Vapour pressure; Raoult's law; ideal and non-ideal solutions — NEET & JEE Questions

Question 1 · difficulty L1 · recall

For an ideal solution, the partial vapour pressure of component i is given by

• A. $p_i=x_i p_i^0$
• B. $p_i=p_i^0/x_i$
• C. $p_i=x_i/p_i^0$
• D. $p_i=p_i^0-x_i$

Question 2 · difficulty L2 · understanding

Two volatile liquids $\mathbf{A}$ and $\mathbf{B}$ form an ideal solution. Consider a 5 molal solution of $\mathbf{B}$ in $\mathbf{A}$ inside a closed container having a total vapour pressure of 100 mm Hg at 300 K . The vapour pressure of pure $\mathbf{A}$ at 300 K is 105 mm Hg . Assume that $\mathbf{A}$ and $\mathbf{B}$ behave as ideal gases in the vapour phase. Given : The gas constant $R=0.08 \mathrm{~L} \mathrm{~atm} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ Molar mass of $\mathbf{A}$ is $50 \mathrm{~g} \mathrm{~mol}^{-1}$ Molar mass of $\mathbf{B}$ is $57 \mathrm{~g} \mathrm{~mol}^{-1}$ Density of liquid $\mathbf{B}$ at 300 K is $0.5 \mathrm{~g} / \mathrm{mL}$ $1 \mathrm{~atm}=760 \mathrm{~mm} \mathrm{Hg}$ The mole fraction of $\mathbf{B}$ in vapour phase which is in equilibrium with this solution is $\_\_\_\_$ .

Question 3 · difficulty L2 · understanding

The Henry's law constant for the solubility of N$$_2$$ gas in water at 298 K is 1.0 $$\times$$ 10$$^5$$ atm. The mole fraction of N$$_2$$ in air is 0.8. The number of moles of N$$_2$$ from air dissolved in 10 moles of water at 298 K and 5 atm pressure is

• A. 4.0 $$\times$$ 10$$^{-4}$$
• B. 4.0 $$\times$$ 10$$^{-5}$$
• C. 5.0 $$\times$$ 10$$^{-4}$$
• D. 4.0 $$\times$$ 10$$^{-6}$$

Question 4 · difficulty L2 · understanding

An ideal solution is one that approximately obeys Raoult's law

• A. over the full composition range
• B. only at one arbitrary composition
• C. only when one component is absent
• D. only when vapour pressure is zero

Question 5 · difficulty L3 · application

A component has mole fraction 0.40 and pure vapour pressure 200 mmHg. If the solution is ideal, its partial pressure is

• A. 80 mmHg
• B. 200 mmHg
• C. 500 mmHg
• D. 120 mmHg

Question 6 · difficulty L3 · application

An ideal binary solution has $x_A=0.30$, $p_A^0=100$ mmHg, $x_B=0.70$ and $p_B^0=200$ mmHg. What is total vapour pressure?

• A. 170 mmHg
• B. 100 mmHg
• C. 200 mmHg
• D. 240 mmHg

Question 7 · difficulty L3 · application

A solution shows a positive deviation from Raoult's law. Its observed vapour pressure is

• A. higher than the ideal value
• B. lower than the ideal value
• C. always zero
• D. exactly equal to the ideal value by definition

Question 8 · difficulty L3 · application

A solution shows a negative deviation from Raoult's law. Which interaction pattern is most consistent?

• A. A-B attractions are stronger than the average of A-A and B-B attractions
• B. A-B attractions are much weaker
• C. There are no intermolecular forces
• D. Every molecule behaves as an ideal gas

Question 9 · difficulty L4 · application

Which thermodynamic behaviour is characteristic of an ideal solution on mixing?

• A. $\Delta H_{mix}\approx0$ and $\Delta V_{mix}\approx0$
• B. Large positive $\Delta H_{mix}$ only
• C. Large negative $\Delta V_{mix}$ only
• D. No entropy change is possible

Question 10 · difficulty L4 · application

If unlike A-B attractions are weaker than A-A and B-B attractions, the solution most likely shows

• A. positive deviation from Raoult's law
• B. negative deviation from Raoult's law
• C. perfect ideality necessarily
• D. zero vapour pressure