Question

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!

Answer 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.
Finally,
%MgSO4.7H2O = [mass MgSO4.7H2O/mass sample]*100 = ??
Post your work if you get stuck.