Question

Assertion: In aqueous solution $pH + pOH = p{K_\omega }$ (not $\,14\,$) at all temperatures.
Reason: Water dissociates more at higher temperatures.
A: Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B: Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C: Assertion is true but Reason is false.
D: Assertion is false but Reason is true.
E: Both Assertion and Reason are false.

Answer 1

When in solution , water has a propensity to dissociate (break up) into ions. Very few of the molecules are dissociated at any one point in any water sample. Dissociation of water affects the pH of the solution. As water is only very weakly acidic and basic, we generally disregard this effect , allowing us to conclude that its contribution to the solution 's pH is negligible compared to that of other acids or bases present.

Complete step by step answer:
Let us know what $pH$, $pOH$ and $p{K_\omega }$ are;
Terms and their definitions:
$pH$- It is the negation of logarithmic value of concentration of Hydrogen ions present in a solution. If the $pH$ of a solution is less than $\,7\,$, then the solution is said to be acidic (For example, Hydrochloric acid, Sulphuric acid and Nitric acid). If the $pH$ of the solution is greater than $\,7\,$, then the solution is said to be basic (For example, Sodium Hydroxide, Calcium hydroxide). If the $pH$ is exactly or approximately equal to $\,7\,$, then the solution is said to be neutral (For example, water).
$pOH$- It is the difference of subtracting $pH$ from $\,14\,$. If the $pOH$ value is less than $\,7\,$, then the solution is said to be basic, which is the exact opposite of $pH$.
$p{K_\omega }$ -It is the sum total of $pH$ and $pOH$ in an aqueous solution. That is $pH$ + $pOH$ = $p{K_\omega }$.
Now, the Assertion might seem kind of true. But it says not $\,14\,$ at all temperatures. But actually it is $\,14\,$, when the temperature of the aqueous solution is at $\,{25^o}C\,$. This is because, at an ideal temperature for a solution, the ${H^ + }$ ions and $O{H^ - }$ ions are in equilibrium, leading to the appropriate sum total of $\,14\,$ from $14.9$ (the majority value of $p{K_\omega }$ ). Hence the Assertion is proved wrong.
Now, for the Reason part;
The dissociation of water is directly proportional to the given temperature. That is, if the temperature increases, the dissociation of water increases. This is because an increase in temperature produces heat energy which can dissociate many ions of water. So, the reason is correct.
Hence, the correct option is Option D- Assertion is false but reason is true.

Note: A few of the molecules are dissociated at any one moment in any water sample. However, when one hydrogen ion reattaches to a hydroxide ion to form a water molecule, there is a continuous change; another water molecule dissociates to replace the solution of the hydrogen ion and the hydroxide ion. At normal temperature the concentration of $\,{H^ + }\,$ ions and the $\,O{H^ - }\,$ ions are in equilibrium.