Question

Which of the following solutions will have pH equal to 1.0?

• A. 55 mL of $\frac{M}{10}$ HCl + 45 mL of $\frac{M}{10}$ NaOH
  
• B. 50 mL of $\frac{M}{5}$ HCl + 50 mL of $\frac{M}{5}$ NaOH
  
• C. 75 mL of $\frac{M}{5}$ HCl + 25 mL of $\frac{M}{5}$ NaOH
  
• D. None of the above

Answer 1

The correct option is C 75 mL of $\frac{M}{5}$ HCl + 25 mL of $\frac{M}{5}$ NaOH


We need to calculate the resulting molar concentration of $H^{+}$ ions to determine the resulting pH of solutions.
Here n-factor of NaOH and HCl is 1 . So Molarity and Normality are equal.
Formula used is:
$M_{mix}=\frac{M_1{V}_1-M_2{V}_2}{V_1+V_2}if{M}_1{V}_1>M_2{V}_2$
Option A: $(0.1\times 55)>(0.1\times 45)$ $N_{mix}=\frac{(0.1\times 55)-(0.1\times 45)}{100}=0.01pH=-log\left(0.01\right)=2\times log\left(10\right)=2$

Option B: Since the concentration and volume of both $NaOHandHCl$ is equal. It forms a neutral solution of pH=7

Option C: $(0.2\times 75)>(0.2\times 25)$
$N_{mix}=\frac{(0.2\times 75)-(0.2\times 25)}{100}=0.1pH=-log\left(0.1\right)=log\left(10\right)=1$