Question

(a) Give Arrhenius definition of an acid and base.
(b) Choose strong acid and strong base from the following: $C{H}_{3}COOH,N{H}_{4}OH,KOH,HCl$

Answer 1

Hint: To find the strong acid and strong base we have to see their ability to dissociate in the water. Strong acids and strong base completely dissociates in the aqueous solution. We can determine the strong acid and strong base on the basis of $K_a/p{K}_a$ and $K_b/p{K}_b$.

Complete step by step solution:
Let us discuss each question one by one
(a) Give Arrhenius definition of an acid and base.
From your chemistry lesson you have learned about the Arrhenius definition of acid and base. According to Arrhenius an acid is any substance which contains hydrogen and releases hydrogen ion in the aqueous solution whereas base is the substance which contains hydroxide and releases hydroxide ion in its aqueous solution.
But there are some limitations of Arrhenius acid base theory due to which it is not accepted so far such as:
-This theory is only applicable to aqueous solution
- All the bases do not have hydroxide ions but still act as a base in its aqueous solution for example ammonia.
(b) Choose strong acid and strong base from the following: $C{H}_{3}COOH,N{H}_{4}OH,KOH,HCl$
As we know that strong acids are those acids which are completely or nearly 100% ionized in their aqueous solution. So, the equation for a strong acid HA can be shown as:
$$HA(aq)+H_{2}O(l)\to H_3{O}^{+}(aq)+A^{-}(aq)$$
-Here hydronium ion $(H_3{O}^{+})$ is the conjugate acid of water and $A^{-}$ is the conjugate base of the acid HA. So, the strong acid yields 100% of $(H_3{O}^{+})$ when a strong acid ionizes completely in the water. We can also determine the strong acid on the basis of $K_a/p{K}_a$ i.e, higher the value of $K_a$ or lower the value of $p{K}_a$the stronger will be the acid.
-Therefore HCl is the strong acid among the following because it completely dissociates to release $H^{+}$ into the solution with $p{K}_a$ value approximately -7 whereas $C{H}_{3}COOH$ is the weak acid because it partially dissociates in the water and its $p{K}_a$ value is 4.7.
-Now, strong bases are those bases which are completely ionized in their aqueous solution. These compounds dissociate in the water to yield one or more than one hydroxide ion. We can also determine the strong base on the basis of $K_b/p{K}_b$, where $p{K}_b=14-p{K}_a$.
-Therefore KOH which is an alkali metal hydroxide is a strong base because it gets completely ionized in their aqueous solution to release $O{H}^{-}$whereas $N{H}_{4}OH$ is considered as the weak base because it gets partially dissociate into its ion in the aqueous solution.

Thus, the correct answer is HCl and KOH.

Note: Here we consider water as a neutral compound because they all are relative to water. Strong acids are only considered as strong when they dissociate significantly in the water. In case if the value of water will be low then the strong acids would be weaker. If we dissolve HCl in $H_{2}S{O}_4$ then in this case HCl will be considered as a weak acid because $H_{2}S{O}_4$ dissociate more easily than HCl in the solution.