# Chemistry Grade 11 - CuSO4 hydrous/anhydrous help!

1. Nov 17, 2005

### peon

ok these are the lab questions. My friends and i have been stuck on it for a whole period and afterschool and im not really sure what im doing.

1. A hydrates MGSO4 salt, weighing 3.211g, is heated until reaching a constant weight. The weight of the anhydrous MgSO4 is 1.570g.

a)Calculate the percent of water in the hydrated salt.

b)Calculate the moles of water removed and the moles of anhydrous MgSO4 remaining.

c) What is the formula for the hydrated MGSO4?

2. Anhydrous CaCl2 is used to remove water from the air within a dessicator and forms a hydrated salt. A 16.43 of anhydrous CaCl2 weighed 21.75g after being left ina dessicator for several months. What is the formula of the hydrated CaCl2?

thanks a lot!!! =)

2. Nov 18, 2005

### Astronuc

Staff Emeritus
Well one mass is for the hydrous state (with H2O) and the other mass is for the anhydous state - so the difference must be the mass of water.

So, assuming % is % by weight (or mass), find the % of water in hydrated MgSO4.

The number of moles, n, is found by dividing the mass by the molar mass - hint: molecular mass of water.

Then the formula for hydrated Mg sulfate is just MgSO4.nH2O, where n is usually an integer. You will also need the number of moles of MgSO4, so that n is an integer.

In part 2 - A 16.43 of anhydrous CaCl2 weighed 21.75g after being left ina dessicator for several months. What is the formula of the hydrated CaCl2?

Same approach as part 1. The difference in mass is the mass of water. Determine moles of water and CaCl2, and write a formula where the number of moles, n, of water is an integer.

3. Nov 21, 2005

### peon

ooo okies thx!

4. Dec 7, 2010

### aeromat

Sorry to bump this topic but I was wondering; how do we find 'n'- the integer value?

5. Dec 8, 2010

### Staff: Mentor

Experimental ratio is never exactly integer, you have to round.

Note that while formula contains n that is an integer, in reality quite often hydrated salts don't have exact composition, as there can exist several hydrates in equilibrium. In effect n determined experimentally depends on the temperature and humidity.