Concentration of Solutions
Percent by Mass
Consider the following problems:
Problem 1. What is the mass percentage of a solution made by dissolving 4 g NaCl in 100 g of water?
Answer
1. Known: mass of NaCl = 4 g; mass of H20 = 100 g
2. Unknown: % (m/m)
3. Process: Use the equation:
\[
\begin{aligned}
\%\,(\mathrm{m/m})
&=
\left(
\frac{
\text{mass of NaCl}
}{
\text{mass of NaCl} + \text{mass of } H_2O
}
\right)
\times 100
\end{aligned}
\]
mass of NaCl = 4 g; mass of H2O = 100 g; mass of NaCl + mass of H2O = 104 g Therefore:
\[
\begin{aligned}
\%\,(\mathrm{m/m})
&=
\left(
\frac{4.0\,\text{g}}{104\,\text{g}}
\right)
\times 100
\end{aligned}
\]
4. Answer: The prepared solution has % NaCl (m/m) of 3.84.
