Yield & Atom Economy Practice Questions

Question 1

3 marks

A student prepared zinc sulfate crystals by adding zinc oxide to dilute sulfuric acid, then filtering and crystallising the solution.

The maximum theoretical mass of zinc sulfate crystals that could be produced was 8.05 g.

The student produced 6.44 g of zinc sulfate crystals.

(a)

Calculate the percentage yield of zinc sulfate crystals.

Use the equation:

$\%\ \text{yield} = \frac{\text{mass of product actually made}}{\text{maximum theoretical mass of product}} \times 100$

Percentage yield = ................. %

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(b)

Give one reason why the mass of zinc sulfate crystals produced was less than the maximum theoretical mass.

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Question 2

3 marks

Ethanol is manufactured by reacting ethene with steam.

The equation for the reaction is:

$\mathrm{C_2H_4(g) + H_2O(g) \rightleftharpoons C_2H_5OH(g)}$

(a)

How does the equation show that the atom economy of the forward reaction is 100%?

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(b)

In this process the percentage yield of ethanol is less than 100%.

Suggest one reason why, using the information in the equation.

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(c)

Give one reason why it is important for industry to use reactions with a high atom economy.

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Question 0

5 marks

Zinc nitrate crystals can be made by reacting zinc carbonate with dilute nitric acid.

The equation for the reaction is:

$\mathrm{ZnCO_3 + 2HNO_3 \rightarrow Zn(NO_3)_2 + H_2O + CO_2}$

This is the method used by a student.

Add zinc carbonate powder to $25\ \mathrm{cm^3}$ of dilute nitric acid until the zinc carbonate is in excess.
Filter the mixture.
Warm the filtrate to evaporate some of the water.
Leave the solution to crystallise, then dry the crystals between two pieces of filter paper.

(a)

The percentage yield of zinc nitrate crystals was less than 100%.

Suggest two reasons why, with reference to the method.

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(b)

Calculate the percentage atom economy for the production of zinc nitrate in this reaction.

Give your answer to 3 significant figures.

Relative formula masses ( $M_r$ ): $\mathrm{ZnCO_3} = 125$ , $\mathrm{HNO_3} = 63$ , $\mathrm{Zn(NO_3)_2} = 189$ , $\mathrm{H_2O} = 18$ , $\mathrm{CO_2} = 44$

Percentage atom economy = ................. %

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Question 0

5 marks

Ammonium sulfate is used as a fertiliser. It is made by reacting ammonia solution with dilute sulfuric acid.

$\mathrm{2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4}$

A technician carried out two experiments to make ammonium sulfate.

(a)

In Experiment 1, the maximum theoretical mass of ammonium sulfate was 9.25 g. The technician obtained 7.03 g of ammonium sulfate.

Calculate the percentage yield of ammonium sulfate in Experiment 1.

Percentage yield = ................. %

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(b)

In Experiment 2, the technician obtained 11.4 g of ammonium sulfate.

The percentage yield in Experiment 2 was 60.0%.

Calculate the maximum theoretical mass of ammonium sulfate that could have been produced in Experiment 2.

Maximum theoretical mass = ................. g

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Question 0

5 marks

A student heated a 6.0 g coil of magnesium ribbon in a crucible until all of the magnesium had reacted with oxygen in the air.

The equation for the reaction is:

$\mathrm{2Mg + O_2 \rightarrow 2MgO}$

Relative atomic masses ( $A_r$ ): $\mathrm{O} = 16$ , $\mathrm{Mg} = 24$

(a)

Calculate the maximum theoretical mass of magnesium oxide that could be produced from 6.0 g of magnesium.

Maximum theoretical mass = ................. g

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(b)

The student collected 8.6 g of magnesium oxide.

Calculate the percentage yield of magnesium oxide.

Percentage yield = ................. %

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Question 0

6 marks

Ethanol can be produced by the fermentation of glucose.

The equation for the reaction is:

$\mathrm{C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2}$

Relative atomic masses ( $A_r$ ): $\mathrm{H} = 1$ , $\mathrm{C} = 12$ , $\mathrm{O} = 16$

(a)

Calculate the percentage atom economy for the production of ethanol by fermentation.

Give your answer to 3 significant figures.

Percentage atom economy = ................. %

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(b)

Calculate the maximum theoretical mass of ethanol that could be produced from 45 g of glucose.

Maximum theoretical mass of ethanol = ................. g

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Question 0

4 marks

Hydrogen is being developed as a fuel for vehicles.

A company can produce hydrogen using one of two routes.

Route A: reacting methane with steam
$\mathrm{CH_4 + 2H_2O \rightarrow CO_2 + 4H_2}$

Route B: electrolysis of water
$\mathrm{2H_2O \rightarrow 2H_2 + O_2}$

Table 1

	Route A	Route B
Percentage atom economy for hydrogen	15.4%	11.1%
Percentage yield of hydrogen	96%	99%
Rate of production of hydrogen	fast	slow
By-product	carbon dioxide	oxygen
Energy needed	heat, from burning methane	large amount of electricity

The company wants to produce hydrogen in the most sustainable way.

Use Table 1 to suggest which route the company should choose.

Give reasons for your answer.

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Atomic Structure & the Periodic Table

Bonds, Structure & Properties of Matter

Quantitative Chemistry

Chemical Changes

Energy Changes

Chemical Change: Rate & Extent

Organic Chemistry

Chemical Analysis

Chemistry of the Atmosphere

Using Resources

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