When 20.0 mL of 0.15 M hydrochloric acid is mixed with 20.0 mL of 0.10 M sodium hydroxide, the pH of the resulting solution is
A)0.00.
B)12.40.
C)1.60.
D)0.82.
E)7.00.

Question

Quizplus · Accepted Answer

The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) is a neutralization reaction, producing water and sodium chloride (NaCl). The moles of HCl are $20.0 \, \text{mL} \times 0.15 \, \text{M} = 3.0 \times 10^{-3} \, \text{moles}$, and the moles of NaOH are $20.0 \, \text{mL} \times 0.10 \, \text{M} = 2.0 \times 10^{-3} \, \text{moles}$. After the reaction, there are $3.0 \times 10^{-3} - 2.0 \times 10^{-3} = 1.0 \times 10^{-3} \, \text{moles}$ of HCl left unreacted. The total volume of the solution is $20.0 \, \text{mL} + 20.0 \, \text{mL} = 40.0 \, \text{mL}$ or $0.040 \, \text{L}$. The concentration of HCl remaining is $\frac{1.0 \times 10^{-3} \, \text{moles}}{0.040 \, \text{L}} = 0.025 \, \text{M}$. The pH is calculated using the formula $\text{pH} = -\log[H^+]$, where $[H^+] = 0.025 \, \text{M}$. Thus, $\text{pH} = -\log(0.025) \approx 1.60$.

When 200 ML of 0