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Practical and data skills

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Which is an independent repeat of an enzyme reaction?

  1. Preparing a fresh reaction mixture under the same specified conditions
  2. Changing enzyme amount for the second pH value
  3. Reading the final gas volume from the same tube three times and taking the mean
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Preparing a fresh reaction mixture under the same specified conditions

Independent preparations help estimate variation between experimental runs.

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A 1.0 mol dm-3 stock is diluted tenfold, then the diluted solution is diluted tenfold again. What is the final concentration?

  1. 0.10 mol dm-3
  2. 0.20 mol dm-3
  3. 0.010 mol dm-3
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0.010 mol dm-3

1.0/10/10 = 0.010 mol dm-3.

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Which display is most appropriate for comparing mean leaf length among three named plant species?

  1. A pie chart of the raw lengths
  2. A bar chart with species as categories
  3. A continuous line with invented values between species
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A bar chart with species as categories

The species are separate categories and mean length is the quantitative response.

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An endpoint is reached in 50 s in trial A and 100 s in trial B, with the same initial amount and endpoint. How do their relative rates compare?

  1. Trial A is twice as fast.
  2. The rates are equal because the endpoints match.
  3. Trial B is twice as fast.
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Trial A is twice as fast.

(1/50)/(1/100) = 2.

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What happens to the standard deviation if the same 2 cm calibration error is added to every observation?

  1. It remains unchanged, while the mean rises by 2 cm.
  2. It increases by 2 cm, the same amount as each reading.
  3. It becomes zero.
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It remains unchanged, while the mean rises by 2 cm.

A shared offset changes the centre but not the deviations around it.

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Which statement about standard deviation is NOT correct?

  1. A larger value means more spread
  2. It has the same units as the data
  3. Adding a constant to all values leaves it unchanged
  4. It is always smaller than the mean
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It is always smaller than the mean

Spread can exceed the mean when values vary widely, so this is not a rule.

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A four-category cross is tested against a fixed 9:3:3:1 expectation. What degrees of freedom apply to the specified one-row test?

  1. 16
  2. 3
  3. 4
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3

Four categories minus one gives 3 degrees of freedom.

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A chi-squared test of a 3:1 ratio gives a value of 5.2. The critical value at p = 0.05 with 1 degree of freedom is 3.84. Which conclusion is correct?

  1. Accept the null hypothesis; the data fit 3:1.
  2. The genes must be linked.
  3. The data prove the ratio is 1:1.
  4. Reject the null hypothesis of a 3:1 ratio.
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Reject the null hypothesis of a 3:1 ratio.

5.2 is above 3.84, so a difference this large would arise by chance less than 5% of the time.

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Which situation matches an unpaired t-test?

  1. Lengths of independently sampled seedlings from two treatment groups
  2. Height of each seedling before and after treatment
  3. Counts of purple and white offspring against a predicted ratio
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Lengths of independently sampled seedlings from two treatment groups

provided the other test assumptions are reasonable. Each group contains different independent individuals.

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A balance reads every mass 0.20 g too high. Which action directly addresses the problem?

  1. Round all readings to the nearest gram.
  2. Check and correct the zero/calibration before measuring.
  3. Repeat the same weighing 20 times without adjustment.
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Check and correct the zero/calibration before measuring.

This targets the systematic offset rather than merely repeating it.

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Why must Benedict's solution be in excess when comparing glucose concentrations?

  1. So that the colour always ends brick-red, whatever the concentration.
  2. So that the mixture heats up faster and reacts fully in the water bath.
  3. Because excess Benedict's solution turns non-reducing sugars into reducing sugars as it heats.
  4. So the amount of precipitate is set by the sugar, not by reagent running out.
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So the amount of precipitate is set by the sugar, not by reagent running out.

If the copper(II) ions ran out, two high concentrations could give the same colour and could not be told apart.

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Write a short answer, then compare it with a sample and check for missing ideas.

Question 1 of 11

Estimate a sugar concentration

5 marks

Describe how you would estimate the concentration of reducing sugar in a fruit juice using Benedict's solution and glucose standards. Then describe how you would show that the juice also contains a non-reducing sugar. Include two risks, each with a precaution and a rating. [5]

Standards, fixed volumes, time and temperature, reagent in excess, compare colours; acid hydrolysis and neutralisation; hazard, risk, precaution, rating.

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