I am enrolled in analytical chemistry after a 4 year break from any chemistry. I was recently given a problem set and I need some help on the following:

A 0.899g sample containing epsom salt, MgSO4 & 7 H2O, and other non-hydrated materials was dehydrated completely in a stream of hot, dry nitrogen. The resulting air stream was passed through a tube containing the desiccant magnesium perchlorate. The original weight of the desiccant tube was 61.997g and, after the determination, the tube weighed 62.318g. What is the % epsom salt in the sample.

I would appreciate any help. Thanks!

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  1. The gain in weight of the perchlorate is the mass of H2O. Convert that to mols by mols = grams/molar mass H2O.
    Convert mols H2O to mols MgSO4.7H2O using the coefficient in the balanced equation below.
    MgSO4.7H2O+ heat ==> MgSO4 + 7 H2O
    mols MgSO4.7H2O = mols H2O x (1 mol MgSO4.7H2O/7H2O). (note how the units that we don't want cancel and leaves the units we want to keep.)
    Convert mols MgSO4.7H2O to grams.
    g = mols x molar mass.
    %MgSO4.7H2O = [mass MgSO4.7H2O/mass sample]*100 = ??
    Post your work if you get stuck.

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