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Percent By Mass of a Hydrate
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.
Problem: A student weighs 200g of the hydrate ZnSO4?*XH2O. The compound is 75% ZnSO4? 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 ZnSO4?.
- 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