# Homework Help: What is the change in entropy of the gas?

1. Jul 30, 2004

### daisyi

One mole of an ideal gas in a cylinder fitted with a piston is made to expand slowly (reversible) from an initial volume of 1x10^3 cm3 to a final volume of 2x10^3 cm3. The cylinder is in contact with a heat reservoir so that, throughout the expansion process, the gas is held at a constant temperature. What is the change in entropy of the gas?

2. Jul 30, 2004

### Staff: Mentor

isothermal process

For an isothermal process, $\Delta S = Q/T$.

3. Jul 30, 2004

### PureEnergy

This problem can be solved using

$$\Delta S = nC_Pln\frac{V_2}{V_1}+nC_Vln\frac{P_2}{P_1}$$

All you need to do now, is to recall the relationship between an ideal gas and the heat capacity. You don't need the temperature to solve this problem.

4. Jul 30, 2004

### PureEnergy

Along with Doc's equation, you'll want to use [itex]\Delta U = Q + W[/tex]

5. Jul 30, 2004

### kenallen

but for isothermal process change in internal energy(delta U) is equal to 0.