Question

What is the $pH$ of $0.001M$ $HCl$?
A. $1$
B. $2$
C. $3$
D. $4$

Answer 1

We need to know that the pH is a chemistry scale for determining the acidity or basicity of aqueous solutions. Acidic solutions (those with a greater concentration of ${H^ + }$ ions) have a lower pH than basic or alkaline solutions. The pH scale is logarithmic, indicating the concentration of hydrogen ions in a solution in reciprocal order. Since the pH formula approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in the solution, this is the case. pH is defined as the negative of the base 10 logarithm of the ${H^ + }$ ion's action.

Complete answer:
Molarity, M is defined as the number of moles of solute per litre of solution and is given by the formula :$M = \dfrac{{number of moles of solute}}{{{\text{volume of solution in litres}}}}$.
HCl is a powerful acid that totally dissociates into ${H^ + }$ and $C{l^ - }$ when dissolved in water.
$pH{\text{ }} = {\text{ }} - log\left[ {{H^ + }} \right]$
Now we can substitute the known values we get,
$pH{\text{ }} = {\text{ }} - log\left[ {0.001} \right]$
On simplification we get,
$pH{\text{ }} = 3$

So, the correct answer is “Option C”.

Additional information:
We have to remember that the molarity is a measurement of the moles in the total volume of the solution, whereas Normality is a measurement of the gram equivalent in relation to the total volume of the solution. The number of moles is also called milliequivalent. Also,the concentration of the constituent atoms can be expressed both in terms of molarity and normality as it is a monovalent compound.

Note:
Note that the concentration of a given compound can be considered to be the concentration of their dissociated ions in case of strong acids and bases only. In case of weak acids or weak bases where they do not dissociate completely, the concentration of the acid or the base will be more than its dissociated ions.