Question

Which of the following salts have a ph value lesser than 7?
Na2CO3
NaCl
AlCl3
(NH4)2CO3

Answer 1

Heya friend!!
Rajdeep here....

When you add water on these salts, they hydrolyze, giving an acid and a base:

1. Na2CO3 + H2O --------> NaOH + H2CO3
We know, sodium hydroxide is a strong base and carbonic acid is a weak acid. So influence of the base is more, due to which Na2CO3 solution is basic in nature, so it's pH value is MORE THAN 7.

2. NaCl + H2O ------> NaOH + HCl
We know, NaOH is a strong base and HCl is a strong acid, so both neutralize each other, due to which NaCl solution is neutral. Hence, its pH value is NEARLY EQUAL TO 7.

3. AlCl3 + H2O ------> Al(OH)3 + HCl
We know, Al(OH)3 is a weak base and HCl is a strong acid, hence influence of acid is more due to which AlCl3 solution is acidic. So it's pH value is LESS THAN 7.

4. (NH4)2CO3 + H2O -----> NH4OH + H2CO3
NH4OH, though a weak base, is an alkali, but H2CO3 is a weak acid, hence ammonium carbonate solution in basic, so its pH value is MORE THAN 7.

Your answer is 3. AlCl3.

Thanks!!