How can I calculate the molarity of a saturated solution?

Question

How can I calculate the molarity of a saturated solution?

1 Answer

Impact of this question

38913 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2014-06-07T01:25:11

If you know the equilibrium constant for the solubility (sometimes called the solubility product constant), then you can set up and solve an equilibrium equation to find the molarity.

Example: What is the molarity of a saturated solution of lead chloride ( $P b C l_{2}$ $PbCl_2$ )? The solubility equilibrium constant is $K_{s p} = 5.89 \times 10^{- 5}$ $K_(sp)=5.89×10^(−5)$

Solution:

Start by writing the chemical equation for dissolving the salt: $P b C l_{2} (s) \leftrightarrow P b^{2 +} (a q) + 2 C l^{-} (a q)$ $PbCl_2(s) harr Pb^(2+)(aq) + 2 Cl^(-)(aq)$

Next write the equation for the equilibrium constant. Remember that only the dissolved aqueous species contribute to the equation: $K_{s p} = [P b^{2 +}] {[C l^{-}]}^{2} = 5.89 \times 10^{- 5}$ $K_(sp)=[Pb^(2+)][Cl^(-)]^2=5.89 times 10^(-5)$

If a saturated solution contains an unknown concentration $x$ $x$ of the salt, then we know that
$(x) {(2 x)}^{2} = 5.89 \times 10^{- 5}$ $(x)(2x)^2=5.89 times 10^(-5)$

The $2 x$ $2x$ term is because the concentration of dissolved chloride ion is alway twice that of lead cations if all the species come from lead chloride.

Solving the last equation for $x$ $x$ , we find that $x = 2.45 \times 10^{- 2}$ $x=2.45 times 10^(-2)$ . Because $x$ $x$ represents the concentration of dissolved salt, the molarity of the saturated solution is $2.45 \times 10^{- 2} M$ $2.45 times 10^(-2) M$ .

Science

Math

Humanities

... and beyond

How can I calculate the molarity of a saturated solution?

1 Answer

Related questions

Impact of this question