A substance, A2B, has the composition by mass of 60% A and 40% B. What is the composition of AB2 by mass? A) 40% A, 60% B B) 50% A, 50% B C) 27% A, 73% B D) 33% A, 67% B E) none of these

ANS: C Use an arbitrary mass, 100 g is an easy number to work with. 60% of 100 g is 60 g, there are two A's. Each A is 30 g 40 g is B, and there is only one, so B is 40 g. AB2, would have a mass of 30 g + 2*40 g = 110 g The new percent by mass composition of A is: 30g110g∗100%=27.3% The new percent by mass composition of B is: 80g110g∗100%=72.7%