What is the percent composition of a compound that contains 17.6 g of iron and 10.3 g of sulfur has a total mass of 27.9 g compound?

The percent by mass of iron in the stated compound is 100(17.6/27.9) or 63.1 %, to the justified number of significant digits. Since the compound contains only two elements, the percent by mass of sulfur is 100 - 63.1 or 36.9 %.

To find the atomic percentages rather than the mass percentages, the mass for each element must be divided by the gram Atomic Mass of that element. 17.6 grams constitutes 17.6/55.457 or about 0.313 gram atoms of iron, and 10.3 grams of sulfur constitutes 10.3/32.06 or about 0.321 gram atoms of sulfur. The atomic ratiOS of the two elements in simple binary compounds such as iron sulfide must have ratiOS of small whole numbers. The specified compound contains about equal numbers of iron and sulfur and therefore has the formula FeS.