Thermodynamics And Thermochemistry Result Question 34
Passage
When $100$ mL of $1.0$ $M $ $HCl$ was mixed with $100$ mL of $1.0$ $ M $ $NaOH$ in an insulated beaker at constant pressure, a temperature increase of $5.7^{\circ} \mathrm{C}$ was measured for the beaker and its contents (Expt. 1). Because the enthalpy of neutralisation of a strong acid with a strong base is a constant $\left(-57.0 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$, this experiment could be used to measure the calorimeter constant. In a second experiment (Expt. 2), $100$ mL of $2.0$ M acetic acid $\left(K_a=2.0 \times 10^{-5}\right.$ ) was mixed with $100$ mL of $1.0$ $M$ $ NaOH$ (under identical conditions to Expt. 1) where a temperature rise of $5.6^{\circ} \mathrm{C}$ was measured.
34. The $\mathrm{pH}$ of the solution after Expt. 2 is
(a) $2.8$
(b) $4.7$
(c) $5.0$
(d) $7.0$
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Answer:
Correct Answer: 34. $(4.7)$
Solution:
A buffer is now formed.
$K_a=\frac{\left[\mathrm{H}^{+}\right]\left[\mathrm{CH}_3 \mathrm{COO}^{-}\right]}{\left[\mathrm{CH}_3 \mathrm{COOH}\right]}=\left[\mathrm{H}^{+}\right]$
$[\because[\mathrm{CH}_3 \mathrm{COOH}]=[\mathrm{CH}_3 \mathrm{COO}^{-}]]$
$\Rightarrow \quad \mathrm{pH}=\mathrm{p} K_a=-\log (2 \times 10^{-5})=5-\log 2=4.7$
BETA
