Question

Which one of the following is a strong acid?

acetic acid

sodium hydroxide

ammonia

phosphoric acid

nitric acid

Answer 1

The larger Ka, more dissociation of HA and so the stronger the acid. The larger Ka is, the more negative the pKa so the lower the pKa, the stronger the acid.

Ka of acetic acid is 1.75 × 10 ^−5. Hence pKa = -log (Ka) = -log (1.75 × 10 ⁻⁵) = 4.75

Ka of phosphoric acid 6.9 × 10 ^−3. Hence pKa = -log (Ka) = -log (6.9 × 10 ⁻³) = 2.16

pKa of Nitric acid = -1.4

Ka of phosphoric acid 5.6 x 10^-10. Hence pKa = -log (Ka) = -log (5.6 x 10^-10) = 9.25

sodium hydroxide is basic in nature.

Hence out of all Nitric acid has the lowest pKa=-1.4. Hence Nitric acid is the strong acid.