Question

If the value of $C_{P}$ for nitrogen gas is $7 \,JK ^{-1} mol ^{-1}$, then the value of $\Delta H$ on heating $28 \,g$ of nitrogen gas from $0^{\circ} C$ to $100^{\circ} C$ at constant pressure will be

• A. 1200 J
• B. 1300 J
• C. 1400 J
• D. 1500 J

Answer 1

Answer: (C)

Heat of reaction at constant pressure is given by the equation
Here $ \Delta H =n C_{p}\left(T_{2}-T_{1}\right) $
$C_{p} =7 \,JK ^{-1} mol ^{-1}$
weight of $N _{2} gas =28\, g$
$T_{1}=0^{\circ} C =273+0=273 \,K $
$T_{2}=100^{\circ} C =273+100=373\, K$
Number of moles
$=\frac{\text { weight of nitrogen }}{\text { molecular weight of nitrogen }}$
$=\frac{28}{14}=2$
Now by using
$\Delta H =n C_{P}\left(T_{2}-T_{1}\right)$
$=2 \times 7(373-273) $
$\Delta H =14 \times 100=1400 \,J$