Question

Which molecule will form a solution with a very low pH?

H3PO4
H2O
NaOH
KOH

Answer 1

The correct answer is A: H3PO4

Answer 2

Answer: Option (a) is the correct answer.

Explanation:

A solution which has more number of hydrogen ions will be acidic in nature and its pH will be less than 7. Whereas a solution which has more number of hydroxide ions will be basic in nature and thus, its pH will be greater than 7.

Also,        pH = $-log \frac{1}{H^{+}}$

Therefore, a low pH means pH will be very less than 7.

The given molecules will dissociate as follows.

      $H_{3}PO_{4} \rightleftharpoons 3H^{+} + PO^{3-}_{4}$

$H_{2}O$ being a neutral molecule will not dissociate and pH of water is 7. Hence, it is a neutral molecule.

      $NaOH \rightleftharpoons Na^{+} + OH^{-}$

      $KOH \rightleftharpoons K^{+} + OH^{-}$

So, we can see that $H_{3}PO_{4}$ gives maximum number of hydrogen ions. Therefore, we can conclude that molecule of $H_{3}PO_{4}$ will form a solution with a very low pH.