Question

10 mL of $ {\text{0}}{\text{.1N HCl}} $ is added to the 990mL solution of NaCl. The pH of the resulting solution is:
(A) Zero
(B) 3
(C) 7
(D) 10

Answer 1

Hint: We need to find the pH of the solution. Since, n factor of HCl is 1. So, molarity would be equal to normality. We shall find the moles of hydrogen ions present and the concentration of hydrogen ions and thus, it pH.

Formula used: $ pH = - {\text{log}}\left[ {{H^ + }} \right] $
 $ {\text{no of moles = M $\times$ V}} $
Here $ M $ is the molarity
 $ V $ is the volume.

Complete step by step answer
We already know the molarity of HCl is equal to its normality.
Number of moles of $ {H^ + } = M \times V = 0.1 \times 10 \times {10^{ - 3}} = {10^{ - 3}} $
So, we get the total volume:
 $ {\text{Total Volume = 10 + 990 mL = 1 L}} $
 $ \left[ {{H^ + }} \right] = \dfrac{{{{10}^{ - 3}}}}{1}M = {10^{ - 3}}M $
Thus, we obtain the pH:
 $ pH = - \log {10^{ - 3}} = 3 $
So, the pH of the solution is 3.
Thus, the correct option is B.

Additional Information
The pH of a solution is a measure of hydrogen ion concentration, which in turn is a measure of its acidity. Pure water dissociates slightly into equal concentrations of hydrogen and hydroxyl ions.
The qualitative ranking of solutions as "more acidic" or "more basic" is possible on the basis of qualitative tests, but such an approach is not useful for quantitative discussion. Quantitatively, the acidity of a solution is measured by and is equal to the concentration of hydronium ion in that solution.

Note
Any aqueous (water-based) liquid can be classified as an acid, base, or neutral. Oils and other non-aqueous liquids are not acids or bases. acids can accept an electron pair or donate a hydrogen ion or a proton in a chemical reaction, while bases can donate an electron pair or accept hydrogen or a proton. The pH scale is a measure of the acidity or alkalinity (basicity) or a solution. Substances that aren't acidic or alkaline (that is, neutral solutions) usually have a pH of 7. Acids have a pH that is less than 7. Alkalis have a pH that is greater than 7. pH is a measure of the concentration of protons $ \left( {{H^ + }} \right) $ in a solution.