Calculate the concentration of sodium formate and formic acid when the buffer is at pH = 3.9.

Question

You are preparing 1L of a 250mm formic acid buffer. The pka of formic acid is 3.75. Calculate the concentration of sodium formate and formic acid (each in mM) when the buffer is at pH = 3.9.

At pH 3.9 the concentration of sodium formate is ______ mM.

At pH 3.9 the concentration of formic acid is ______ mM.

Answer 1

Use the henderson equation:

pH=pKa+log([formate]/[formicacid])

[formate]=? mM

[formic acid]= 250mM

let the ratio of [formate]/[formicacid] be equal to x

3.9=3.75+log₁₀(x)

Solve for x

log₁₀(x)=0.15

x=10^0.15

x=1.4125

[formate]/[formic acid]=1.4125

x/250mM=1.4125

250(1.4125)=353.125=x

x=[formate]=353.125 mM

check:

3.9=3.75+log₁₀(353.125/250)

The expression is true

You can also solve by just making [formate]=x and get the same answer:

3.9=3.75+log₁₀(x/250)

x=353.125 mM