Question

When 100 mL of 1.0 M Na3PO4 is mixed with 100 mL of 1.0 M AgNO3, a yellow precipitate forms and [Ag ] becomes negligibly small. Which of the following is a correct listing of the ions remaining in solution in order of increasing concentration

(A) PO43- < NO3- < Na+
(B) PO43- < Na+ < NO3-
(C) NO3- < PO43- < Na+
(D) Na+ < NO3- < PO43-
(E) Na+ < PO43- < NO3-

Answer 1

A. PO43- < NO3- < Na+

Step-by-step explanation:

Here’s the reaction equation:

Na3PO4 + 3AgNO3 = Ag3PO4 + 3Na+ + 3NO3-

All of the Ag and PO43- in the solution reacted to form the Ag3PO4 which is the yellow precipitate, leaving little or no Ag or PO43- left in the solution. This narrows down the answer to options A and B.

Since the concentrations and the volumes of the reactants are the same, the number of moles will also be the same.

Na3PO4 will release 3 moles of Na+ while AgNO3 releases 1 mole of NO3-. Hence, the Na+ would be more in solution than the NO3-, narrowing the answer further down to option A.

PO43- < NO3- < Na+