Ba(NO3)2, Mg(NO3)2, and AgNO3

Question

Let's say that you start with an aqueous solution of various compounds and you want to separate them. The strategy consists of choosing a chemical that will selectively precipitate one component of the mixture at a time. You separate the precipitate by physical methods (e.g. filtration, centrifugation), and then continue reacting the rest of the mixture with another chemical that will selectively precipitate another component, and so forth.

For this exercise, let's say that you begin with an aqueous mixture containing the following compounds: Ba(NO₃)₂, Mg(NO₃)₂, and AgNO₃

You also have separate solutions, one of each:  K₂SO₄ (aq), KCl (aq), and KOH (aq) Let's call these the challenge reagents.

Your task is to propose a step-by-step procedure for sequentially separating the cations in your mixture. You must include

1. What challenge reagent to add,
2. Which cation will precipitate,
3. What is the equation for that reaction, and
4. How will you separate the precipitate from the rest of the mixture so you can go on to the next challenge reagent.

Answer 1

The solution contains Ba(NO₃)₂, Mg(NO₃)₂, and AgNO₃

Add KCl first. It will precipitate AgCl. BaCl₂ and MgCl₂ are soluble.

AgNO₃(aq) + KCl(aq) → AgCl(s) + KNO₃(aq)

net ionic Ag⁺(aq) + Cl^-(aq) → AgCl(s)

Centrifuge, the solid AgCl will be on the bottom, The solution of BaCl₂ and MgCl₂ can be decanted.

To the decanted solution that contains BaCl₂ and MgCl₂ add K₂SO₄. BaSO₄ will precipitate and MgSO₄ is soluble.

BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)

net ionic: Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

Centrifuge, the solid BaSO₄ will be on the bottom, The solution of MgCl₂ can be decanted.

OR

you can use KOH for the second step. Ba(OH)₂ is soluble and Mg(OH)₂ will precipitate.

MgCl₂(aq) + 2KOH(aq) → 2KCl(aq) + Mg(OH)₂(s)

net ionic: Mg²⁺(aq) + 2OH^-(aq) → Mg(OH)₂(s)

Centrifuge, the solid Mg(OH)₂ will be on the bottom, The solution of Ba(OH)₂ can be decanted.

You can use filtration instead of centrifugation in both cases. The solid will remain on the filter paper.