# Homework Help: Basic Molarity

1. Oct 1, 2009

### Zhalfirin88

1. The problem statement, all variables and given/known data
21.76g Cr(III) Sulfate is dissolved in enough water to give 500.0 mL of solution. 10.00 mL of this solution is pipetted into a 250.0mL volumatric flask and diluted to the mark.

a) Calculate the molarity of Cr(III) Sulfate in original solution
b) Calculate the molarity of sulfate ion in the diluted solution.

3. The attempt at a solution
I figured out a) but I'm having trouble with b) where do I start?

2. Oct 1, 2009

3. Oct 1, 2009

### Zhalfirin88

I don't get it, what's the final and initial volumes for this problem? And for the molarity, would it be the one I solved for in a) ?

4. Oct 1, 2009

### symbolipoint

Those are literally in the problem description.

5. Oct 1, 2009

### Zhalfirin88

That doesn't help.

The link he posted said the final volume is the sum of all volumes. Now, the sum of all what volumes? In class, there hasn't been a single mention of densities so I don't understand your link.

6. Oct 2, 2009

### Staff: Mentor

From the question: you take 10 mL of the solution and add enough pure water to fill it up to 250 mL. What is the final volume?

Note: densities are used when you are not given final volume and you have to calculate it. In your case final volume is given verbatim in the question so you can use equation 12.2 from the linked page, no need to go any further.

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