Q.Which one of the following is incorrect for ideal solution?

Step 1. For an ideal solution, by definition there is no volume change and no heat exchange on mixing: $\Delta V_{mixing}=0$ and $\Delta H_{mixing}=0$. Since $\Delta H=\Delta U+\Delta(PV)$ and $\Delta V=0$ here, $\Delta U_{mixing}=0$ too -- so (a) and (b) are both TRUE statements for an ideal solution.

Step 2. An ideal solution obeys Raoult's law exactly across the whole range, so the observed vapour pressure always equals the Raoult's-law-predicted vapour pressure: $\Delta P=P_{observed}-P_{calculated}=0$ -- so (c) is also a TRUE statement. …

Check each thermodynamic quantity against the three defining conditions of an ideal solution ($\Delta V=0$, $\Delta H=0$, exact Raoult's law obedience); recognise that …

• Assuming that because $\Delta H_{mixing}=0$, $\Delta G_{mixing}$ must also be 0 -- forgetting the $-T\Delta S$ term, which is always negative for any mixing process. …