# CHEM

I don't understand how to find the formula of a hydrate. Here's one of the question. if you guide me through it I think I'll be able to do the rest of them. Thanks :)

When 100g of ZnSO4x�œH2O is heated, 43.86g of water is given off. Calculate the formula of the hydrate.

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1. ZnSO4xH20 �¨ ZnSO4 + xH2O

No. of moles of H2O = mass �€ molar mass
= 43.86 �€ 18
= 2.4367 mols

No. of moles of ZnSO4xH20
= mass �€ molar mass
= 100 �€ (161 + 18x) mols
No. of moles of ZnSO4xH20
= 1/x X no. of moles of H2O
= 1/x X 2.4367
= i.e. 2.4367/x mols
Therefore, 2.4367/x = 100 �€ (161 + 18x)

Solving the equation,
100x = 2.4367 (161+18x)
56.1394x = 392.3087
x = 6.990
= 7 (whole no.)
Hence the formula is ZnSO4. 7H2O
There are 7 water molecules.

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posted by Jen
2. Please note that those missing symbols mean divide. i.e.
no. of moles = mass/molar mass

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posted by Jen

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