When an ionic compound dissolves in water, it dissociates into its component ion

Question

When an ionic compound dissolves in water, it dissociates into its component ions. So how do we specify the concentration of such a solution? Is it the concentration of the cation? The concentration of the anion? The sum of these two concentrations? IT IS NONE OF THESE. Unless stated otherwise, the concentration of a solution of an ionic compound is the concentration of the original undissociated solute. In a 2 M solution of magnesium phosphate: What is the molarity of magnesium ions? What is the molarity of phosphate ions? In this chapter you will learn about properties of solutions that depend on the total concentration of solute particles. What is the total molarity of particles in this solution (the sum of the two concentrations above)?

Explanation / Answer

Given

Molarity of Mg3(PO4)2 is 2 M ( mol/L )

Disassociation will give Mg2+ and PO43-

Mg3(PO4)2 ----> 3 Mg2+ + 2 PO43-

1 mole 3 mole 2 mole

so Molarity of  Mg2+ = (3/1) * 2 (mol/L ) = 6 mol/L

(a) Answer: 6 M is molarity of magnesium ions

molarity of PO43- = (2/1) * 2 mol/L = 4 mol/L

(b) Answer: 4 M is molarity of phosphate ions

Total molarity of all particles when disassociated = 6 mol/L + 4 mol/L = 10 mol/L

(c) Answer : 10 M of total particles are present

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