Ionisation of acids and bases; Ostwald's dilution law — NEET & JEE Questions

Question 1 · difficulty L1 · recall

Ostwald's dilution law is primarily applicable to

• A. weak electrolytes
• B. strong electrolytes at all concentrations
• C. non-electrolytes only
• D. metals

Question 2 · difficulty L2 · understanding

For a weak acid, what generally happens to its degree of ionisation when the solution is diluted?

• A. It decreases
• B. It increases
• C. It remains exactly constant
• D. It becomes zero

Question 3 · difficulty L2 · understanding

For a weak monoprotic acid $HA$ of initial concentration $C$ and degree of ionisation $\alpha$, the equilibrium concentration of $H^+$ is approximately

• A. $C$
• B. $C\alpha$
• C. $C/\alpha$
• D. $\alpha/C$

Question 4 · difficulty L2 · understanding

For a weak electrolyte, if the degree of ionisation $\alpha$ is very small, the approximation $1-\alpha\approx1$ is used mainly to

• A. simplify the equilibrium-constant expression
• B. make the electrolyte behave as a strong electrolyte
• C. set the ion concentration equal to zero
• D. remove concentration from the calculation

Question 5 · difficulty L3 · application

A weak monoprotic acid has concentration $0.10\,M$ and degree of ionisation $0.020$. What is $[H^+]$?

• A. 0.0002 M
• B. 0.0020 M
• C. 0.020 M
• D. 0.10 M

Question 6 · difficulty L3 · application

A weak acid has $K_a=1.0\times10^{-5}$ and concentration $0.10\,M$. Using the small-ionisation approximation, what is its degree of ionisation?

• A. 0.001
• B. 0.010
• C. 0.10
• D. 1.0

Question 7 · difficulty L3 · application

For a weak acid with constant $K_a$, the concentration is reduced from $0.10\,M$ to $0.025\,M$. Under the small-ionisation approximation, by what factor does the degree of ionisation change?

• A. It becomes half
• B. It doubles
• C. It becomes four times
• D. It remains unchanged

Question 8 · difficulty L3 · application

A weak base $BOH$ has concentration $0.20\,M$ and degree of ionisation 0.05. What is the equilibrium concentration of $OH^-$?

• A. 0.0025 M
• B. 0.010 M
• C. 0.040 M
• D. 0.20 M

Question 9 · difficulty L4 · application

A weak acid has $K_a=4.0\times10^{-6}$ and concentration $0.040\,M$. Estimate its degree of ionisation.

• A. 0.001
• B. 0.005
• C. 0.010
• D. 0.100

Question 10 · difficulty L4 · application

A weak monoprotic acid is 2% ionised in a $0.10\,M$ solution. Approximately what is its $K_a$?

• A. $4.0\times10^{-6}$
• B. $4.0\times10^{-5}$
• C. $2.0\times10^{-3}$
• D. $2.0\times10^{-2}$