I'm in Year 13 and this is a past paper question which I don't have a mark scheme for and I'm confused on the chemistry.

Hydrated potassium chromium(III) sulfate has the formula $\ce{KCr(SO4)2.xH2O}$

An experiment using heating can determine $x$.

Give a step by step description of how you would carry out the experiment

Include all measurements taken

Outline how you would use your results to find $x$.

You are provided with solid $\ce{KCr(SO4)2.xH2O}$ and apparatus usually found in a school laboratory

I think mostly I want to know how would begin with this type of question, like what part of my knowledge do you think this is testing me on because I don't know where to start.

  • $\begingroup$ I think the key lies in the word "heating" in line 2. What happens to a hydrated compound as you heat it (assuming it doesn't decompose)? It's simple if you knew there were one water of hydration, so imagine what would happen to the weight of the a mono-hydrated compound as you heated it? Then if there were more waters of hydration, repeat? That's a pretty big hint, bet let me know if I've just confused you. $\endgroup$
    – airhuff
    Feb 13, 2017 at 19:39

1 Answer 1


All the clues you need to solve the problem have already been given. A key clue is the second line; the word heating. What do you know about heating and how it affects water content? Use this as a starting point.

Thenceforth, the question’s solution should suggest itself.


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