- Specification route
- RP3
- Question bank
- 14 questions
- Course stage
- AS / Year 12
Sample questions
A student needs to produce $20\ cm^3$ of a $0.2\ mol\ dm^{-3}$ sucrose solution from a $1.0\ mol\ dm^{-3}$ stock solution. What volumes of stock solution and distilled water are required?
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Use the formula $C_1V_1 = C_2V_2$. $(1.0 \times V_1) = (0.2 \times 20)$.
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Answer: $4\ cm^3$ of stock solution and $16\ cm^3$ of distilled water.
Required practical 3. To find the volume of stock needed: $(0.2 / 1.0) \times 20\ cm^3 = 4\ cm^3$. The rest of the volume ($20 - 4 = 16\ cm^3$) is made up of distilled water.
In an experiment investigating the water potential of plant tissue, why must the potato cylinders be blotted dry with a paper towel before being weighed?
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Droplets of water clinging to the outside of the potato will add false weight to the scale reading.
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Answer: To remove excess surface water, ensuring that only the actual mass of the potato tissue is recorded.
Required practical 3. Surface water varies between samples and adds unpredictable mass. Blotting dry removes this variable, ensuring the mass measured represents the water inside the tissue.
When investigating the water potential of potato tissue, why is it vital to calculate the *percentage* change in mass rather than just the change in mass in grams?
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If a $10g$ potato gains $1g$ and a $100g$ potato gains $1g$, the effect of osmosis is very different, even though the absolute mass change is the same.
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Answer: Because the initial starting masses of the potato cylinders will slightly vary; using percentages allows for valid comparisons between them.
Required practical 3. Since it's impossible to cut cylinders to the exact same starting mass, calculating the percentage change standardizes the data, making comparisons between different samples valid.
A student plots a calibration curve with sucrose concentration on the x-axis and percentage change in mass on the y-axis. What does the point where the line of best fit crosses the x-axis (x-intercept) represent?
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At the x-intercept, the y-value (percentage change in mass) is exactly zero.
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Answer: The concentration of sucrose that is isotonic to the potato tissue, meaning there is no net movement of water by osmosis.
Required practical 3. The x-intercept represents $0\%$ change in mass. This indicates no net movement of water, meaning the water potential of the solution equals the water potential of the potato tissue.
A student needs to make a dilution series of sucrose solutions: $1.0$, $0.8$, $0.6$, $0.4$, and $0.2\ mol\ dm^{-3}$. They have a stock solution of $1.0\ mol\ dm^{-3}$ and distilled water. To make $10\ cm^3$ of the $0.6\ mol\ dm^{-3}$ solution, what volumes should they mix?
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The required concentration ($0.6$) is $60\%$ of the stock concentration ($1.0$). Therefore, $60\%$ of the final volume must be stock.
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Answer: $6\ cm^3$ of the $1.0\ mol\ dm^{-3}$ stock solution and $4\ cm^3$ of distilled water.
Required practical 3 / AT c. Using $C_1V_1 = C_2V_2$: $(1.0) \times V_1 = (0.6) \times 10$. Therefore, $V_1 = 6\ cm^3$ of stock. The remainder ($10 - 6 = 4\ cm^3$) is water.
When investigating osmosis using potato cylinders, why must all the cylinders be cut using the same size cork borer and trimmed to exactly the same length?
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Osmosis happens across a surface. If one cylinder is much wider, it has more surface for water to cross.
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Answer: To ensure they all have a consistent surface area to volume ratio, so the rate of osmosis is not affected by physical dimensions.
Required practical 3 / PS 2.4. Surface area to volume ratio affects the rate of osmosis. Using the same cork borer and length controls this variable, ensuring validity.
Why should all potato cylinders used in an osmosis experiment ideally be taken from the exact same potato?
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An old, dry potato will have a very different internal water concentration than a fresh, damp potato.
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Answer: To ensure they are genetically identical and have grown in the same conditions, so their initial internal water potentials are as similar as possible.
Required practical 3. Different potatoes may have different ages, starch contents, and water potentials due to different environmental histories. Using one potato controls the biological variable of initial water potential.
A potato cylinder is placed in distilled water ($0.0\ mol\ dm^{-3}$ sucrose). After 30 minutes, what will happen to its mass and why?
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Distilled water is pure water. It has the highest possible water potential (zero).
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Answer: Its mass will increase because the water potential of the distilled water is higher than the potato tissue, so water enters the cells by osmosis.
Required practical 3. Distilled water has a water potential of $0\ kPa$ (the highest possible). The potato cells have a negative water potential due to dissolved solutes. Water moves down the gradient into the potato by osmosis, increasing its mass.
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