The pH of a 0.025M solution of HCN is 3.45. Calculate Ka for HCN. A weak monopro

Question

The pH of a 0.025M solution of HCN is 3.45. Calculate Ka for HCN.

A weak monoprotic acid has a Ka of 1.6 X 10-5. Determine the pH, pOH, and the concentration of all species at equilibrium if the initial concentration of acid is 0.045.

Determine the pH, [H3A], [H2A-], [HA-2], and [A-3] for a 0.1M solution of H3A. The respective pKa1, pKa2, and pKa3 values are 3.11, 5.15 and 7.34.

Carbonic acid, H2CO3, is an important component of blood. It is a diprotic acid.

Write the two chemical reactions that describe the acid-base behavior of carbonic acid.

For each reaction, label each species as acid, base, conjugate acid or conjugate base.

Carbon dioxide dissolves in water, forming carbonic acid. Write a chemical reaction describing this reaction.

Will the pH of a solution increase or decrease if the partial pressure of CO2 is increased over a solution? Why?

Explanation / Answer

1) Ka of HCN = [H+][CN-] / [HCN]

- log[H+] = 3.45

   [H+] = 10-3.45

    Reaction:       HCN ------> H+ + CN-

    Initial             0.025           0       0

    Equilibrium 0.025-x     x        x

We know, x = 10-3.45 = 2.84x10-3

Ka = x2 / 0.025-x = (2.84x10-3)2 / 0.025-0.00284

                              = 3.63x10-4

2) Weak monoprotic acid ----> XH

reaction:          XH ---------> X- + H+

Initial             0.045             0      0

Equilibrium   0.045-x           x      x

Ka = [X-][H+] / [XH] = 1.6x10-5

=> x2 / 0.045-x   = 1.6x10-5

=> x = 8.49x10-4

[H+] = [X-] = 8.49x10-4

[OH-] = 10-14 / 8.49x10-4 = 1.18x10-11

[XH] = 0.045-0.000849 ~ 0.045

pH = 3.07

3) The given Ka values are wrong, they are not practically possible since it is difficult to remove second H+ from the acid than first and hence the Ka1 should be much greater than Ka2 and Ka3.

4)

    a) H2CO3 + H2O ? H3O+ + HCO3-

    Excess acid (H3O+) in the body is neutralized by HCO3-... H2CO3 + H2O ? H3O+ + HCO3-

    Excess base (OH-) reacts with the carbonic acid (H2CO3)... H2CO3 + OH- ? H2O + HCO3-

     b) H2CO3 + H2O ? H3O+ + HCO3-

           salt                  acid     base

         H2CO3 + OH- ? H2O + HCO3-

             acid      base                salt

     c) CO2 + H2O ---> H2CO3 ----> H+ + HCO3-

     d) By increasing the partial pressure of the CO2 gas the pH increase or decease depends on the nature of solution, if its highly basic then pH decreases, as it acts as a base, if the solution is acidic, then its pH increases as it acts as an acid.

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