Question

9. What is the pH for 0.20 M HCIO solution? K = 2.5 x 10-7 a) 4.05 b) 3.70 c) 4.80 d) 7.00 e) 7.40 10. What is the pH of a mixture containing 0.30 M HNO, and 0.15 M NaNO, (K, for HNO2 is 4.5 x 104) a) 4.35 b) 3.05 c) 4.65 d) 5.01 e) 4.05 11. Acid strength increases in the series: HCN <HF <HSO.. Which of these species is the strongest base? a) SO42- b) F- c) CN- d) HSO, e) H2SO4

Answer 1

9)

use:
Ka = Kw/Kb
Kw is dissociation constant of water whose value is 1.0*10^-14 at 25 oC
Ka = (1.0*10^-14)/Kb
Ka = (1.0*10^-14)/2.5*10^-7
Ka = 4*10^-8

HClO      + H2O ----->     ClO-   +   H+
0.2                    0         0
0.2-x                  x         x


Ka = [H+][ClO-]/[HClO]
Ka = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes

Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((4*10^-8)*0.2) = 8.944*10^-5

since c is much greater than x, our assumption is correct
so, x = 8.944*10^-5 M



So, [H+] = x = 8.944*10^-5 M


use:
pH = -log [H+]
= -log (8.944*10^-5)
= 4.0485
Answer: 4.05

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