[bennybenji]

What mass of barium chloride(BaCl2) will remain after a 15.0g sample of the hydrated salt BaCl2.2H2O is heated to drive off all the water?

Mr(BaCl2) = 208.24
Mr(H2O) = 18.02

Do I start from BaCl2.2H2O since the ratio is 1:2
I thought you have to work backwards since we are given how many water molecules there are?
Also the mass of water is not given.

The final answer is 12.8g

[Borek]

Start with reaction equation.

[bennybenji]

Which reaction equation?

I thought you need to find the mass of water??

[Arkcon]

All of the information you need is there.  Lets try it this way: 

You've got a sack of bolts and nuts of a certain mass.  Every bolt has 2 nuts on it, each nut weighs 18.  Each bolt weighs 208.24.  You unscrew each nut and throw them all away, what does the sack of empty bolts weigh now?

You've twice said the mass of water is not given, but it is -- 18.02.  If they didn't feel like giving it to you (and they might not next time) it's easy to calculate -- atomic mass oxygen = 16 + atomic mass hydrogen 1.01, twice.

[EDIT]
Damn, and I checked too

[Borek]

Which reaction equation?

Reaction that happens when you heat the substance in question.

[bennybenji]

The molar mass of water is 18.02.
So i cant find the mass of water yet.

Also how do i work backwards since i know the molar masses from BaCl2 and H20 but i need to find the mass of BaCl2??

[Borek]

BaCl₂.2H₂O = BaCl₂ + 2H₂O

This is your reaction equation. It was described in the question - you are heating hydrated barium chloride driving off water. You know mass of the hydrated barium chloride, all you have to do is to calculate mass of anhydrous product.

http://www.chembuddy.com/?left=balancing-stoichiometry&right=stoichiometric-calculations