Homework Help: Chemistry

Posted by S on Monday, January 23, 2012 at 8:20pm.

A student mixed 15.0 mL of a solution of 0.0200 M ferric chloride FeCl3, with 25.0 mL of water.

Calculate the moles of iron ion, Fe +3 in the resulting solution.

(I think this is a Dilution problem, but I wasn't really sure how to start it out. Any help would be greatly appreciated.)

Here is what I started off with, but then I got confused:

M= moles solute/ 1 L solution

Moles of solute = (M)(1 L solution)
Moles Fe +3 = (0.0200M)(0.015L)
= 0.0003

Can I receive some clarification please? :) Thank you.

Chemistry - DrBob222, Monday, January 23, 2012 at 8:29pm
Assuming the volumes are additive (that is, is 15.0 + 25.0 = 40.0 mL), then
0.0200 x (15.0/40.0) = ?M
Or you can use the dilution formula as follows:
15.0*0.0200 = 40.0*M
M = 15.0*0.0200/40.0.
Same answer either way.

If you had followed through with your work you were headed in the right direction.
moles Fe^3+ = M x L = 0.02 x 0.015L = 0.0003 moles.
Then M of new soln = mol/L = 0.0003/0.040L = 0.0075 M. See if this is the same answer as both of the two above.
Chemistry - S, Monday, January 23, 2012 at 9:33pm
Wouldn't I just leave the answer as 0.0003 moles since the question is asking for the amount of the moles of the iron ion Fe +3 ? :) That's when I got confused.
Chemistry - DrBob222, Monday, January 23, 2012 at 10:18pm
You are right and I am wrong. Your remark about being a dilution problem started my mind in the direction of molarity and I went in that direction. However, you are absolutely right, moles is what the problem asks you do do and M x L = 0.0003 moles is correct.
Chemistry - S, Monday, January 23, 2012 at 10:47pm
Thanks so much for your feedback! :)

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