Question

Calculate the pH of the buffer that results from mixing 60.0 mL of 0.25 M HCHO2(pKa= 3.74) and 15.0 mL of 0.500 M NaCHO

Answer 1

Answer:

$pH=3.44$

Explanation:

Hello.

In this case, since the formic acid (HCHO2) is a weak acid and it is mixed with its conjugate base (CHO2⁻) in the form of NaCHO2, we can compute the pH via the Henderson-Hasselbach equation:

$pH=pKa+log(\frac{[Base]}{[Acid]} )$

Whereas the concentration of both acid and base are computed considering final total volume of 75.0 mL after adding the volume of each solution as shown below:

$n_{base}=0.500mol/L*0.0150L=7.50x10^{-3}mol\\\\n_{acid}=0.25mol/L*0.0600L=1.5x10^{-2}mol$

And the resulting concentrations upon the final volume:

$[Base]=\frac{7.50x10^{-3}mol}{0.0750L}=0.1M$

$[Acid]=\frac{1.5x10^{-2}mol}{0.0750L}=0.20M$

Thus, the pH is:

$pH=3.74+log(\frac{0.10M}{0.20M} )\\\\pH=3.44$

Best regards.