Question

3.Arrange the following according to increasing acid strength(i) Ka= 2.5 + 10-15(ii) Ka= 9.0 + 10-9(ini) pKa=7.5(iv) % dissociation =100(a) iv, in, in, i(b) ii, I, in, iv(c) i, in, iv, i(d) i, i, ini, iv(e) ili, iv, il, i

Answer 1

(d) i, ii, iii, iv

Step-by-step explanation

A large Ka value indicates a strong acid because it means the acid is largely dissociated into its ions. A large Ka value also means the formation of products in the reaction is favored. A small Ka value means little of the acid dissociates, so you have a weak acid.

In addition, the smaller the pKa value, the stronger the acid.

Note: pKa = - log ka

To arrange the following according to increasing acid strength, first, convert the Ka to pKa using the formula above.

(i) Ka= 2.5 + 10⁻¹⁵

pKa = - (log 2.5 + log 10⁻¹⁵)

pKa = -(0.40 - 15)

pKa = -0.40 + 15 = 14.6

(ii) Ka= 9.0 + 10⁻⁹

pKa = - (log 9.0 + log 10⁻⁹)

pKa -(0.95 - 9)

pKa = -0.95 + 9 = 8.05

(iii) pKa = 7.5

(iv) % dissociation = 100

This implies the acid dissociates completely in water. Strong acids have a large dissociation constant, so they dissociate completely in water.

The smaller the pKa value, the stronger the acid.

Therefore the arrangement of the above according to increasing acid strength is:

(d) i, ii, iii, iv