Exercises: Percent Composition and Empirical and Molecular Formulas

Define percent composition by mass of a compound. Then write the expression you would use to calculate the percent by mass of oxygen in a compound, using the terms "mass of oxygen in one mole of compound" and "molar mass of the compound."

Calculate the percent composition by mass of hydrogen and oxygen in water, $\text{H}_2\text{O}$. Give your answers to three significant figures.

Classify each formula as an empirical formula only, a molecular formula only, or possibly both: $\text{C}_6\text{H}_{12}\text{O}_6$, $\text{CH}_2\text{O}$, $\text{H}_2\text{O}$, $\text{C}_2\text{H}_4$. Give a one-sentence reason for each.

Aaliyah is studying fertilizers. Calculate the percent by mass of nitrogen in ammonium nitrate, $\text{NH}_4\text{NO}_3$, and in urea, $\text{CO(NH}_2)_2$. Which compound is the richer source of nitrogen per gram? Show your work.

In the lab, Mateo analyzes a 2.500 g sample of a compound and finds that it contains 1.000 g of carbon, 0.168 g of hydrogen, and the rest is oxygen. Find the mass of oxygen, convert each mass to moles, and determine the empirical formula of the compound.

A compound has the empirical formula $\text{CH}_2\text{O}$ and a molar mass of 180.16 g/mol. Determine its molecular formula. Then explain why the empirical formula alone cannot tell you the molar mass of a compound.

Priya analyzes an unknown iron oxide and finds that it is 69.9% iron by mass, the rest being oxygen. (a) Determine the empirical formula of the oxide. (b) Calculate the percent iron by mass in $\text{FeO}$ and in $\text{Fe}_2\text{O}_3$, and use your results to argue which of these two compounds Priya has. (c) Explain why a 100.0 g sample is a convenient assumption for this kind of problem.

A compound is analyzed by combustion. A 1.000 g sample is found to contain 0.5217 g of carbon, 0.1304 g of hydrogen, and 0.3479 g of oxygen. Its molar mass is determined to be about 46 g/mol. (a) Find the empirical formula. (b) Find the molecular formula. (c) Another compound has the same empirical formula but a molar mass near 92 g/mol. Write its molecular formula and explain whether its percent composition by mass differs from that of the first compound.