# Homework Help: Moles of solute particles are present in 5.07 mL of 0.688 M

1. Sep 30, 2009

### Neophyte

1. The problem statement, all variables and given/known data

How many moles of solute particles are present in 5.07 mL of 0.688 M Na3PO4? Use scientific notation with 3 significant figures!!!!

2. Relevant equations

3. The attempt at a solution
0.00507 L of soln
0.688 M
M= mol solute / L of soln
0.688 = x / 0.00507
.688 * 0.00507
x = 3.49E-3

What went wrong :yuck:

2. Sep 30, 2009

### Bohrok

Re: Molarity

Moles of solute particles should be osmolarity: the molarity of aall particles which would be the molarities of the two ions.

3. Sep 30, 2009

### Neophyte

Re: Molarity

So it is
3.49E-3 * 4 = 1.40E-2 ?

4. Sep 30, 2009

### Bohrok

Re: Molarity

Do you have the answer to the problem?

5. Sep 30, 2009

### Neophyte

Re: Molarity

No, it was simply marked incorrect so I am unsure of the actual answer.

I just looked up osmolarity after you mentioned it. My book unfortunately neglected it :(. This is the first encounter with it so I was not sure if the method of obtaining it was correct. "the osmolarity of a simple solution is equal to the molarity times the number of particles per molecule"

I had the molarity = 3.49E-3
(Na3 = 3 PO4 = 1) = 4
3.49E-3 * 4

If it said "How many moles of solute are present" would it have been just finding the molarity?

6. Sep 30, 2009

### Bohrok

Re: Molarity

Yes, "moles of solute" would be molarity, but moles of solute particles would mean osmolarity.

7. Oct 1, 2009

### Staff: Mentor

Re: Molarity

You may treat it as a nitpicking (and I won't comment) but this is not the correct result

I mean - on some level it works, but phosphate is a strong base, it reacts with water and Na+ to some extent reacts with OH-. Thus the real result is slightly different - total concentration of dissolved entities (be it ions or neutral molecules) is about 2.804M, and number of moles is 14.2*10-3 (not 14.0*10-3).

Even that is only an approximation, but better than the initial one

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