# Homework Help: Chemistry Problem

1. Sep 26, 2007

### higherme

[SOLVED] Chemistry Problem

The question is
Calculate the mass of pure Na2CO3 needed for the preparation of 250.00mL of a 0.005M standard carbonate solution.

when it says PURE Na2CO3, what molarity does it mean? is it 1M??

my guess is to use M1V1 = M2V2 to solve
my final volume is 250mL
final concentration is 0.005 M
and if the pure Na2CO3 means 1M then that would be what i would be using as my stock right? then i would solve for the volume of that stock i need. After that I would use the density of Na2CO3 to find the mass

is that right?

2. Sep 26, 2007

### chemisttree

Find out how many moles of sodium carbonate you will need to make up this solution.
Hint: #moles = molarity(moles/L)/volume(L)

3. Sep 26, 2007

### higherme

so I figured that I will need 0.005M/0.25L = 0.02 mol of carbonate

now I use the molar mass of sodium carbonate to find the mass ??

4. Sep 26, 2007

### eli64

yes, correct.

5. Sep 26, 2007

### higherme

Thanks!

6. Sep 27, 2007

### higherme

wait.. i thought molarity is found by (mol/L) x L?
and not (mol/L) / L

????

7. Sep 27, 2007

### higherme

i mean moles is found by (mol/L) x L

8. Sep 27, 2007

### chemisttree

9. Sep 27, 2007

### higherme

before, you said that #moles = molarity(moles/L)/volume(L)

i thought #moles = molarity(moles/L) x volume(L)??

10. Sep 28, 2007

### eli64

higherme you are correct, Molarity = mol/L;
so mol = M x L

probably a typo in the reply before. then multiply by Molar mass to get grams

11. Sep 28, 2007

### chemisttree

Yes higherme, you are correct. Definitely a 'typo' (screwup) on my part...