Percentage Composition — Prelims Questions

What is the percentage by mass of oxygen in $H_2SO_4$ (sulfuric acid)? (Atomic masses: H=1, S=32, O=16)

A compound contains 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula? (Atomic masses: C=12, H=1, O=16)

A compound has an empirical formula of $CH_2$ and a molar mass of $84,\text{g/mol}$. What is its molecular formula? (Atomic masses: C=12, H=1)

A hydrated salt, $MgSO_4 cdot xH_2O$, contains 51.2% water by mass. What is the value of 'x'? (Atomic masses: Mg=24, S=32, O=16, H=1)

Which of the following statements about percentage composition is INCORRECT?

A sample of a compound contains 2.0 g of hydrogen and 16.0 g of oxygen. What is the percentage by mass of hydrogen in this compound? (Atomic masses: H=1, O=16)