What is the percent by mass of Y?

Question

If 3.9 g of element X reacts with 15.2 g of element Y to form the compound XY, what is the percent by mass of X in the compound? 

What is the percent by mass of Y?

Answer 1

We will have to assume that the reaction is a 100% yield, meaning that all of the mass of both elements is completely transferred into the new compound XY.

To find percent by mass, we need to know the mass of the part (in this case, elements X and Y, respectively) and the mass of the whole. The mass of X is 3.9 g, and the mass of Y is 15.2 g, so the mass of the new compound XY must be the sum of these two numbers, which comes out to 19.1 g XY.

To find the percent by mass of X, we divide 3.9 g X by 19.1 g XY and multiply by 100 to get 20.4%. This means that by mass, 20.4% of this compound is element X.

To find the percent by mass of Y, we divide 15.2 g Y by 19.1 g XY and multiply by 100 get 79.6%. This means that by mass, 79.6% of this compound is element Y.

Hope that clears it up.

Answer 2

Conservation of mass

3.9 g x + 15.2 g Y ==> 19.1 g XY (conservation of mass dictates that mass on left = mass on right of equation

Percent by mass of Y = mass of Y/total mass (x100%) = 15.2 g/19.1 g (x100%) = 79.6%

Percent by mass of x = 100 - 79.6 = 20.4% (also obtained from 3.9 g/19.1 g (x100) = 20.4%