${A} \underset{{T}\, 300\, {~K}}{\longrightarrow} {B}$

$[\Delta {G}]_{{P}, {T}}=-49.4\, {~kJ} / {mol}$

$\Delta {H}_{{rxn}}=51.4 \,{~kJ} / {mol}$

$\Delta {S}_{{rxn}}=?$

$\Rightarrow$ From the relation $[\Delta {G}]_{{P}, {T}}=\Delta {H}-{T} \Delta {S}$

$\Rightarrow \Delta S_{\text {rxn }} =\frac{\Delta H_{r x n}-[\Delta G]_{p, T}}{T}$

$=\frac{[51.4-(-49.4)] \times 100}{300} \,\frac{{J}}{{mol} {K}}$

$\Rightarrow \Delta {S}_{{rxn}} =336\, \frac{{J}}{{mol} {K}}$