If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.89, which of the following is TRUE? If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.89, which of the following is TRUE? [HCHO2] > [NaCHO2] [HCHO2] = [NaCHO2] [HCHO2] >> [NaCHO2] [HCHO2] < [NaCHO2] It is not possible to make a buffer of this pH from HCHO2 and NaCHO2.

Question

Chemistry

Jasper

26 September, 23:01

If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.89, which of the following is TRUE? If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.89, which of the following is TRUE? [HCHO2] > [NaCHO2] [HCHO2] = [NaCHO2] [HCHO2] >> [NaCHO2] [HCHO2] < [NaCHO2] It is not possible to make a buffer of this pH from HCHO2 and NaCHO2.

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Answers (1)

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Rachael Bowman · Answer 1

The pH is greater than the pKa. There is more conjugate base than acid in solution. The answer is [HCHO2] < [NaCHO2]

Explanation:

Step 1: Data given

pKa of HCHO2 = 3.74

pH = 3.89 (HCHO2/NaCHO2)

This buffer contains formic acid (CHOOH), a weak acid; and HCOONa (the salt of its conjugate base, the formate anion, HCOO-)

Step 2: Calculate [HCOO-]/[HCOOH]

pH = pKa + log [HCOO-]/[HCOOH]

3.89 = 3.74 + log[HCOO-]/[HCOOH]

log[HCOO-]/[HCOOH] = 0.15

[HCOO-]/[HCOOH] = 10^-0.15 = 1.41

This means in the buffer there is 1.41 times more HCOO - (HCOONa) as HCOO (HCOOH) so [HCOONa] > [HCOOH]

The pH is greater than the pKa. There is more conjugate base than acid in solution. The answer is [HCHO2] < [NaCHO2]