Question

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

acid base

Ka name formula Kb name formula

6.8x10-4 hydrofluoric acid HF 1.8x10-5 ammonia NH3

4.9x10-10 hydrocyanic acid HCN 1.7x10-9 pyridine C5H5N

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.

Answer 1

Answer:

HF ≥ HCN ≥ C₅H₅N ≥NH₃

Explanation:

Higher $K_a$ higher strength of acidity because of higher degree of dissociation .

From the given value of $K_a$ , it is clear that this value is higher for

HF that HCN  , so acidic strength of HF is more than HCN .

Similarly higher $K_b$ represents higher strength of basicity . The higher the basicity , the lower the acidity .

From the given value of $K_b$ it is clear that this value is higher for NH₃ than C₅H₅N. So NH₃ is more basic than C₅H₅N .

Hence NH₃ is less acidic than  C₅H₅N.

The acidity of given compounds can be arranged as follows

HF ≥ HCN ≥ C₅H₅N ≥NH₃

We know that higher the acidity lower the pH . This order will be reverses in case of value of pH. Hence HF will produce lowest pH solution and NH₃ will produce highest pH.

In order of increasing pH , these compounds can be arranged as follows .

HF ≥ HCN ≥ C₅H₅N ≥NH₃