Consider a buffer made from the weak acid HA and its conj base, A^-, which pair

Question

Consider a buffer made from the weak acid HA and its conj base, A^-, which pair of concentrations results in the most effective buffer? 0.60M HA; 0.60 MA^- 0.30M HA; 0.60M A^- 0.90M HA; 0.20M A^- 0.20M HA; 0.90M A^- Calculate the pH at the equivalence point for the titration of 100ml of 0.10M HCN with 100 mL of 0.10 M NaOH? Ka = 4.9 times 10^-10 3.0 7.0 11.0 8.5 Calculate the molar ratio of CH_3NH_2 to CH_2 NH_3Cl required to create a buffer of pH = 10.24 K_b = 4.4 times 10^-4 0.60 0.15 0.26 0.40 A certain weak base, B^-, has a k_b of 1.0 times 10^-9. What is the pH of a buffer prepared by dissolving an equal molar mixture of its' base, B^-, and acid, BH^+, in water? 8.70 5.30 7.18 6.03

Explanation / Answer

Q8

Buffer equation

pH = pKa + loG(A-/HA)

so the best value is when A- = HA so log(1) = 0 an dhas plenty of capacity

so...

choose A, in which M = 0.6 for each specie

Q9

pH in equivalence

TotalV = 100+100 = 200

mmol of CN- formed = M1V1/(VT) = 100*0.1/200 = 0.05 M of CN-

CN- + H2O <-> HCN + OH-

Kb = [HCN ][OH-]/[Cn-]

Kb = Kw/Ka = (10^-14)/(4.9*10^-10) =2.04081*10^-5

2.04081*10^-5 = x*x/(0.05-x)

x = 0.001

[OH- = 0.001

pOH = -log(OH) = -log(0.001) = 3

ph = 14-3 = 11

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