Question

1. For each of the following, tell whether the acid is strong or weak.

a) Acetic acid b)HCl c) H3PO4 d)H2SO4 e)HCN f)H2CO3

2. Write the formula for the conjugate base of each acid.

a) H2SO4 c)HI e)NH4

3. Write the formula for conjugate acid for each base.

a) OH- b)NH3 c) CO3^2-

4. For each equilibrium, label the stronger acid, stronger base, weaker acid, weaker base. For which reaction(s) does the position of equilibrium lie toward the right? For which does it lie towards the left?

a) H3PO4+OH- ------> H2PO4 + H20 c) HCO3 + OH- --------> CO3^2 +H20

5. In each pair, select the stronger acid.

a) Pyruvic acid (pKa = 2.49) or lactic acid (pKa= 3.08) c) Benzoic Acid (Ka = 6.5 x 10^-5) or lactic acid (Ka = 8.4 x 10^-4)

Answer 1

1.(a): Acetic acid when dissolved in water is very weakly dissociated to CH₃CO₂^-(aq) and H₃O⁺. Ka for the dissociation of acetic acid is 1.8*10^-5. Hence acetic acid is a weak acid.

(b): HCl is completely dissociated in aqueous solution. Hence it is a strong acid.

(c): H₃PO₄ is a weak acid because it is weakly dissociated to H₂PO₄^-(aq), HPO₄^2-(aq) and PO₄^3-(aq) in aqueous solution.

(d): H2SO4 is a strong acid because it is almost dissociated to HSO₄^-(aq) and then to SO₄^2-(aq) when dissoled in water.

(e): HCN when dissolved in water is very weakly dissociated to CN^-(aq) and H₃O⁺. Ka for the dissociation of HCN is 6.2*10^-10. Hence HCN is a weak acid.

(f): H2CO3 is also weakly dissociated when dissoled in water. Hence it is a weak acid.