Question

pH of water is 7 when any substance $Y$ is dissolved in water then $pH$ becomes 13. Substance $Y$ is a salt of:
A) Strong acid and strong base
B) Strong acid and weak base
C) Weak acid and weak base
D) Weak acid and strong base

Answer 1

We know that in chemistry $pH$ which can be abbreviated as potential of hydrogen is a scale with respect to the acidity or basicity of a substance is determined. The $pH$ scale shows lower values for acidic solution while the basic solutions have higher $pH$ values.
We also know that the $pH$ values 0 indicate a strong acid while a $pH$ value of 14 indicates a very strong base. This is how the strength of an acid or base is determined.

Complete answer:
We know that the $pH$ scale is a scale that was devised to measure the acidity or basicity of any solution based on the concentration of ${H^ + }$ ions in its aqueous solution. We know that if an acid is able to dissociate completely into ${H^ + }$ then it is a strong acid, if not it is a weak acid. The definition of the strong base is also almost similar only that the ion dissociated from a strong base will be $O{H^ - }$ .
Thus, we can say that the concentration of ${H^ + }$ ions in an aqueous solution determines the strength of an acid / base. Mathematically a $pH$ number of a solution is,
$pH = - {\log _{10}}\left[ {{H^ + }} \right]$
At $25^\circ C$ the $pH$ of water is 7 which is assumed to be neutral. Now consider a compound $Y$ is dissolved in water to make its $pH = 13$ . We know that at $pH$ greater than 7, the solution is a base. Since the solution has become basic due to the addition of $Y$ we can say that $Y$ is itself a basic salt.
We also know that a salt is formed by reaction between an acid and a base that neutralizes the product. If the salt we have obtained, $Y$ is basic then, the constituents of the salt $Y$ will be a strong base and a weak acid.
Thus $Y$ is a salt of strong base and weak acid.
Hence option D is correct.

Note:
In acid-base titrations, if:
The base is strong and the acid is weak. The resulting salt is basic.
The acid is strong and the base is weak. The resulting salt is acidic.
The acid is weak and the base is also weak; the result can either be acidic, basic or neutral.
The acid and base are strong, then the result will be neutral.