PercentBymassofhydrate

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                        Percent By Mass of a Hydrate
Annie Herchen 2005

What does that mean?
Some compounds have water included in their compound, and they are called hydrates. The formula has the coumpound, like Zinc Nitrate and then the moles of water that are in the compound. (ex. Zn(NO₃)₂_*6H₂O)

However, on some problems the information given does not unclude the number of moles of water in the hydrate. The problem will give the percent by mass of the compound in the entire hydrate.

Steps for the Math:

Understand what the problem is asking for.
Multiply the percent by mass of the anhydrate compound and the total mass of the hydrate.
Subtract the grams of the anhydrate compound from the total mass.This gives you the grams of water in the hydrate.
Convert the grams into moles by multiplying the molar mass of water and the grams of water.
State the answer.

Problem: A student weighs 200g of the hydrate Zn(SO₄)_*XH₂O. The compound is 75% Zn(SO₄) by mass. How many moles of water are in the hydrate?

Steps for the Math:

The X before the water in the compound is the variable the problem asks for.
200g x .75 = 150g of Zn(SO₄).
200g - 150g = 50g of water.
Molar Mass of water: 2(1.0079g) + 16.00g = 17.0158g/mol.
50g x 1 mol/17.0158g = 2.938 moles ~ 3 moles
So there are 3 moles of water in Zn(SO₄) hydrate. The final formula is Zn(SO₄)_*3H₂O.

Now try this one on your own:

Problem:
For an experiment, a scientist weighs 250g of Au(PO₄)_*XH₂O. The hydrate is 60% Au(PO₄) by mass. How many moles of water are in the hydrate?

Answer: 6 moles