Question

Three solutions P, Q, and R have a $pH$ value of 3.5, 5.2, and 12.2 respectively. Which one of these is a:
i) weak acid
ii) strong alkali

Answer 1

Think about what the term $pH$ itself means and what it signifies about the amount of ${{H}^{+}}$ and $O{{H}^{-}}$ ions present in any given solution. Identify the scale of this parameter and classify the given solutions accordingly.

Complete step by step answer:
We know that the term $pH$ defines the amount of ${{H}^{+}}$ ions that are present in any given solution. The scale of $pH$ is logarithmic, so a solution with $pH=2$ will be 10 times as acidic compared to a solution with $pH=3$ but 100 times as acidic compared to solution with $pH=4$. The value of $pH$ is defined using the concentration of ${{H}^{+}}$ ions that may be present in the solution. The formula that defines $pH$ is: $pH=-\log [{{H}^{+}}]$
$pH$ is a 14-point scale where 1 is the most acidic and 14 is the most basic. The point where the substance or solution is considered to be neutral is when the $pH$ is 7. So anything with a $pH$ below 7 will be acidic and anything with a $pH$ above 7 will be basic. Gathering the given facts, we can say that a solution with $pH=1$ will be most acidic while ones with a value of around $6 Similarly, with basic substances, the substances with$pH=14$will be the most basic or strong alkalis and the ones with a value of around$7Now, onto the problem, the $pH$ values of the given solutions P, Q, and R are 3.5, 5.2, and 12.2 respectively. So, according to what we know till now:
i) Solution Q will be a weak acid with a $pH$ value of about 5.2. Compared to this, solution P is a strong acid with a $pH$ value of 3.5
ii) Solution R will be considered as a strong alkali since it is closer to 14 than 7 on the 14-point scale with a $pH$ value of 12.2.

Note: Remember that a $pOH$ value is also defined. It is derived from the concentration of $O{{H}^{-}}$ ions that may be present in the solution. It is the exact opposite of a $pH$ value. The sum of the $pH$ and $pOH$ values of the same solution is always 14. The formula that goes along with this is: $pH+pOH=14$.