Question

A)Rank the following acids in order of decreasing acidity given their K_a values.
      -       1.       2.       3.       4.      

H₂CO₃ K_a = 4.3 x 10⁻⁷

      -       1.       2.       3.       4.      

CH₃COOH  K_a = 1.8 x 10⁻⁵

      -       1.       2.       3.       4.      

HCN   K_a = 4.9 x 10⁻¹⁰

      -       1.       2.       3.       4.      

HNO₂ K_a = 4.5 x 10⁻⁴

B) What is the K_a of acid HA when the initial concentration of HA is 0.23M and the pH is 3.83?

Express your answer in scientific notation and be mindful of significant figures.

Answer 1

A

H₂CO₃ Ka = 4.3 x 10⁻⁷ = 4300 x 10⁻¹⁰      (3)

CH₃COOH Ka = 1.8 x 10⁻⁵ = 1,80,000 x 10⁻¹⁰   (2)

HCN Ka = 4.9 x 10⁻¹⁰ = 4.9 x 10⁻¹⁰   (4)

HNO₂ Ka = 4.5 x 10⁻⁴   =45,00,000 x 10⁻¹⁰ (1)

HNO₂ >CH₃COOH > H₂CO₃ > HCN

B

HA   H⁺ + B^-

pH = 3.83

-log [H⁺] = 3.83

[H⁺] = 10^-3.83 = 1.48 x 10^-4

So,  [B^-] = 1.48 x 10^-4

[HA] = 0.23 = 2.3  x 10^-1

K_a = (1.48 x 10^-4 x 1.48 x 10^-4) / 2.3  x 10^-1

= 9.5 x 10^-8