Question

Four statements are given-
(i) On increasing the temperature water becomes acidic
(ii) pH of \[{10^{ - 8}}M\;HCl\] is $8$
(iii) aqueous solution of \[N{H_4}Cl\] is acidic
(iv) solubility of salt in aqueous solution depends on temperature
Which of the following options is correct?
(1) i, iii, iv
(2) ii, iii iv
(3) iii, iv
(4) ii, i, iii

Answer 1

Hint: As we all know that according to Le Chatelier’s principle if at equilibrium, one of the factors is changed the equilibrium will try to shift in that position where change is lower so as to counter that change.

Complete Step by step answer: As we all know that Le Chatelier’s principle is a key concept of chemical equilibria and when we change any one of the factors in equilibrium, its position basically changes accordingly to counter that change. We can change the equilibrium chiefly by changing the temperature.
If we increase the temperature of water or its pH level, the water molecules will prompts the equilibria to lower the temperature by absorbing heat which creates more hydrogen ions and hydroxide ions and thus results in lowering the pH of water but it does not mean that the water will become acidic because a solution can only be acidic when hydrogen ions will be more than hydroxide ions which is not the case with water as concentration of hydrogen and hydroxide ions remains the same, thus the first statement is false.
We also know that pH is determined on the basis of hydrogen ion concentration, it is the negative logarithm of hydrogen ion concentration and also measures how acidic or alkaline a solution is. In the second part we are given with pH and we know that $HCl$ is a strong acid and hence it will not have a pH $8$ which is mostly considered as a basic pH. Therefore this statement is incorrect.
In the third option, we know that ammonium chloride will dissociate into ammonium and chlorine ions and the salt of this compound is a mixture of weak base ($N{H_3}$) and a strong acid ($HCl$). So due to the presence of chloride ions, the aqueous solution of \[N{H_4}Cl\] is slightly acidic with a pH $4.6 - 6.0$. Hence it is a true statement.
Now the last part says that the solubility of salt in aqueous solution depends on temperature which is true because as we know that solubility of a substance is the amount of that substance required for the formation of a saturated solution in a given amount of solvent. Solubility varies with the temperature, when the temperature is increased, solubility increases. Hence this statement is also true.

So from the above explanation we can say that the correct answer is (3).

Note: At a temperature of about ${25^ \circ }C$, pH of the acidic solution is generally less than $7$, neutral solution like pure water has a pH $7$value and the basic solutions have pH more than $8$. Although acidic strength can also be determined by other parameters also such that the electron accepting capability.