# Homework Help: Complex molarity problem.

1. Oct 31, 2012

### pasido

1. The problem statement, all variables and given/known data
http://i.imgur.com/Rz0qB.png

2. Relevant equations
The answer is B, but I would like to know how to find the answer.
Molarity = moles * volume

3. The attempt at a solution
Yep, I tried for 15+ minutes and couldn't get the darn units to cancel out. A worked out solution would be very helpful.

Last edited by a moderator: Oct 31, 2012
2. Nov 1, 2012

### sjb-2812

Is there a reason you have provided an image, rather than text?

How many moles of Na+ do you need?

3. Nov 1, 2012

### symbolipoint

The relevant equation, "Molarity = moles * volume", is wrong.

4. Nov 3, 2012

### ARlTHMOMANIA

Molarity = mole per liter or mol/L

Determine the ratio of Na+ in Na3PO4 by observing the formulas (*Hint 1/3 or 3/1). Multiply this by your Molar concentration of Na+ given in the problem.

Next, set up a ratio of:

[Na*(1/3 or 3/1) (M)] / Na3PO4 (M)

or

Na3PO4 (M) / [Na*(1/3 or 3/1) (M)]

(reason whether the answer should be greater than 1 or less than 1 to determine which ratio to use).

Finally multiply this by the volume you're trying to prepare.