Molecular mass from colligative properties — NEET & JEE Questions

Question 1 · difficulty L1 · recall

Which colligative property is commonly written as $\Delta T_f=K_fm$ for a dilute solution of a non-electrolyte?

• A. Depression in freezing point
• B. Elevation in boiling point
• C. Osmotic pressure
• D. Relative lowering of vapour pressure

Question 2 · difficulty L2 · understanding

Why can colligative properties be used to determine molar mass?

• A. They depend primarily on the number of solute particles rather than their chemical identity
• B. They depend only on solute colour
• C. They are independent of concentration
• D. They measure atomic number directly

Question 3 · difficulty L2 · understanding

For a dilute non-electrolyte solution, elevation in boiling point is directly proportional to

• A. molality
• B. molar mass
• C. solvent vapour pressure only
• D. mass of solvent regardless of solute

Question 4 · difficulty L2 · understanding

Why is osmotic pressure particularly useful for determining molar masses of proteins and polymers?

• A. It can be measured at room temperature for very dilute solutions
• B. It requires boiling the solute
• C. It is independent of solute concentration
• D. It destroys macromolecules before measurement

Question 5 · difficulty L3 · application

A non-electrolyte of mass $1.0\,\text{g}$ is dissolved in $100\,\text{g}$ water. The freezing point is lowered by $0.186\,\text{K}$. If $K_f=1.86\,\text{K kg mol}^{-1}$, what is the molar mass?

• A. 50 g mol$^{-1}$
• B. 100 g mol$^{-1}$
• C. 186 g mol$^{-1}$
• D. 200 g mol$^{-1}$

Question 6 · difficulty L3 · application

A $2.0\,\text{g}$ non-electrolyte is dissolved in $100\,\text{g}$ benzene. The boiling point rises by $0.506\,\text{K}$. If $K_b=2.53\,\text{K kg mol}^{-1}$, what is the molar mass?

• A. 50 g mol$^{-1}$
• B. 100 g mol$^{-1}$
• C. 200 g mol$^{-1}$
• D. 253 g mol$^{-1}$

Question 7 · difficulty L3 · application

A solution containing $1.0\,\text{g}$ of a non-electrolyte in $0.50\,\text{L}$ has osmotic pressure $0.246\,\text{atm}$ at $300\,\text{K}$. Take $R=0.082\,\text{L atm mol}^{-1}\text{K}^{-1}$. What is the molar mass?

• A. 50 g mol$^{-1}$
• B. 100 g mol$^{-1}$
• C. 200 g mol$^{-1}$
• D. 400 g mol$^{-1}$

Question 8 · difficulty L3 · application

A solute associates in solution. Compared with the value expected for the same analytical molality without association, the observed colligative property will generally be

• A. smaller
• B. larger
• C. unchanged
• D. negative in every case

Question 9 · difficulty L4 · application

A $0.10\,m$ solution of an electrolyte has $i=2.0$. If $K_f=1.86\,\text{K kg mol}^{-1}$, what freezing-point depression is expected?

• A. 0.093 K
• B. 0.186 K
• C. 0.372 K
• D. 1.86 K

Question 10 · difficulty L4 · application

A solute has true molar mass 120 g mol$^{-1}$ but gives an apparent molar mass of 240 g mol$^{-1}$ from a colligative measurement. What is the van't Hoff factor?

• A. 0.50
• B. 1.0
• C. 2.0
• D. 4.0