Discussion Forum

1)

When 100 mL of 1.0 M  HCl was mixed with 100mL   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. I). Because the enthalpy of neutralisation of a strong acid with a strong base is a constant  (-57.0kJmol-1), this experiment could be used to measure the calorimeter constant.In a second  experiment  (Expt.2), 100mL  of 2.0 M  acetic acid  (K_a = 2.0 x 10^-5) was mixed with 100 mL of 1.0 M   NaOH  (under identical conditions  to  Expt. 1)  where a temperature rise of 5.6° C was measured

The pH  of the solution after Expt.2 is 

Answer:

Option B

Explanation:

$CH_{3}COOH+NaOH \rightarrow CH_{3}COONa+H_{2}0$

200mol        100 mo                          0                   0

100mol          0                            100mmol          

A buffer is now formed.

                    $K_{a}=\frac{[H^{+}][CH_{3}COO^{-}]}{[CH_{3}COOH]}=[H^{+}]$

                               $[\because [CH_{3}COOH]=[CH_{3}COO^{-}]]$

                             $\Rightarrow pH=pK_{a}=-\log (2\times 10^{-5})$

                         = 5- $\log 2$   = 4.7