Exercises: Composition of Mixtures and Pure Substances

Classify each as a pure substance or a mixture, and if a mixture, as homogeneous or heterogeneous: (a) distilled water, (b) saltwater, (c) granite, (d) gold.

Calculate the mass percent of oxygen in water, $\text{H}_2\text{O}$. Use molar masses of 1.008 g/mol for H and 16.00 g/mol for O.

A 50.0 g sample of a trail mix contains 12.5 g of raisins. What is the mass percent of raisins in the mix?

A compound is analyzed and found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Determine its empirical formula. Use molar masses of 12.01, 1.008, and 16.00 g/mol.

A 2.500 g sample of a compound of iron and oxygen contains 1.749 g of iron. Determine the empirical formula of the compound. Use molar masses of 55.85 g/mol for Fe and 16.00 g/mol for O.

Jayden has a 10.0 g mixture of sand and sodium chloride. He dissolves the salt in water, filters out the sand, and dries it. The dry sand has a mass of 6.4 g. (a) What is the mass percent of NaCl in the original mixture? (b) Explain why the mass percent of NaCl in pure table salt would not vary from sample to sample, while the percent in this mixture can.

A hydrocarbon sample is burned completely. A 1.000 g sample produces 3.138 g of $\text{CO}_2$ and 1.285 g of $\text{H}_2\text{O}$. (a) Calculate the mass of carbon and of hydrogen in the sample and confirm that no other element is present. (b) Determine the empirical formula. (c) The molar mass is about 70 g/mol. Determine the molecular formula. Use molar masses of 44.01 g/mol for $\text{CO}_2$, 18.02 g/mol for $\text{H}_2\text{O}$, 12.01 g/mol for C, and 1.008 g/mol for H.

A 5.00 g sample of a mixture of calcium carbonate, $\text{CaCO}_3$, and sodium chloride, NaCl, is heated strongly. Only $\text{CaCO}_3$ decomposes: $\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$. The solid residue has a mass of 3.90 g. (a) Calculate the mass of $\text{CO}_2$ released. (b) Calculate the mass and mass percent of $\text{CaCO}_3$ in the original mixture. (c) Explain why this method would give wrong results if the NaCl also lost mass on heating. Use molar masses of 100.09 g/mol for $\text{CaCO}_3$ and 44.01 g/mol for $\text{CO}_2$.