1. To prepare a buffer solution with a pH of 3.7. Which of the following conjugated pairs would be the best option?
A. HSO4- / SO42- Ka = 1.2 x10-2
B. HCN / CN- Ka = 4.0 x 10-10
C. HCHO2 / CHO2- Ka = 1.8 x 10-4
D. H2CO3 / HCO3 Ka = 4.2 x 10-7
E. HClO / ClO- Ka = 2.9 x 10-8
To prepare a buffer solution with a pH of 3.7. The pKa value of the weak acid should be near to 3.7.
pKa = -logKa
A. HSO4- / SO42- Ka = 1.2 x10-2
pKa = -log(1.2x10-2) = 1.92
This pKa is much lower than 3.7. So, this option is wrong.
B. HCN / CN- Ka = 4.0 x 10-10
pKa = -log(4.0x10-10) = 9.40
This pKa is much higher than 3.7. So, this option is also wrong.
C. HCHO2 / CHO2- Ka = 1.8 x 10-4
pKa = -log(1.8x10-4) = 3.74
This pKa is nearly equal to 3.7. So, this may be the best conjugated pair
D. H2CO3 / HCO3 Ka = 4.2 x 10-7
pKa = -log(4.2x10-7) = 6.34
This pKa is higher than that of 3.7. So, this option is wrong.
E. HClO / ClO- Ka = 2.9 x 10-8
pKa = -log(2.9x10-8) = 7.54
Thia pKa is much higher than that of 3.7. So, this option is also wrong.
Hence,
Answer: C
Let me know if you have any queries regarding this
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