So in this problem we're trying to find the mass of calcium chloride that

would contain 25 grams of chlorine.

The key thing we have to remember here is that the percentages are going to

be true for a sample, regardless of the sample size.

So if I calculate the percentage of chlorine in any calcium chloride sample,

it's going to be the same for all of them.

And so I can actually do that,

find the percentage of chlorine in calcium chloride by using our molar mass of

the compound relative to the molar mass of the chlorine in that compound.

So I'm going to first look at the molar mass of the whole compound,

I have 1 calcium which has a mass of 40.01.

And then I have chlorine, which has a mass of 35.45, and there are 2 of them.

And when I add these values together, I end up with 110.91 grams per mol.