Hydrated Compounds.....HOW DO YOU SOLVE THEM!? I got the answer!!

11/9/15

Many of us are confused on how to solve these type of questions and frankly I was too! It takes a little brain power and practice to solve it!

Suppose a student does an experiment to determine the formula of the hydrate: Cr(NO3)3 x nH2O

Mass of Test Tube: 16.114g
Mass of Test Tube +  Cr(NO3)3 x nH2O: 34.909g
Mass of Test Tube  +  Cr(NO3)3 x nH2O (1st heating): 27.335g
Mass of Test Tube  +  Cr(NO3)3 x nH2O (2nd heating): 27.335g 

Find n.

Now where the heck to we start?!?!

First of all what is the hydrated compound? Well it's the compound with the water added to it!! Hence hydrated. It's Cr(NO3)3 x nH2O. 

Second of all what is the anhydrated compound? It's the compound without the water. Hence ANhydrated. It's Cr(NO3)3. It's also known as the salt.

So we have a test tube that weighs 16.114g. Is this mass part of the compound? NO!! So we must subtract it from any compound to get the mass of what is inside of it!

That will give us 39.909g - 16.114g = 18.795 g of the hydrated compound

Then we want to figure out the mass of the anhydrated compound (without the water).
So we must take the last mass of the last heating which is 27.335g. But WAIT!!!!! You have to remember to subtract the mass of the test tube which is 16.114g. 

That will give us 27.335-16.114g = 11.221g of the anhydrated compound. 

So we know the mass of the hydrated compound (with water) and the mass of the anhydrated compound (without water), so next we need to figure out what mass of water was driven off/evaporated. That means we need to subtract the hydrated mass from the anhydrated to give us the mass of the water.

That will give us 18.795g - 11.221g = 7.574g of H2O

Now we need to convert the H2O and anhydrated into moles.

7.574g H2O x (1mol / 18.02 molar mass) = 0.4203 g (KEEP IN MIND OF SIG FIGS!!!!!)

11.221 g Cr(NO3)3 x (1mol / 238.03 molar mass) = 0.047141 g (KEEP IN MIND OF SIG FIGS!!!!!)

A little trick to know is that is the anhydrate is supposed to be smaller than the water!
0.047141 g is smaller than 0.4203g.

The last step is to divide by the anhydrate. 

0.047141 g / 0.047141 g = 1 


0.4203g / 0.047141 g = 8.916 (KEEP IN MIND OF SIG FIGS!!!!!)

n = 8.916


Hope that helped!!!

Examples of problems and solutions for Hydrated Compounds