Equal volumes of 0.10\text{ mol dm}^{-3} solutions of methanoic acid (\text{HCOOH}) and hy… — JAMB UTME

Equal volumes of $0.10\text{ mol dm}^{-3}$ solutions of methanoic acid ( $\text{HCOOH}$ ) and hydrochloric acid ( $\text{HCl}$ ) are prepared at $25^\circ\text{C}$ . Which of the following statements correctly accounts for the lower electrical conductivity observed in the methanoic acid solution?

A
Methanoic acid has a lower molar concentration of dissolved acid molecules in solution than hydrochloric acid.
Methanoic acid undergoes incomplete ionization in water, resulting in a lower concentration of mobile hydronium ions.Answer
C
Methanoic acid requires an indicator to undergo complete ionization in an aqueous environment.
D
Methanoic acid molecules neutralize hydronium ions rapidly to reform un-ionized acid molecules, reducing ionic mobility.

Answer

Methanoic acid undergoes incomplete ionization in water, resulting in a lower concentration of mobile hydronium ions.

Electrical conductivity in aqueous solutions depends on the concentration of free, mobile ions. Hydrochloric acid is a strong monobasic acid that dissociates completely in water to yield a high concentration of

\text{H}_3\text{O}^+

and

\text{Cl}^-

ions. In contrast, methanoic acid is a weak monobasic acid that ionizes only partially in aqueous solution, leaving most acid molecules un-ionized. This lower ion concentration directly accounts for the reduced electrical conductivity.

Step-by-Step Solution

Analyze acid strength and ionization behavior of both acids

Hydrochloric acid (

\text{HCl}

) is a strong acid that ionizes completely:

\text{HCl}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightarrow \text{H}_3\text{O}^+_{(aq)} + \text{Cl}^-_{(aq)}

. Methanoic acid (

\text{HCOOH}

) is a weak acid that ionizes partially:

\text{HCOOH}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{HCOO}^-_{(aq)} + \text{H}_3\text{O}^+_{(aq)}

Electrical conductivity in aqueous solutions depends on the concentration of mobile ions present.

Compare ion concentrations at equal molar concentration

For

0.10\text{ mol dm}^{-3}

\text{HCl}

[\text{H}_3\text{O}^+] = 0.10\text{ mol dm}^{-3}

. For

0.10\text{ mol dm}^{-3}

\text{HCOOH}

[\text{H}_3\text{O}^+] \ll 0.10\text{ mol dm}^{-3}

due to partial ionization.

Fewer ions per unit volume results in lower electrical current transport through the methanoic acid solution.

Key Concept

Relative strength of acids depends on the extent of ionization in water, where weak acids establish an equilibrium yielding fewer mobile ions than strong acids of equal concentration.

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