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Pupil
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posted by  electropositive on 7/10/2008 9:48:42 PM  |  status: Live  

Finding Molarity of a Solution

Course Textbook Chapter Problem
General Chemistry Chemistry: The Central Science (10th) by Brown, Lemay, Bursten 13 N/A
Question Details:
I am going over a practice exam, and I am having particular trouble with what seemed to be an easy question.  I have the correct answer from the key, but I can not seem to arrive at it on my own.  I am looking for help with the method used to find it.

The question is:   

A solution is prepared by dissolving 23.7g of CaCl2 in 375 g water.  The density of the resulting solution is 1.05 g/mL.  The molarity of Cl- in this solution (in M) is:
A) 0.214
B) 0.562
C) 1.12
D) 1.20
E) 6.64 x 10-2

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Sage
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posted by Red Raiders on 7/10/2008 9:51:11 PM  |  status: Live
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Asker's Rating: Somewhat Helpful   
Response Details:
The answer is A!!!
 
rate livesavver!!!
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Pupil
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posted by electropositive on 7/10/2008 10:08:55 PM  |  status: Live
Asker's Rating: N/A-Posted by Person Asking Question   
Response Details:
according to my answer key...the correct answer is C.

I would love a step-by-step on how to reach C!
Mentor
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posted by AJ <3 on 7/10/2008 10:13:16 PM  |  status: Live
Asker's Rating: Lifesaver   
electropositive's comment:
"Exactly what I was looking for! (and really fast!)"
Response Details:
First, write out the reaction's formula:
 CaCl2 + H2O  Ca(OH)2 + HCl
Now balance the formula:
CaCl2 + H2O  Ca(OH)2 + 2HCl
We need to find the Molarity, so that means we need to get .
Use stoichiometry/ mole to mole relationships to find the molesof the HCl product:
 

Save that for later.
Next, we need to find the volume. We know that our solution if prepared from 23.7g of CaCl2 in 375 g water. That means the total mass of our solution is just 23.7g + 375g which is 398.7g. Now we also know the density of the solution, so we are able to solve for the volume:


Now that we have our moles of Chloride ion and the volume of the solution, just solve for it molarity:
   
And there's your answer, C) 1.12
 
Please ask for further assistance.
Please ask for further assistance. AJ
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