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Question 8
Medium

Sarah wanted to investigate the effect of pH on the rate of carbon dioxide production by yeast. She set up the apparatus shown below:

A diagram showing a test tube containing a mixture of yeast, glucose solution, and a pH buffer. There is a layer of oil resting on top of this solution. A delivery tube leads from the top of this stoppered test tube into another test tube containing water, where bubbles of gas are seen escaping from the end of the tube. The first test tube is situated inside a water bath to maintain a constant temperature.

a.

The oil layer prevents the entry of oxygen into the glucose and yeast mixture. Explain why this is necessary.

[2]
b.

Sarah varied the pH of the mixture between pH 3 and pH 9 using buffer solutions.

Sketch the expected shape of the graph if the rate of carbon dioxide production (y-axis) is plotted against pH (x-axis).

[3]
c.

State the dependent variable in this experiment.

[1]
d.

State the independent variable in this experiment.

[1]
e.

Give two variables, other than the oil layer, that Sarah would need to keep the same in her experiment.

[2]
f.

Suggest one way that Sarah could improve the reliability of her experiment.

[1]
g.

Suggest how Sarah could improve the accuracy of her measurement of the rate of carbon dioxide production.

[1]
h.

Write the word equation for the anaerobic respiration of yeast.

[3]

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