The question is incomplete; the complete question is;
Which of the following statements is/are true for a 0.10 M solution of a weak acid HA?
a. [ H+] >> [ A−]
b. [ H+] = [ A−]
c. The pH is 1.00.
d. The pH is less than 1.00.
Answer:
b. [ H+] = [ A−]
Explanation:
Given the acid as HA, we know that being a weak acid, its dissociation in water can never be 100%. If it were a strong acid, then it could have undergone a 100% dissociation in solution. The conjugate base of a weak acid is a always a weak base hence A^- is expected to act as a weak base. At the same concentration, weak acids have a higher pH value than strong acids. Hence if the pH of a strong acid HA is 1, then the pH of a weak acid HA must be greater than 1.
But, we look at the equation for the dissociation of the weak acid HA
HA(aq)⇄H^-(aq) + A^-(aq). This implies that the HA dissociates in a 1:1 ratio therefore; [H+] = [ A−], hence the answer given above.
