Question

The scenes below depict the relative concentrations of H3PO4, H2PO4 , and HPO42- during a titration with aqueous NaOH, but they are out of order. (Phosphate groups are purple, hydrogens are blue, and Nations and water molecules are not shown.) = H,PO = H2PO = HPO 2- (a) List the scenes in the correct order. First (select) A (select) 4 (select)

Answer 1

(a) List the scenes in the correct order

First

C

B

D

A

Last

(b) pH = 2.14

(c) 69.00 mL NaOH

Explanation

(a) Scene C is the first scene because all the molecules in the solution are H₃PO₄ molecules

Scene B is the second scene because half the molecules of H₃PO₄ are deprotonated to H₂PO₄^-. This means NaOH has neutralized half H₃PO₄ molecules and solution is at first equivalence point.

At equivalence point, pH = pK_a1

pH = -log(7.2 x 10^-3)

pH = 2.16

Scene D is the third scene because half the molecules of H₂PO₄^- are deprotonated to HPO₄^2-. This means NaOH has neutralized half H₂PO₄^- molecules and solution is at second equivalence point.

Scene A is the last scene because all the molecules are HPO₄^2-