Question

1.

A buffer solution is 0.313 M in HCN and 0.220 M in NaCN . If K_a for HCN is 4.0×10^-10, what is the pH of this buffer solution?

2. A buffer solution is 0.473 M in H2C204 and 0.366 M in KHC2O4 . If Ka1 for H2C2O4 is 5.9*10^-2, what is the pH of this buffer solution?

3.

A buffer solution is 0.392 M in NaHC2O4 and 0.329M in NaC2O4 . If Ka for HC2O4- is 6.4*10^-5, what is the pH of this buffer solution?

pH =

Answer 1

To find pH of a buffer solution we use Henderson-Hasselbalch equation

Where

pKa=-log Ka

1. Concentration of weak acid HCN=0.313 M

Concentration of NaCN=0.220 M=Concentration of conjugate base CN^{​​​​​​-}

Ka for HCN=4.0x10^-10

Substituting values in Henderson-hasselbalch equation

pH=-log (4.0x10^-10)+log(0.220 M/0.313 M)=9.398-0.153=9.245

2. Concentration of weak acid H₂C₂O₄=0.473 M

Concentration of KHC₂O₄=0.366 M=Concentration of conjugate base HC₂O₄^-

Ka for H₂C₂O₄=5.9x10^-2

Substituting values in Henderson-hasselbalch equation

pH=-log (5.9x10^-2)+log(0.366 M/0.473M)= 1.229-0.111=1.118

3. Concentration of weak acid HC₂O₄^-=concentration of NaHC₂O₄=0.392 M

Concentration of Na₂C₂O₄=0.329 M=Concentration of conjugate base C₂O₄^2-

Ka for HC₂O₄^2-=6.4x10^-5

Substituting values in Henderson-hasselbalch equation

pH=-log (6.4x10^-5)+log(0.329 M/0.392 M)=4.194-0.076=4.118