5.3.1 Percentage error, percentage yield and atom economy Quiz: Pearson Edexcel Chemistry, Unit 5

The 20 questions

1. What is the percentage yield of a reaction?

   • The mass of the reactants divided by the mass of the product, multiplied by 100 to give the percentage
   • The theoretical mass divided by the actual mass of the product, multiplied by 100 to give a percentage value
   • The moles of product divided by the moles of reactant, multiplied by 1000 to give the yield in per mille
   • The actual mass or moles of product divided by the theoretical mass or moles, multiplied by 100

2. A reaction has a theoretical yield of 8.0 g and produces 6.0 g. What is the percentage yield?

   • 75%
   • 133%
   • 80%
   • 60%

3. A volume of 25.0 cm3 is measured with an uncertainty of plus or minus 0.05 cm3. What is the percentage uncertainty?

   • 0.2%
   • 0.05%
   • 2%
   • 0.02%

4. A mass of 0.50 g is measured to plus or minus 0.01 g. What is the percentage uncertainty?

   • 0.02%
   • 2%
   • 0.2%
   • 5%

5. How can the percentage uncertainty in a titre be reduced?

   • Use a larger titre volume with the same burette, since the absolute uncertainty stays about the same
   • Use a smaller titre volume with the same burette, since the reading is then more precise and easier to read
   • Use a balance instead of a burette, since a balance gives a more precise volume for the titration result
   • Use a more concentrated indicator, which changes colour more sharply and so reduces the uncertainty in the titre

6. How can random errors in an experiment be reduced?

   • Use a different method for each measurement
   • Repeat measurements and calculate a mean
   • Ignore results that differ from the mean
   • Calibrate the balance against a known mass

7. Which is an example of systematic error?

   • A balance that reads 0.02 g too high on every measurement
   • A volume read at different points on the meniscus
   • A reading taken from a slightly different angle each time
   • A temperature that fluctuates randomly during an experiment

8. Which factor is the most likely reason for a percentage yield below 100%?

   • The balance was zeroed correctly before each weighing, which means the masses recorded are accurate
   • The reaction produced more product than expected because the reagents were more pure than the calculation assumed
   • Some product is lost during transfer or purification
   • The reaction used an excess of one reagent, which always makes the yield exceed the theoretical value

9. What does atom economy measure?

   • The mass of waste produced per gram of product, which is calculated from the mass of the product
   • The speed at which the reactants are used up during the course of the reaction in the flask
   • The proportion of reactant atoms that end up in the desired product
   • The percentage of the reaction that has reached equilibrium at the end of the experiment in the flask

10. A reaction theoretically gives 0.50 mol of product and actually gives 0.40 mol. What is the percentage yield?

    • 125%
    • 40%
    • 20%
    • 80%

11. 10.0 g of calcium carbonate is heated and gives 4.0 g of calcium oxide. What is the percentage yield? (CaCO3 = 100, CaO = 56)

    • 56%
    • 40%
    • 100%
    • 71%

12. A student measures a titre of 24.50 cm3, but the true value is 25.00 cm3. What is the percentage error?

    • 24.5%
    • 0.5%
    • 4.0%
    • 2.0%

13. A two-step synthesis has yields of 80% and 65%. What is the overall percentage yield?

    • 65%
    • 145%
    • 72.5%
    • 52%

14. Which change most reduces the percentage uncertainty in a mass measurement?

    • Measuring the mass twice and adding the results
    • Using a larger mass with the same balance
    • Using a smaller mass with the same balance
    • Using a glass beaker instead of a weighing boat

15. A student's calculated yield is over 100%. Which is the most likely reason?

    • All reactants were used up with no loss at any stage of the process, which gives the maximum possible yield
    • The reaction went to completion exactly, so the yield was calculated correctly and should be exactly 100 percent
    • The balance was zeroed correctly before each weighing, so the masses should be accurate and give the right answer
    • The product is impure, for example containing water or solvent

16. Explain why atom economy matters even when the percentage yield is 100%.

    • A reaction with 100% yield always has an atom economy of 100%, so the two measures always give the same value
    • Atom economy only applies to reactions that do not produce a gas, so it is irrelevant to most reactions
    • A reaction with 100% yield can still produce many by-products, so most reactant atoms are wasted
    • A 100% yield means no waste can be produced, so the atom economy must also be 100% for the reaction

17. Explain the difference between random and systematic error.

    • Random errors always shift results in one direction, while systematic errors vary unpredictably from one reading to the next
    • Random errors vary unpredictably and are reduced by repeats; systematic errors shift all results one way and need calibration
    • Systematic errors are caused only by human mistakes, and random errors arise from the apparatus alone
    • Both types of error are reduced by taking more readings, because every error averages out to zero in the end

18. A set of results agrees closely with each other but differs from the accepted value. What does this suggest?

    • An error in the balance reading only, because the balance is the only instrument that could be wrong
    • Random error only, because the results agree closely with one another, which shows that the errors are random
    • A systematic error, because the results are precise but not accurate
    • No error at all, because the results are consistent with one another and so must be accurate and correct

19. A 2-step synthesis has an overall yield of 50%. The first step has a yield of 100%. What is the yield of the second step?

    • 100%
    • 50%
    • 150%
    • 25%

20. Which expression defines percentage uncertainty?

    • (absolute uncertainty x measured value) / 100
    • (measured value / absolute uncertainty) x 100
    • (measured value - absolute uncertainty) / 100
    • (absolute uncertainty / measured value) x 100

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