Question

Find the work done when $5$ moles of hydrogen contracts isothermally from $15$ L to $5$ L against a constant pressure of 1 atm at ${25}^{0}C$.

• A. $685.3cal$
• B. $-848.2cal$
• C. $422.5cal$
• D. $-240.5cal$

Answer 1

The correct option is D $-240.5cal$

We know, work done for an isothermal irreversible process is given as:
$W=-P_{ext}(V_2-V_1)$

where $P_{ext}$= external pressure = 1 atm
$V_2$= final volume = $5L$
$V_1$ = initial volume = $15L$
so, $(V_2-V_1)=-10L$
$W=-1\times -10=10Latm$
and $1Latm=101.3J$
So, $W=10\times 101.3J=1010.3J$
We know $1cal=4.2J$
$W=\frac{1010.3}{4.2}cal$
$W=240.5cal$