Solutions- stoichiometry help! (Urgent!!)

[theiblife]

25.0g of LiCl and 55..0g of CaCl2 is mixed together with water to make a solution. The final [Cl-] is 0.632 mol dm-3. What is the final volume of the solution?

The correct answer is 2.5 dm3.

I solved for moles for both compounds first. C1V1 = C2V2 , which is the same as n =C2V2.. so I need concentration of each compound right? How do I figure that out from the final concentration of Cl? 

[kw0573]

The correct strategy may look different from what you are attempting, at a first glance, but it's not altogether different from the concept of "mol of Cl^- ions is conserved". Now concentration and volume is not the only way to balance the mols. Another way is to use the mass you are given, and molar mass (which you can find out). There may be a trick relating the mols of a compound and the mols of ions. Once you get mol of Cl^- you can use the definition of molarity to find the combined volume. 

[theiblife]

Sorry, I might have phrased that wrong. I used the molar mass of both compounds to solve for the moles. I know, but the final concentration of Cl, doesn't that mean the concentration of Cl in LiCl plus the concentration of CaCl2. So don't we need the concentration of each compound to find the concentration of both Cl?

[kw0573]

Aha! If you already have the mols of Cl^- in the final solution, then you only need to apply the definition of molar concentration to find out the volume. Certainly,CV = CV is useful if you know three of the four variables, but since you only know one (the final concentration), that's a sign to look for a different strategy. 

[theiblife]

Oh thanks so much! I realized what I was doing wrong--I found out moles of LiCl and CaCl2 and with that I could have found out moles of Cl-there are 2 mol Cl in CaCl2 and use that to find volume..