[renia45]

To find the concentration of BaCl2 I think I would divide by 0.0100L right? Sorry I think I mixed up the numbers but had the right idea. haha
And the concentration of Ba2+ is the same as the concentration of BaSO4 since it is a 1:1 ratio?
Ok thanks so much!!! This question has been bothering me for the longest time.
I greatly appreciate your help and time.
Thanks again 

[DrCMS]

To find the concentration of BaCl2 I think I would divide by 0.0100L right?

How big was the sample 10ml or 20ml?

[renia45]

It said: "A 20.0 mL solution of 0.200M Na2SO4 is added to a 10.0mL water sample which contains the unknown concentration of BaCl2" so I would use 0.0100 L in that case as the 0.0200 L would be used only in finding the moles of Na2SO4.

Also, since there is only one Ba in BaCl2 then when I have the moles of BaCl2 then this would be the moles of Ba2+ and then I would divide by 0.0200 L and that is the concentration which then needs to be divided by 100 (because of the 100 fold part) correct?