When `100mL` of `1.0M HCl` was mixed with `100 mL` of `1.0 M NaOH` in an insulated beaker at constant pressure, a temperature increase of `5.7^(@)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 constant `(-57.0kJmol^(-1))` this experiment couldbe used to measure the calorimeter constant. In a second experiment (Expt.2), `100mL` of `2.0M` acetic acid `K_(a)=2.0xx10^(-5))` was mixed with `100mL` of `1.0M NaOH` (under identical conditions to Expt. 1) where a temperature rise of `5.6^(@)C` was measured.
Enthalpy of dissociation (in `kJ mol^(-1)`) of acetic acid obtained from the Expt. 2 is

Options

A. 2.8
B. 4.7
C. 5
D. 7

Answer

B

Explanation

`CH_(3)COOH + OH^(-) rarr CH_(3)COO^(-) + H_(2)O`
`implies CH_()COOH rarr CH_(3)COO^(-) + H^(+) implies K_(a) = [CH_(3)COO^(-)][H^(+)]/[CH_(3)COOH] = 100 xx y/100`
`implies [H^(+)] = y = K_(a) = 2 xx 10^(-5) = 10^(0.3) xx 10^(-5) = 10^(-4.7) implies pH = 4.7`