A student is given a solution A of hydrochloric acid (HCl) of unknown concentration and a solution B of an alkali XOH of unknown concentration. They are asked to determine the concentrations of these

The atomic mass of H is $1 \, \text{g mol}^{-1}$.
The atomic mass of Cl is assumed to be $35.5 \, \text{g mol}^{-1}$.

$$\text{Molar mass of HCl} = \text{Atomic mass of H} + \text{Atomic mass of Cl}$$ $$\text{Molar mass of HCl} = 1 \, \text{g mol}^{-1} + 35.5 \, \text{g mol}^{-1} = 36.5 \, \text{g mol}^{-1}$$

The mass concentration of HCl is given as $3.80 \, \text{g dm}^{-3}$.

$$\text{Molar concentration} = \frac{\text{Mass concentration}}{\text{Molar mass}}$$ $$\text{Concentration of A (HCl)} = \frac{3.80 \, \text{g dm}^{-3}}{36.5 \, \text{g mol}^{-1}}$$ $$\text{Concentration of A (HCl)} \approx 0.1041 \, \text{mol dm}^{-3}$$

Rounding to three significant figures:

$$\boxed{\text{0.104 mol dm}^{-3}}$$

The mass concentration of HCl is given as $3.80 \, \text{g dm}^{-3}$.

$$\text{Mass of HCl} = \text{Mass concentration} \times \text{Volume}$$ $$\text{Mass of HCl} = 3.80 \, \text{g dm}^{-3} \times 0.020 \, \text{dm}^3$$ $$\text{Mass of HCl} = 0.076 \, \text{g}$$ $$\boxed{\text{0.076 g}}$$