Assuming equal concentration of conjugate base and acid, which one of the follow
ID: 1000144 • Letter: A
Question
Assuming equal concentration of conjugate base and acid, which one of the following mixture is suitable for making a buffer solutions with an optimum pH of 7-7.1? A) CH_3COONa/CH_3COOH, Ka = 1.8 times 10^-5 B) KHS/H_2S, Ka = 8.9 times 10^-8 C) Na_2CrO_4/NaHCrO_4 Ka = 3.0 times 10^-7 D) NaCl/HCl E) None of the above. 14) Which one of the following solutions will make the best buffer? A) 0.10 M HNO_2 B) 1.0 M NaHSO_4 and 1.0 M H_2SO_4 C) 0.20 M HONH_2 D) 0.40 M HF and 0.10 M NaF E) 0.50 M HCl and 0.10 M NaCl 15) If the pKa of HCHO_2 is 3.74 and the pH of an HCHO_2/NaCHO_2 solution 3.11, which of the following is true? A) [HCHO_2] > [NaCHO_2] B) [HCHO_2]Explanation / Answer
13. C). The other options (excluding D) would make a buffer in the 4-4.5 pH range, according to Henderson-Hasselbalch equation.
14. D) Buffers are made of an acid and its conjugate base. Only B, D and E remains. Since the option with the weakest acid is D, this is the correct option.
15. A) Since pH - pKa is a negative number, the log[(A-)/(HA)] means the concentration of the undissociated acid, i.e. HCHO2 is higher than the conjugate base concentration, according to the Henderson-Hasselbalch equation.
16. B) The solubility will increase because the available amount of H3O+ ions will decrease as OH- is added (water is formed), thus favoring the right side of the equilibrium.
17. B) First we determine the molar solubility in moles/L: 0.430 mg/mol * 1 g/1000 mg * 1 mol/190.5 g = 2.26x10^-6 mol/L. Ksp = [Cu+] [I-] = 5.10 x 10^-12
18. D) deltaG > 0 indicates the process is not spontaneous. This means it will not go either to the left or to the right, which is the equilibrium condition (K = Q).
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