So,
First of all, let's find the molar mass of CaCl2:
We want to find the percent composition of Cl in the compound. So:
We have 70.9 grams of Cl2 in a total of 110g. What we're going to do is to divide these amounts and then multiply by 100:
If we round to the nearest whole number, that's 64%.
So the answer is 64%.