Question

The molar specific heat of helium at constant volume is $3cal/mo{l}^{o}C$ . Heat energy required to raise the temperature of 1gm helium gas by $1^{o}C$ at constant pressure is :

• A. $1.2cal$
• B. $1.25cal$
• C. $3cal$
• D. $4cal$

Answer 1

The correct option is B $1.25cal$

Heat added for a constant pressure process is,

$dQ=dU+dW$

$n{C}_p\Delta T=n{C}_v\Delta T+nR\Delta T$

Given 1 gm of Helium, number of moles$=1/4=0.25$

$R=2cal/mol-K$

$dQ=0.25\left[3\right(1)+2(1\left)\right]=0.25\left(5\right)=1.25cal$

Option B.