Calculate the concentration of: (a) H3O+ and (b) the pH of buffer solution prepared by mixing 200. mL of 0.10 M NaF and 100. mL of 0.050 M HF, Ka =7.2 x 10-4 for HF.

Question

Calculate the concentration of:

(a) H₃O⁺ and  (b) the pH of buffer solution prepared by mixing 200. mL of 0.10 M NaF and 100. mL of 0.050 M HF, Ka =7.2 x 10^-4 for HF.

Answer 1

in order to find the pH of a buffer solution you use the Henderson-Hasselbach equation

pH=pKa + log ([base]/[acid])

pKa= -log(Ka)= -log(7.2E-4)=3.1427

[base]= moles of base/total volume

moles of NaF=0.1M x 0.2L=.02moles 0.02 mol/.3L=0.067M

[acid]= moles of acid/total volume

moles HF= 0.05 M x 0.1L= 5E-4 mole 5E-4 mol/.3L=1.67E-3 M

pH=3.1427 + log (0.067/1.67E-3)

pH=4.75

pH=-log(H3O+)

H3O+= 10^-4.75= 1.79E-5 M H3O+