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Reliable tube mass measurements
A student investigated the concentration of salt in solution Z. The student used a method involving osmosis and tubing made of partially permeable membrane.
This is the method used.
- Cut six pieces of tubing to the same length.
- Tie one end of each piece of tubing.
- Put the same volume of solution Z into each piece of tubing.
- Tie the other end of each piece of tubing to form a sealed tube.
- Record the mass of each tube.
- Place each tube into a different concentration of salt solution.
- After 2 hours, remove each tube from the salt solutions.
- Record the mass of each tube.
The student dried the outside of each tube with a paper towel before recording the mass.
Why was it important to dry the tubes?
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Marking points
- 1 Explains that liquid left on the outside would alter/increase the measured mass (or invalidate the result).
Full-mark answer
Any water or salt solution left on the outside of a tube would add to its measured mass, so the change in mass would not be valid.
Why this answer loses marks
The tubes were dried to make them dry.
This repeats the method but misses that surface liquid would add to the balance reading and make the mass result invalid.