Answer:
8.69 is the pH at the equivalence point
Explanation:
Formic acid, HCHO₂, reacts with NaOH as follows:
HCHO₂ + NaOH → NaCHO₂ + H₂O.
At the equivalence point you will have in the reaction just NaCHO₂ and H₂O. The concentration of NaCHO₂ will be:
<em />
<em>Moles: </em>0.0278L * 0.797mol/L = 0.02216moles
To reach the equivalence point it is necessary to add:
0.02216mol * (1L / 0.928mol) = 0.0239L
Total volume in the equivalence point:
0.0278L + 0.0239L = 0.0517L
Concentration: 0.02216moles / 0.0517L = 0.429M
The equilibrium of NaCHO₂, CHO₂⁻, in water is:
CHO₂⁻(aq) + H₂O(l) ⇄ OH⁻(aq) + HCHO₂(aq)
Where Kb, 5.56x10⁻¹¹ is defined as:
5.56x10⁻¹¹ = [OH⁻] [HCHO₂] / [CHO₂⁻]
In the equilibrium, it is produced X OH⁻ and HCHO₂, and as concentration of NaCHO₂ is 0.429M:
5.56x10⁻¹¹ = [X] [X] / [0.429M]
2.383x10⁻¹¹ = X²
4.88x10⁻⁶ = X = [OH⁻]
As pOH = -log [OH⁻]
pOH = 5.31
And pH = 14 - pH
pH = 8.69 is the pH at the equivalence point
