Question

To determine the percentage of water in a hydrated salt, a student heated a 1.2346 g sample of the salt for 30 minutes; when cooled to room temperature, the sample weighed 1.1857 g. After the sample was heated for an additional 10 minutes and again cooled to room temperature, the sample weighed 1.1632 g. Which of the following should the student do next?(A) Use the smallest mass value to calculate the percentage of water in the hydrated salt.(B) Repeat the experiment with a new sample of the same mass and average the results.(C) Repeat the experiment with a new sample that has a different mass.(D) Reheat the sample until its mass is constant.(E) Use the average of the mass values obtained after the two heatings to calculate the percentage of water in the hydrated

salt.

Answer 1

The correct answer is option D: Reheat the sample until its mass is constant.

Step-by-step explanation:

The student should reheat the sample until its mass will be constant. This to avoid that the percentage contains other substances than only water. During heating, hydrated salt loses its water. He only wants to know the percentage of water, so until the mass keeps changing a lot ( isn't constant), means there is still both water and salt. If the mass doesn't change anymore, means that all the water has been heated and evaporated. Then he can calculate the percentage of salt in the sample, and thus the percentage water as well.