Question

What do you mean by a concentration of a solution? Mention two ways, of expressing the concentration of a solution.

Answer 1

Molarity:

• “Molarity (M) is defined as the number of moles of solute dissolved in one liter (or one cubic decimetre) of solution.”

Molality:

• “Molality: Molality (m) is defined as the number of moles of the solute per kilogram (kg) of the solvent.”

Concentration of a solution:

• “The composition of a solution can be described by expressing its concentration”.
• The concentration of the solution can be determined quantitatively in several ways.

1. Mass percentage (w/w):

• The mass percentage of a component of a solution is defined as:

$\mathrm{Mass}\%\mathrm{of}\mathrm{the}\mathrm{component}=\frac{\mathrm{Mass}\mathrm{of}\mathrm{the}\mathrm{component}\mathrm{in}\mathrm{the}\mathrm{solution}}{\mathrm{Total}\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{solution}}\times 100$

For example, the concentration of the solution in (w/w) will be as follows if the mass of the component of 10.0 g and the total mass of the solution is 80.0 g.

$$\begin{gathered} \mathrm{Mass}\%\mathrm{of}\mathrm{the}\mathrm{component}=\frac{10.0\mathrm{g}}{80.0\mathrm{g}}\times 100 \\ \mathrm{Mass}\%\mathrm{of}\mathrm{the}\mathrm{component}=12.5\%(\mathrm{w}/\mathrm{w}) \end{gathered}$$

2. Volume percentage (V/V):

• The volume percentage is defined as:

$\mathrm{Volume}\%\mathrm{of}\mathrm{the}\mathrm{component}=\frac{\mathrm{Volume}\mathrm{of}\mathrm{the}\mathrm{component}}{\mathrm{Total}\mathrm{volume}\mathrm{of}\mathrm{solution}}\times 100$

For example, a solution is made by dissolving 250 ml of the component in 1.50 L of solution, then the resulting %(v/v) will be as follows:

$$\begin{gathered} \mathrm{Volume}\%\mathrm{of}\mathrm{the}\mathrm{component}=\frac{0.250\mathrm{L}}{1.50\mathrm{L}}\times 100 \\ \mathrm{Volume}\%\mathrm{of}\mathrm{the}\mathrm{component}=16.7\%(\mathrm{v}/\mathrm{v}) \end{gathered}$$