Question

$2$ moles of solid benzoic acid undergoes combustion at $300 \,K$.
$C _{6} H _{5} COOH(s) +\frac{15}{2} O _{2} (g) \rightarrow 7 CO _{2} (g) +3 H _{2} O(l)$.
The value of $\Delta H -\Delta U$ value is

• A. $-1.247 \, kJ$
• B. $-2.494 \, kJ$
• C. $+2.494 \, kJ$
• D. $+1.247 \, kJ$

Answer 1

Answer: (B)

$C _{6} H _{5} COOH(s) +\frac{15}{2} O _{2} (g) \rightarrow 7 CO _{2} (g) +3 H _{2} O(l)$
$\Delta H -\Delta U =\Delta n _{ g } RT$
$=\frac{-\frac{1}{2}(8.314) \times 300}{1000}$
$=-1.247 \,kJ$ for two moles it is $-2.494\, kJ$