Question

For the diprotic weak acid H2A, Ka1 = 3.3 × 10^-6 and Ka2 = 5.3 × 10^-9. What is the pH of a 0.0450 M solution of H2A? What are the equilibrium concentrations of H2A and A2- in this solution? STRATEGY: Make a table of molar concentrations for each successive dissociation step. H2A(aq) ? H+(aq) + HA-(aq) Initial: 0.0450 0 0 Change: -x +x +x HA-(aq) ? H+(aq) + A2-(aq) Initial: x x 0 Change: -y +y +y Ka1 = x^2 / (0.0450 - x) ? x^2 / 0.0450 x^2 = Ka1 × 0.0450 x = ?((3.3 × 10^-6)(0.0450)) = 0.00039 M Express Ka2 in terms of x and y, then show the simplified expression (when y is assumed to be negligible compared to x). Finally, give the numerical value of y. Ka2 = ? ? / ? ? ? ? ? complete (no simplifications) simplified y = ? ? M

          For the diprotic weak acid H2A, Ka1 = 3.3 &times; 10^-6 and Ka2 = 5.3 &times; 10^-9. What is the pH of a 0.0450 M solution of H2A? What are the equilibrium concentrations of H2A and A2- in this solution?
STRATEGY: Make a table of molar concentrations for each successive dissociation step.
H2A(aq) ? H+(aq) + HA-(aq)
Initial: 0.0450 0 0
Change: -x +x +x
HA-(aq) ? H+(aq) + A2-(aq)
Initial: x x 0
Change: -y +y +y
Ka1 = x^2 / (0.0450 - x) ? x^2 / 0.0450
x^2 = Ka1 &times; 0.0450
x = ?((3.3 &times; 10^-6)(0.0450)) = 0.00039 M
Express Ka2 in terms of x and y, then show the simplified expression (when y is assumed to be negligible compared to x). Finally, give the numerical value of y.
Ka2 = ? ? / ? ? ? ? ?
complete (no simplifications) simplified
y = ? ? M

For the diprotic weak acid H2A, Ka1 = 3.3 times; 10^-6 and Ka2 = 5.3 times; 10^-9. What is the pH of a 0.0450 M solution of H2A? What are the equilibrium concentrations of H2A and A2- in this solution?
STRATEGY: Make a table of molar concentrations for each successive dissociation step.
H2A(aq) ? H+(aq) + HA-(aq)
Initial: 0.0450 0 0
Change: -x +x +x
HA-(aq) ? H+(aq) + A2-(aq)
Initial: x x 0
Change: -y +y +y
Ka1 = x^2 / (0.0450 - x) ? x^2 / 0.0450
x^2 = Ka1 times; 0.0450
x = ?((3.3 times; 10^-6)(0.0450)) = 0.00039 M
Express Ka2 in terms of x and y, then show the simplified expression (when y is assumed to be negligible compared to x). Finally, give the numerical value of y.
Ka2 = ? ? / ? ? ? ? ?
complete (no simplifications) simplified
y = ? ? M

Added by Andr-S J.

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