Question

Which of the following solutions would be least acidic? Assume all of the acids are the same concentration and at 25°C. The acid is followed by its Ka.

a) Hydrofluoric acid, 3.5. 10-4
b) Hydrocyanic acid, 4.9. 10-10
c) Nitrous acid, 4.6. 10-4
d) Unable to be determined by Ka

Answer 1

Option (b) Hydrocyanic acid, 4.9×10^-10

Step-by-step explanation:

Data obtained from the question include:

Ka of Hydrofluoric acid = 3.5×10^-4

Ka of Hydrocyanic acid = 4.9×10^-10

Ka of Nitrous acid = 4.6×10^-4

To know which acid is least acidic, we shall determine the the pKa value for each acid.

This is illustrated below:

For Hydrofluoric acid

Ka = 3.5×10^-4

pKa =..?

pKa = –Log Ka

pKa = –Log 3.5×10^-4

pKa = 3.5

For Hydrocyanic acid

Ka = 4.9×10^-10

pKa =..?

pKa = –Log Ka

pKa = –Log 4.9×10^-10

pKa = 9.3

For Nitrous acid

Ka = 4.6×10^-4

pKa =..?

pKa = –Log Ka

pKa = –Log 4.6×10^-4

pKa = 3.3

Summary:

Acid >>>>>>>>>>>>> Ka >>>>>>>> pKa

Hydrofluoric acid >> 3.5×10^-4 >> 3.5

Hydrocyanic acid >> 4.9×10^-10 > 9.3

Nitrous acid >>>>>>> 4.6×10^-4 >> 3.3

NB: The smaller the pKa value, the more acidic the compound is and the larger the pKa value, the less acidic the compound will be.

From the above calculations, Hydrocyanic acid has the highest pKa value.

Therefore, Hydrocyanic acid is the least acidic compound