How many moles of acetic acid should be added to 100 ml of 0.6 M formic acid solution such that the — Ionic Equilibrium Chemistry Question
Question
How many moles of acetic acid should be added to 100 ml of 0.6 M formic acid solution such that the percentage dissociation of formic acid remains unchanged? $K_a$ for acetic acid = $1.8 \times 10^{-5}$ and $K_a$ for formic acid = $2.4 \times 10^{-4}$.
💡 Solution & Explanation
For the degree of dissociation of formic acid to remain strictly mathematically equivalent when another acid is mixed without significantly altering volume (an isohydric assumption framework often used in such classic problems), the concentration of added acetic acid ($C_2$) must satisfy the isohydric condition $K_1C_1 = K_2C_2$. Thus, $(2.4 \times 10^{-4})(0.6) = (1.8 \times 10^{-5})C_2$, which yields $C_2 = 8.0 \text{ M}$. The moles of acetic acid required for $V = 100 \text{ ml} = 0.1 \text{ L}$ is $n = C_2 \times V = 8.0 \times 0.1 = 0.8 \text{ moles}$. Therefore, correct answer is A.