What is the concentration at the first equivalence point?

Question

You titrate 50.0 mL of 0.100 M H2A (a diprotic acid) with 0.250 M KOH. What is the concentration of HA- at the first equivalence point?

Answer 1

Look at the reaction taking place:

H₂A + KOH ==> KHA + H₂O ==> K₂A + H₂O

acid.....base........1st eq................2nd eq.........

Assuming H2A is a strong acid, we have the following relationship...

H₂A <==> H⁺ HA^-

HA^- <==> H⁺ + A^2-

moles H₂A initially present = 50.0 ml x 1 L/1000 ml x 0.100 mol/L = 0.005 moles

moles KOH added to reach first equivalence point = 1/2 x 0.005 = 0.0025 moles

moles HA^- @ first equivalence point = 0.0025 moles

volume of KOH needed to provide 0.0025 moles: (x L)(0.250 mol/L) = 0.0025 mols and x = 0.01 L = 10 ml

volume of solution @ first equivalence point = 50.0 ml + 10.0 ml = 60.0 ml = 0.060 L

[HA^-] = 0.0025 mols / 0.060 L = 0.417 M