Chemistry Math Question

Question

100.0 mL of a .40 M solution of unknown weak acid is combined with 50 ml of a .25 M strong base, and the final pH of the solution is measured to be 5.25. What is the pK_a of the unknown acid?

Answer 1

HA = weak acid

BOH = strong base

HA + BOH <==> BA + H2O

moles acid = 0.1 L x 0.4 mol/L = 0.04 moles

moles base = 0.05 L x 0.25 mole/L = 0.0125 moles

After reaction...and with a total volume of 250 ml = 0.15 L...

moles acid = 0.04 - 0.0125 = 0.0275 moles

Final con. = 0.0275/0.15 = 0.183 HA

moles BA = 0.0125 moles

F install con. = 0.083 M

pH pKa + log (salt/acid)

5.25 = pKa log (0.083/0.183) = pKa+ (-0.34)

pKa = 5.59