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Question
A solution of NH4Cl and NH4OH acts as a buffer.
Solution
NH4Cl is a strong electrolyte hence, it dissociates completely. NH4OH is a weak electrolyte and dissociates to a small extent. Its dissociation is further suppressed by the common ion `NH_4^+`provided by NH4Cl in the solution.
`NH_4CI_((aq)) ⇌ NH_(4(aq))^+ + CI_max^-`
`NH_4OH_((aq)) ⇒ NH_(4(aq))^+ +OH_((aq))^-`
This solution acts as a basic buffer and maintains its pH around 9.25. It resists the change in pH on the addition of a small amount of acid or alkali. This can be explained as below:
Upon adding a small amount of HCl to this solution, H+ ions of HCl get neutralized by OH– ions already present and more of NH4OH molecules get ionized to compensate for the loss of OH– ions. Thus, pH practically remains unchanged.
Upon adding a small amount of NaOH to this solution, OH ions of NaOH combine with `NH_4^+` ions already present to form weakly ionized NH4OH. Thus, pH of the solution remains practically unchanged.
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