Question

a. What is the pH of a solution containing 0.2 M HA and 0.1 M A-  ?   Ka of HA is 0.002.

b.

What ratio of NaCHO2:HCHO2 would be required to make a buffer with pH 2.97 ?

Formic Acid, HCHO2   pKa = 3.74

Answer 1

a)

Ka = 2*10^-3

pKa = - log (Ka)

= - log(2*10^-3)

= 2.699

we have below equation to be used:

This is Henderson–Hasselbalch equation

pH = pKa + log {[conjugate base]/[acid]}

= 2.699+ log {0.1/0.2}

= 2.40

Answer: 2.40

b)

we have below equation to be used:

This is Henderson–Hasselbalch equation

pH = pKa + log {[conjugate base]/[acid]}

2.97 = 3.74+log {[NaCHO2]/[HCHO2]}

log {[NaCHO2]/[HCHO2]} = -0.77

[NaCHO2]/[HCHO2] = 0.170

Answer: 0.170