Use the 5% rule to determine whether or not the equilibrium concentration of the

Question

Use the 5% rule to determine whether or not the equilibrium concentration of the acid can be approximated by its makeup concentration.

(a) 0.43 M cyanic acid (HCNO, pKa = 3.46) WebAssign will check your answer for the correct number of significant figures. % The approximation is valid. The approximation is not valid. Incorrect: Your answer is incorrect.

(b) 0.0068 M hydrazoic acid (HN3, pKa = 4.60) WebAssign will check your answer for the correct number of significant figures. % The approximation is valid. The approximation is not valid.

(c) 5.7 M arsenic acid (H3AsO4, pKa = 2.26) WebAssign will check your answer for the correct number of significant figures. % The approximation is valid. The approximation is not valid.

Explanation / Answer

HCNO pKa = 3.46

Ka = 10^-3.46 = 3.47 x 10^-4

concentration C = 0.43 M

HCNO ----------------> H+ + CNO-

0.43                                0          0

0.43-x                              x           x

[H+] = sqrt (Ka x C)

        = sqrt (3.47 x 10^-4 x 0.43)

        = 0.012 M

equilibrium concentration of [HCNO] = 0.43 - 0.012 = 0.42 M

% ionization = ([H+] / initial ) x 100 = 2.84 %

this 2.84 % < 5 % . so approximation is valid.

b)

Hydrazoic acid pKa = 4.60

Ka = 10^-4.60 = 2.51 x 10^-5

concentration C = 0.0068 M

[H+] = sqrt (Ka x C)

        = sqrt (2.51 x 10^-5 x 0.0068)

        = 4.13 x 10^-4 M

% ionization = ([H+] / initial ) x 100 = 6.08 %

this 6.08 % > 5 % . so approximation is not valid.

c)

[H+] = sqrt (Ka x C)

        = sqrt (5.495 x 10^-3 x 5.7)

        = 0.177 M

equilibrium concentration of [H3AsO4] = 5.7 - 0.177 = 5.5 M

% ionization = ([H+] / initial ) x 100 = 3.10 %

this 3.10 % < 5 % . so approximation is valid.

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