A student carried out an extended investigation into how the concentration of yeast affects fermentation, using the balance apparatus in Fig. 21.1. Five fla...
Question 1 Report
A student carried out an extended investigation into how the concentration of yeast affects fermentation, using the balance apparatus in Fig. 21.1. Five flasks were set up, each with the same volume and concentration of glucose but a different mass of yeast, plugged with cotton wool. Each flask was weighed at the start and again after three hours, all kept at 30 °C. As the yeast respired anaerobically, carbon dioxide escaped and the mass fell. The mass lost by each flask is shown in Table 21.1, and a printed grid was provided for plotting mass lost against mass of yeast.
| Mass of yeast / g | Start mass / g | Mass after 3 h / g | Mass lost / g |
|---|---|---|---|
| 1.0 | 200.0 | 198.8 | 1.2 |
| 2.0 | 200.0 | 197.8 | 2.2 |
| 3.0 | 200.0 | 197.0 | |
| 4.0 | 200.0 | 196.6 | 3.4 |
| 5.0 | 200.0 | 196.5 | 3.5 |
(a) State the independent variable and the dependent variable [2]
(b) Name the gas that escaped, causing the mass to fall [1]
(c) Complete Table 21.1 by calculating the mass lost with 3.0 g of yeast [1]
(d) Using Table 21.1, describe how you would plot mass lost against mass of yeast on the printed grid [3]
(e) Describe the relationship shown by the results [2]
(f) Explain why the mass lost levelled off at higher masses of yeast [2]
(g) Explain why cotton wool was used to plug each flask [2]
(h) Describe how the student made this a fair test [2]
(i) Describe two ways to make the results more reliable [2]
(j) Calculate the mean rate of mass loss for the 3.0 g flask, in g per hour. Show your working [2]
(k) State the process taking place in the flasks [1]
(l) State two variables that were kept constant [2]
Answer Details
(a) The independent variable is the mass of yeast (the factor deliberately changed) [1]; the dependent variable is the mass lost (the factor measured as a result) [1].
(b) The escaping gas is carbon dioxide [1], produced as the yeast ferments the glucose; it leaves through the cotton wool so the flask gets lighter.
(c) Mass lost with 3.0 g of yeast \(=200.0-197.0=3.0\text{ g}\) [1]. The completed table is:
| Mass of yeast / g | Start mass / g | Mass after 3 h / g | Mass lost / g |
|---|---|---|---|
| 1.0 | 200.0 | 198.8 | 1.2 |
| 2.0 | 200.0 | 197.8 | 2.2 |
| 3.0 | 200.0 | 197.0 | 3.0 |
| 4.0 | 200.0 | 196.6 | 3.4 |
| 5.0 | 200.0 | 196.5 | 3.5 |
(d) Put the mass of yeast on the x-axis and mass lost on the y-axis, both labelled with units [1], choose a scale that spreads the points over at least half the grid [1], then plot each point accurately and draw a smooth best-fit line [1]. Plotted, the data give:
(e) As the mass of yeast increased, the mass lost increased [1], but the increase became smaller and the line levelled off at the higher masses [1].
(f) Beyond about 3 g the glucose becomes the limiting factor: it is being fermented as fast as possible / is being used up [1], so adding more yeast makes no further difference to the amount of carbon dioxide released [1].
(g) The cotton wool lets the carbon dioxide escape so the mass can fall [1], while still keeping out contamination / other microbes [1].
(h) Any two kept the same: the volume and concentration of glucose; the temperature (30 °C); the time (3 h); the same type of flask. [2]
(i) To make the results more reliable: repeat each mass of yeast and take a mean [1]; and use the same accurate balance / weigh carefully so readings are consistent [1].
(j) Mean rate of mass loss for the 3.0 g flask \(=\dfrac{\text{mass lost}}{\text{time}}=\dfrac{3.0}{3}=1.0\text{ g per hour}\). Working [1]; answer 1.0 g/h [1].
(k) The process in the flasks is anaerobic respiration (fermentation). [1]
(l) Any two constants: concentration or volume of glucose; temperature; time. [2]
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