6. Which pair of substances produces a buffer solution when equal volumes of 0.1
ID: 695513 • Letter: 6
Question
6. Which pair of substances produces a buffer solution when equal volumes of 0.1 M solutions are combined? (A) NH, and CH COOH (B) NaNO, and HNO, ntropy? (C) NaCIO, and HCI (D) NaF and HF 7. An aqueous solution is 0.10 M in both Ca and Ph As Na-C0 aq) is added, which will precipitate first, Ca or Pb", as the carbonate? Assume the volume doese't change. Soubiityuct Constants, Kp: aCO, 5x10. Pbco, 14 x 10 d by the curve? (A) Ca (B) Pb2 (C) They will precipitate at the same time, since they both have the same concentration. (D) Neither, Na CO, will precipitate first 8. What is the pH at the equivalence point of a titration of HCI and the weak base pyridine, Csli,N? (A)7.0 (B) above 7.40 (C) below 7.0 roxide um hydroxide rochloric acid acid (D) The Ks of pyridine is required. 9. A pH 7.2 buffer is prepared by dissolving equimolar amounts of H-PO, and HPO in water: ich change in of D? The addition of HCI to this buffer results in (A) a decrease in the [H PO:] (B) a decrease in the [HPO4 (C) an increase in the [HPO4 (D) no change in the concentration of [H PO] or 10. What is the pli of the solution prepared by adding 3.343 g of solid NH.CI to 250.0 mL of 0.25 M aqueous ammonia? Assume the volume does not change. lonization Constant NH, Ki-1.8 x 10-5 Molar mass NHCI 53.49 g/mol (A) 4.74(C)9.26 (D) 11.33 (B) 4.93Explanation / Answer
6.
HF is a weak acid, NaF is its conjugate base.
They will form buffer when equal volumes are mixed.
Option d)
7.
Ksp of PbCO3 < Ksp of CaCO3
When CO32- ions are added to solution, PbCO3 will start precipitating first.
Option b)
8.
At equivalence point, pyridine will form its conjugate strong acid.
So pH < 7
Option c)
9.
H2PO4- + H2O <--> HPO4- + H3O+
Addition of HCl will shift the equilibrium backward.
[HPO4-] will decrease.
Option b)
10.
[NH4+] = 3.343 g / 53.49 g/mol / 0.25 L = 0.25 M
[NH3] = 0.25 M
pOH = pKb + log([NH4+]/[NH3])
pOH = 4.74
pH = 14 – 4.74
= 9.26
Option c)
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