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Dissociation - help14

Ionic compounds can be either strong electrolytes or weak electrolytes. A strong electrolyte will completely dissociation into ions when dissolved in water. That is to say, every last particle of the compound will undergo dissociation in order to form the cation and the anion. There are no un-dissociated particles remaining. If one knows the concentration of the ionic compound, then one can use the balanced dissociation equation to predict the concentration of cation and anion in the aqueous solution. Learn more about how to predict the ion concentrations in the How to Think About This Situation section.

There are three similar versions of this question. Here is one of those versions:

Version 1:

The ionic compound sodium phosphate is a strong electrolyte that completely dissociates when dissolved in water. Determine the concentration of the two ions in an aqueous solution of 1.20 M sodium phosphate.

When ionic compounds dissolve in water, they dissociate. That is, they break apart into ions. There are two such categories of these ionic compounds - strong electrolytes and weak electrolytes. This question applies to strong electrolytes. A strong electrolyte will completely dissociate. That is, if you mix up a solution containing 2.5 moles of the strong electrolyte in 1.0 L of water, then all 2.5 moles of the ionic compound will break apart into ions. The dissociation process is complete in the sense that all the particles (100% of them) dissociate and there are no undissociated particles of the ionic compound remaining.

The dissociation process is often described by a balanced dissociaton equation. This was the focus of the second activity of this Concept Builder. For barium chloride (BaCl₂), the dissociation equation is ...

BaCl_{2 (s)} ==> Ba²⁺ _(aq) + 2 Cl^- _(aq)

Being able to write and interpret such an equation is critical to success on the third activity of this Concept Builder. The equation shows that every particle of BaCl₂ that dissolves will dissociate into one barium ion and two chloride ions. And so the solution contains just as many barium ions as there were barium chloride particles dissolving. And it contains twice as many chloride ions. So if 2.5 moles of BaCl₂ are dissolved into a liter of water, 2.5 moles of Ba²⁺ ions and 5.0 moles of Cl^- ions will be produced. A 2.5 M solution of BaCl₂ will result in concentrations of 2.5 M of Ba²⁺ and 5.0 M Cl^-.

While the above discussion pertains to barium chloride (BaCl₂), the principles and the process apply to any ionic compound that is a strong electrolyte. Use the balanced dissociation equation to determine the ratio of moles of ions to moles of the ionic compound. Then use the mole ratios as multipliers to determine the concentration of products.