Chemistry: Finding concentration.

[Coldkilla]

Alrighty then. Doing some late night studying for my final in 2 weeks, I've come across a type of problem that keeps throwing me off. Its not in my professors notes, and nore have I seen it online in this form, only much simplier forms. Although these types of questions seem very easy, I just can't figure it out!

What is the concentration (in M) of the chloride ion when 18.5 mL of a 0.581 M solution of sodium chloride is combined with 16.2 mL of a 0.61 M solution of iron(III) chloride to a final volume of 50.8 mL?

How can I solve this? I know the formula M= MolesSolute/L Solution but when you've got more than 1 molecule, I'm getting thrown for a loop.

 

[TuxDave]

So is it.

18.5mL of sodium chloride + 16.2mL of iron solute + 16.1 mL of water to total 50.8mL? Otherwise I have no clue where 50.8 came from.

Edit Going to sleep but whatever it turns out to be, can't you figure out how many moles of chloride ions comes from the 18.5mL of solution, and how many moles come from 16.2mL of irone chloride of that concentration. Then total moles divided by total volume = overall concentration?

 

[Paperdoc]

Step 1 is to calculate from each of the input solutions (NaCl and FeCl3) how many moles of chloride ion are contributed. Remember that one mole of FeCl3 provides THREE moles of chloride ion. Step 2, add those up and you know how many moles of Cl- ions. Step 3 simply divide by the number of litres of final solution volume to get moles (of chloride ion) per litre of solution, symbol M.

Oh by the way, remember to express your answer with the correct number of significant figures. The least accurate piece of information you have at the start is the "0.61M solution ...", so your answer should have only 2 significant digits in it. No answer like 1.4447987352865 (not the answer - just a made-up number), which is what your calculator will show.

Actually, the question as stated solves a troublesome problem for you, although they have omitted one detail. The "problem" is that, when you mix together known volumes of two solutions, the total volume of the mixture is NOT simply the sum of the two input volumes. And you can't predict how to fudge the simple sum into the truth. They just did not tell you how, but what they did "behind the scenes" is they added more plain water to the mixture until they had an accurately known final total volume of 50.8 ml. And it is exactly that volume you need to do the last calculation.