Question

HX ($\mathbf{\mu }\mathbf{=}\mathbf{150}\mathbf{}\mathbf{g}\mathbf{/}\mathbf{mol}$and HY($\mathbf{\mu }\mathbf{=}\mathbf{50}\mathbf{}\mathbf{g}\mathbf{/}\mathbf{mol}$) are weak acids. A solution of 12.0g/Lof HX has the same pH as one containing 6.00g/Lof HY. Which is the stronger acid? Why

Short answer

HX is a stronger acid.

Step-by-step solution

To find the stronger acid

Given information:

$$\begin{gathered} {\mathrm{MM}}_{\mathrm{HX}}=150\mathrm{g}/\mathrm{mol} \\ {\mathrm{MM}}_{\mathrm{HY}}=50.0\mathrm{g}/\mathrm{mol} \\ \mathrm{HX}:12.0\mathrm{g}/\mathrm{Ln}\mathrm{and}\mathrm{HY}:6.00\mathrm{g}/\mathrm{L} \end{gathered}$$

First, solve for the concentration of each weak acid.

HX:$\left[\mathrm{HX}\right]=\frac{12.0\mathrm{g}\times {\displaystyle \frac{1\mathrm{mol}}{150\mathrm{g}}}}{\mathrm{L}}=0.08\mathrm{M}$

HY:$\left[HY\right]=\frac{6.0\mathrm{g}\times {\displaystyle \frac{1\mathrm{mol}}{50\mathrm{g}}}}{\mathrm{L}}=0.12\mathrm{M}$

Compare$\text{[HY] > [HX]}$ . They have the same pH, therefore, they produced the same amount of$\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\right]$. A stronger acid will dissociate more, producing more$\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\right]$. Since they produced the same amount of$\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\right]$the stronger acid must have the lower concentration since it dissociated a largeramount of$\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{+}}\right]$compared to its initial concentration.

Therefore, the stronger acid is [HX].