A solution has a volume of 95 cm³ and a density of 1 g/cm³. The temperature of the solution increases from 43.7°C to 49.6°C when a reaction occurs. If the specific heat capacity of the solution is 4.2

The volume of the solution is $95 \text{ cm}^3$.
The density of the solution is $1 \text{ g/cm}^3$.

$$\text{Mass of solution} (m) = \text{Volume} \times \text{Density}$$ $$m = 95 \text{ cm}^3 \times 1 \text{ g/cm}^3 = 95 \text{ g}$$

The initial temperature is $43.7^\circ \text{C}$.
The final temperature is $49.6^\circ \text{C}$.

$$\Delta T = \text{Final temperature} - \text{Initial temperature}$$ $$\Delta T = 49.6^\circ \text{C} - 43.7^\circ \text{C} = 5.9^\circ \text{C}$$

Since a change of $1^\circ \text{C}$ is equal to a change of $1 \text{ K}$, $\Delta T = 5.9 \text{ K}$.

The specific heat capacity of the solution ($c$) is $4.2 \text{ J/g K}$.

$$Q = mc\Delta T$$ $$Q = 95 \text{ g} \times 4.2 \text{ J/g K} \times 5.9 \text{ K}$$ $$Q = 2354.1 \text{ J}$$

Since the temperature increased, the reaction is exothermic, meaning the reaction released this heat. So, $Q_{\text{reaction}} = -Q_{\text{solution}} = -2354.1 \text{ J}$.
Convert to kilojoules:

$$Q_{\text{reaction}} = -2354.1 \text{ J} \times \frac{1 \text{ kJ}}{1000 \text{ J}} = -2.3541 \text{ kJ}$$

The mass of copper deposited is $3.83 \text{ g}$.
The molar mass of copper ($\text{Cu}$) is $63.5 \text{ g/mol}$.

$$\text{Moles of Cu} (n) = \frac{\text{Mass}}{\text{Molar mass}}$$ $$n = \frac{3.83 \text{ g}}{63.5 \text{ g/mol}} \approx 0.06031 \text{ mol}$$

$$\Delta H = \frac{Q_{\text{reaction}}}{n}$$ $$\Delta H = \frac{-2.3541 \text{ kJ}}{0.06031 \text{ mol}}$$ $$\Delta H \approx -39.03 \text{ kJ/mol}$$

Rounding to three significant figures:

$$\boxed{-39.0 \text{ kJ/mol}}$$

21. The atomic numbers of nitrogen and fluorine are 7 and 9 respectively.