KEY2CHEM

Using Mass Data to Determine Purity

Pure substances have a constant composition throughout, while mixtures have varied composition. When working with mixtures, mass percent data can provide insight into how much of the pure substance is present in a mixture. 

\(\text{mass percent purity} = \frac{\text{mass of pure substance}}{\text{mass of sample}} \times 100\%\)


Example 1. 

A \(10.0\text{ g}\) piece of jewelry contains \(5.8\text{ g}\) of gold. What is the mass percent of gold in the jewelry?

A. \(5.8\%\)

B. \(58\%\)

C. \(0.58\%\)

 

Solution

B. \(58\%\)

\(\begin{align}\text{mass percent purity} &= \frac{\text{mass of pure substance}}{\text{mass of sample}} \times 100\%\\\text{mass percent gold} &= \frac{\text{mass of gold}}{\text{mass of jewelry}} \times 100\%\\\text{mass percent gold} &= \frac{5.8\text{ g}}{{10.0\text{ g}}} \times 100\% = 58\%\end{align}\)


Example 2.

A tablet of ibuprofen pain reliever weighs \(0.500\text { g}\). If the active ingredient in the tablet is \(200 \text{ mg}\), what mass percent of the tablet is the active ingredient ibuprofen?

A. \(100\%\)

B. \(0.25\%\)

C. \(40\%\)

 

Solution

C. \(40\%\)

\(\begin{align}\text{mass percent purity} &= \frac{\text{mass of pure substance}}{\text{mass of sample}} \times 100\%\\\text{mass percent ibuprofen} &= \frac{\text{mass of ibuprofen}}{\text{mass of tablet}} \times 100\%\\\text{mass percent ibuprofen} &= \frac{0.200\text{ g}}{{0.500\text{ g}}} \times 100\% = 40\%\end{align}\)


Example 3.

A \(5.0\text{ kg}\) sample of impure iron ore is found to be \(21\%\) iron by mass. How many grams of iron are in the ore sample?

A. \(1050\text{ g}\)

B. \(105\text{ g}\)

C. \(525\text{ g}\)

 

Solution

A. \(1050\text{ g}\)

\(\begin{align}\text{mass percent purity} &= \frac{\text{mass of pure substance}}{\text{mass of sample}} \times 100\%\\\text{mass percent iron} &= \frac{\text{mass of iron}}{\text{mass of ore}} \times 100\%\\21\%&= \frac{\text{ mass of iron}}{{5000\text{ g}}} \times 100\% \\1050\text{ g}&= \text{mass of iron}\end{align}\)