Predicting the qualitative acid-base properties of salts

Question

Problem Page

Consider the following data on some weak acids and weak bases: 

acid	Ka
name	formula
hydrocyanic acid	HCN	×4.910−10
nitrous acid	HNO2	×4.510−4
base	Kb
name	formula
ammonia	NH3	×1.810−5
hydroxylamine	HONH2	×1.110−8

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 NaNO2	Choose 1-4 rate of strength	0.1 M KI	Choose 1-4 rate of strength	0.1 M HONH3Br	Choose 1-4 rate of strength	0.1 M NH4Cl	Choose 1-4 rate of strength

Answer 1

The formatting of your question makes it extremely difficult to follow the table, but suffice it to say that since it appears that all salts are at 0.1 M, we can look only at the Ka and Kb values. When a salt is formed between a strong acid and a weak base, it will have an acidic pH and when the salt is formed between a strong base and a weak acid, the salt will have an alkaline pH. It appears that the salts in question are NaNO₂, KI, HONH₃Br and NH₄Cl. Taking them one at a time, we have...

NaNO₂ - salt from a weak acid (HNO₂) and a strong base (NaOH) - pH will be >7 (alkaline)

KI - salt from a strong acid (HI) and a strong base (KOH) - pH will be neutral = 7

HONH₃Br - salt from a strong acid (HBr) and a weak base (HONH₂) - pH will be <7 (acidic)

NH₄CN - salt from a weak acid (HCN) and a weak base (NH₃) - pH will depend on the Ka and Kb. Since Kb for NH₃ is greater than the Ka for HCN,( or Kb CN^- is greater than Ka NH4⁺), this salt should have a pH >7 (alkaline).

So, the only acidic salt would be HONH₃Br, so it would get a ranking of "1"

Next comes the neutral salt KI, with a ranking of "2"

NaNO₂ would have the next lowest pH so ranking "3"

NH₄CN would have the highest pH with a ranking of "4"

This is all based on hydrolysis of the salts. For example, in determining ranking between NaNO2 and NH4CN, one looks at hydrolysis where NO₂^- ==>HNO₂ + OH^- and compares it to CN^- ==> HCN + OH^-

The comparison is based on the respective Kb for NO₂^- and CN^-.