A student used the apparatus shown to investigate the combustion of octan-1-ol - HSC - SSCE Chemistry - Question 25 - 2023 - Paper 1

To determine the mass of octan-1-ol burnt, we first calculate the heat absorbed by the water. The heat absorbed, represented as $q$, can be calculated using the formula:

$q = \text{mass of water} \times c \times \Delta T$

where:

• mass of water = 205 g
• specific heat capacity ($c$) of water = 4.18 J kg⁻¹ K⁻¹
• change in temperature ($\Delta T$) = final temperature - initial temperature = 60.4°C - 23.7°C = 36.7 K.

Substituting these values into the equation:

$q = 205 \text{ g} \times 4.18 \text{ J kg}^{-1} \text{ K}^{-1} \times 36.7 \text{ K} = 31448 \text{ J}$

Next, since 1 kcal = 4184 J, we convert joules to kilojoules:

$q = \frac{31448}{1000} = 31.448 \text{ kJ}$

Using the molar enthalpy of combustion of octan-1-ol, we can find the moles burnt:

$n(\text{octan-1-ol}) = \frac{-31.448 \text{ kJ}}{-5294 \text{ kJ mol}^{-1}} = 5.94 \times 10^{-3} \text{ mol}$

Now, we calculate the mass of octan-1-ol burnt using the molar mass:

$m(\text{octan-1-ol}) = n \times \text{molar mass} = 5.94 \times 10^{-3} \text{ mol} \times 130.23 \text{ g mol}^{-1} = 0.774 \text{ g}$