Question

Calculate the pH of the solution that results when 20 mL of 0.2M HCOOH is mixed with 25mL of 0.2M NaOH solution.(post-equivalence point)​

Answer 1

The pH of the solution : 12

Further explanation

Reaction

HCOOH + NaOH ⇒ HCOONa + H₂O

mol HCOOH =

`\tt 20~ml* 0.2~M=4~mlmol`

mol NaOH =

`\tt 25~ml* 0.2~M=5~mlmol`

Mol NaOH>mol HCOOH ⇒ at the end of the reaction there will be a strong base remains from mol NaOH, so that the pH is determined from [OH⁻]

ICE method :

HCOOH + NaOH ⇒ HCOONa + H₂O

4 5

4 4 4 4

0 1 1 1

Concentration of [OH⁻] from NaOH :

`\tt (1~mlmol)/(20+25~ml)=0.02`

pOH=-log[OH⁻]

pOH=-log 10⁻²=2

pH+pOH=14

pH=14-2=12