Assume the atomic mass of element X is 22.99 amu. A 17.15-g sample of X combines with 14.17 g of another element Y to form a compound XY. Calculate the atomic mass of Y.

The formula of the compound is XY. This means that the relation between the moles is 1: 1. One mole of X per one mole of Y.

From the information about the element X you can determine the number of moles of X (which is the same that the number of moles of Y).

# of moles of X = weigth of X / atomic mass of X = 17.15 g / 22.9 g/mol = 0.74598

Now the atomic mass of Y = weight of Y / # of moles of Y = 14.17 g / 0.74598 mol = 19 amu