Question

pOH of ${H_{2}}O$ is $7.0\;$ at $298\;K$ . If water is heated at $350\;K$ , which of the following should be true?
(A) pOH will decrease
(B) pOH will increase
(C) pOH will remain $7.0\;$
(D) Concentration of ${H^ + }$ ions will increase but that of $O{H^ - }$ will decrease

Answer 1

pH and pOH are the scales which are used to determine the acidity or basicity of the chemical compounds or substances. pH is calculated with respect to the concentration of hydrogen $({H^ + })$ ions present in the solution while pOH is calculated with respect to the concentration of hydroxide $(O{H^ - })$ ions present in the solution.

Complete answer:
We know that pH and pOH are two scales which are used to determine the acidity or basicity of the chemical compounds or substances. pH is calculated with respect to the concentration of hydrogen $({H^ + })$ ions present in the solution.
$pH = - \log ({H^ + })$
pOH is calculated with respect to the concentration of hydroxide $(O{H^ - })$ ions present in the solution.
$pOH = - \log (O{H^ - })$
The reaction is given as:
${H_2}O \rightleftharpoons {H^ + } + O{H^ - }$
In the above reaction, the water molecule is dissociating in the hydrogen and hydroxide ions. When the temperature is increased to $350\;K$ , the dissociation or ionization of water molecules into these ions increases. Hence, the concentration of hydrogen ions and hydroxide ions increases in the aqueous solution and from the formulas of pH and pOH we can conclude that the value of pH and pOH will decrease.
Hence, the correct option is (A) pOH will decrease.

Note:
We should remember that ${K_W}$ is the dissociation or ionization constant of water and its value always remain ${10^{ - 14}}$ at the standard temperature and pressure conditions but as the temperature increases, the value of ${K_W}$ also increases because the concentration of hydrogen ions and the hydroxide ions also increases in the solution.