Question

1. Calculate the concentration of the 25mL of phosphoric acid Which is neutralized with 7.8 mL of 0.2M NaOH?

2. In a solution of NH3, the concentration of NH3 is 0.01 M, Kb is 1.8 x 10-5. What is the concentration of the hydroxide ion? NH3→ NH4 ++ OH-

3. Calculate the Ka for formic acid, HCHO2 if the H+ concentration is 1.5 x 10-3 M and the concentration of the HCHO2 is 0.0125M at equilibrium.

4. The ionization constant for the weak acid HA is 4.0 x 10-8. Calculate the equilibrium concentration of H+, A- and HA in a 0.050 M solution of the acid.

Answer 1

1) H3PO4 + 3 NaOH -------------------> Na3PO4 + 3H2O

M1 V1 / n1 = M2 V2 / n2

M1 x 25 / 1 = 0.2 x 7.8 / 3

M1 = 0.0208 M

concentration of the 25mL of phosphoric acid = 0.0208 M

2)

[OH-] = sqrt (Kb x C) = sqrt (1.8 x 10^-5 x 0.01)

       = 4.24 x 10^-4 M

3)

HCHO2 --------------------> H+ + CHO2-

0.0125-x                             x            x

x = 1.5 x 10^-3

Ka = x^2 / 0.0125 -x

Ka = (1.5 x 10^-3)^2 / (0.0125 -1.5 x 10^-3)

Ka = 2.05 x 10^-4

4)

[H+] = [A-] = sqrt (Ka x C)

                    = sqrt (4.0 x 10^-8 x 0.050)

                   = 4.47 x 10^-5 M

[H+] = [A-] =4.47 x 10^-5 M

[HA] = 0.05 -4.47 x 10^-5 = 0.0499 M