Which combination of substances would be best for preparing an aqueous buffer so

Question

Which combination of substances would be best for preparing an aqueous buffer solution with a pH of 4.00, given the K_a values: CH_3COOH 1.8 times 10^-5 HF 6.6 times 10^-4 C_6H_5COOH 6.3 times 10^-5; CH_3CH_2NH_3^+ 2.3 times 10^-11 CH_3COOH and CH_3COONa HF and KF C_6H_5COOH and C_6H_5COONa CH_3CH_2NH_3Cl and CH_3CH_2NH_2 The solubility of Cd(OH)_2 in water is 1.70 times 10^-5 mol/L. The K_sp value for Cd(OH)_2 is: 1.97 times 10^-14 4.91 times 10^-15 5.78 times 10^-10 1.70 times 10^-5 none of these

Explanation / Answer

9) Buffer solution are a mixture of a weak acid/base and their corresponding salt(conjugate base) that resist any significant change in pH when a small amount of strong acid/base is added to it.For addition,of 10% strong acid/base pH changes by < ±1 .

pH=pka±log [base]/[acid]

So pH=pka +log 0.1=pka+1 (when 10% strength of acid is increased]

Or,pH=pka-log 10/1=pka-1 (when 10% strength of base is increased]

pH=pka±1(criteria for selecting buffer)

a)pka=-log(1.8*10^-5)=4.74

b)pka=-log(6.6*10^-4)=3.18

c)pka=-log(6.3*10^-5)=4.2

d)pka=-log(2.3*10^-11)=10.64

nearest pka=4.2 so C6H5COOH/C6H5COONa is the best choice for buffer.pH=4.2+1=5.2 or 4.2-1=3.2,3.2-5.2 is the closest range for pH change corresponding to pH=4.

10)Cd(OH)2ß-> Cd2+(aq)+2OH-(aq)

Let the solubility of Cd(OH)2 be S

[Cd2+]=S, [OH-]=2S

Ksp=[Cd2+][OH-]^2=(2S)^2*S=4S^3

Or,ksp=4*(1.7*10^-5)^3=19.65*10^-15=1.96*10^-14

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