to identify the metal in an unknown metal bicarbonate, MHCO₃. They dissolved 1.94 g of the unknown metal bicarbonate into solution and reacted it with hydrochloric acid, under standard conditions. The volume of
carbon dioxide released was recorded every minute.

The student’s results are shown in Table 1.

Table 1

Plot these results on the grid in Figure 1 and draw a line of best fit.

Figure 1

The metal bicarbonate reacts with hydrochloric acid according to the following equation.

MHCO₃ (aq) + HCl (aq) → MCl (aq) + CO₂ (g) + H₂O (l) 

Use the information in part (a) to calculate the number of moles of metal bicarbonate.

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

Record your answer to the appropriate number of significant figures.

Use the information from parts a and b to calculate the M_r of the metal bicarbonate.

Use your answer from part (c) to deduce the identity of the metal in the unknown metal bicarbonate, MHCO₃.

(If you have been unable to calculate an answer for part (c), you may assume an M_r for the unknown metal bicarbonate of 194. This is not the correct answer.)

Succinic acid is a diprotic acid as shown in Figure 1.

Figure 1

A student performs an investigation to determine the structural formula of succinic acid. The equation for the neutralisation of succinic acid, represented as H₂A, with sodium hydroxide is:

H₂A (aq) + 2 NaOH (aq) → Na₂A (aq) + 2 H₂O (l) 

1. A student prepared a 250 cm³ standard solution using 1.42 g of succinic acid.
2. They transferred 25.0 cm³ of the succinic acid solution to a conical flask.
3. The succinic acid solution was titrated against 0.10 mol dm⁻³ sodium hydroxide solution.
4. Their titration results are shown in Table 1.

Table 1

Use the results to calculate the number of moles of succinic acid in the standard solution.

Using your answer to part (a), calculate the M_r of the succinic acid sample.

Deduce the value of n in HOOC(CH₂)_nCOOH to the nearest whole number and state the structural formula for the succinic acid.

At the start of the rough titration, the student had not removed the funnel from the burette. The funnel was removed before the final rough titration reading was taken.

Explain what effect, if any, this will have on the final reading of the rough titration.

The burette used by the student for their titration has an error of ±0.05 cm³. It was used to determine the initial and final reading of the titration.

Calculate the maximum percentage uncertainty in using the burette, using an average titre of 23.70 cm³. Note, this is not the answer for part (a).

Give your answer to 2 decimal places.

A student investigates the mass of calcium carbonate in an indigestion tablet by titration.

Three indigestion tablets were reacted in a beaker containing 50.0 cm³ of 1.00 mol dm^-3 hydrochloric acid. 

The student titrates 25.0 cm³ of the standard solution from part (a) with 0.10 mol dm^-3 sodium hydroxide solution. The average titre was found to be 24.90 cm³.

CaCO₃ (s) + 2 HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g) 

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l) 

Calculate the mass of calcium carbonate (M_r = 100.1), in mg, present in one indigestion tablet. Give your answer to the appropriate number of significant figures. 

The maximum error for both the pipette and the burette used in the student’s titration was ± 0.05 cm³. 

Use information from part b) and an average titre value of 24.50 cm³ to justify which piece of equipment has the lowest percentage uncertainty in its measurement.

4.25 g of an unknown metallic element, Z, reacts completely with 1000 cm³ of oxygen at 100 kPa according to the following equation:

2Z + O₂ → 2ZO

The volume of one mole of gas in these reaction conditions is 24.0 dm³.

Determine the identity of the unknown metallic element, Z.

An unknown acid has the empirical formula CH₂O and a density of 1.05 g cm^-3.

1.71 cm³ of this acid is shown to combust completely with 1486 cm³ of oxygen.

Write the balanced symbol equation, including state symbols, for the reaction of 0.03 moles of this unknown acid with oxygen under standard conditions.

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

A Group 2 metal sulfate, XSO₄, undergoes thermal decomposition to produce the metal oxide, XO, and two other gaseous products with an atom economy of 56.52%. The metal sulfate is known to be sparingly soluble in water.

Using a balanced symbol equation, justify if the metal, X, is strontium.

Explain the effect that increasing the temperature for the thermal decomposition reaction in part (c) will have on the percentage atom economy for the reaction.