Question

The change in entropy of 2 moles of an ideal gas upon isothermal expansion at 243.6 K from 20 L until the pressure becomes 1 atm is :

• A. $1.385cal/K$
• B. $-1.2cal/K$
• C. $-1.2cal/K$
• D. $2.77cal/K$

Answer 1

The correct option is D $2.77cal/K$

From ideal gas equation,$p_1=\frac{nRT}{V_1}$
$=\frac{2\times 0.0821\times 243.6}{20}=2atm$
$△S=nRln\frac{p_1}{p_2}$
$△S=nRln\frac{2}{1}$
$=2\times 2ln2$
$=2\times 2\times 0.693[\because ln2=0.693]=2.77cal/K$