Question - What volume of $0.1 \ce{M}$ $\ce{HCl}$ solution should be added to a $\ce{500 mL}$ of $\ce{0.5 M }$ $\ce{HCOOH }$ solution in order to prepare an acid solution of $\text{pH}$ =$1.5.$ $K_a$ of formic acid is $2×10^{-4}$ at $\ce{298 K?}$
I am not sure how to proceed with this question. Iam also confused how equilibrium reaction will
I guessed it would be $$\ce{HCOOH}+\ce{HCl \rightarrow salt}$$ Let $V$ be volume of HCl. At $t=\text{equilibrium,}$ $$[\ce{salt}] = \dfrac{0.1V}{0.25-0.1V}$$
Now, $$\ce{pH}= \ce{pK_a} + \log\dfrac{0.1V}{(0.25-0.1V)}\Rightarrow \boxed{V=2.5L}$$ But answer is $V=0.208 L$
Am I completely wrong somewhere? Can someone give tell me how to proceed?
$\textbf{NOTE:I am not asking full solution.}$ $\textbf{Just telling how to proceed is fine.}$