Consider the following data on some weak acids and weak bases: acid base name fo

Question

Consider the following data on some weak acids and weak bases: acid base name formula name formula | 4.5x10 4am monia NH3 1.8x 10 nitrous acid HNO hydrofiuoric acidHF 6.8 x 10 pyridine CHN1.7 Use this data to rank the following solutions in order of increasing pH. In other words, select a '1' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so on solution pH 0.1 M NaCl choose one 0.1 M KF 0.1 M NH4Br 0.1 M NaNO2 choose one v choose one choose one v

Explanation / Answer

The order goes like:

0.1 M NH4Br --------------- 1 (Least pH)

0.1 M NaCl ------------------ 2

0.1 M KF --------------------- 3

0.1 M NaNO2 --------------- 4 (Highest pH)

For understanding this, we need to understand the formation of the above salts.

1. NH4Br is formed from the neutralisation reaction of a weak base NH4OH or NH3 and a strong acid HBr. Since the acid is strong, hence the salt is an acidic salt and therefore its pH is the least (less than 7).

2. NaCl is formed from the neutralisation reaction of a strong base NaOH and a strong acid HCl. Since both the acid and the base is strong, hence the salt is a neutral salt and therefore its pH is 7 which comes after NH4Br.

3. KF is formed from the neutralisation reaction of a strong base KOH and a weak acid HF. Since the base is strong, hence the salt is a basic salt and therefore its pH is more (More than 7).

4. NaNO2 is formed from the neutralisation reaction of a strong base NaOH and a weak acid HNO2. Since the base is strong, hence the salt is a basic salt and therefore its pH is more (More than 7).

Now if we compare KF and NaNO2 we would find that incase of KF, the acid is HF with Ka = 6.8 x 10-4. On the other hand, incase of NaNO2, the acid is HNO2 with Ka = 4.5 x 10-4.

Ka for HF > Ka for HNO2 (Ka = dissociation constant of an acid)

Higher the Ka value , stronger the acid. This is because more the Ka, more the dissociation of the acid into H+ ions. More the concentration of H+ ions in solutions, more the acidity. Hence HF is stronger acid than HNO2. Therefore though both KF and NaNO2 are basic salts, still KF would be lesser basic as compared to NaNO2.

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