# Homework Help: How can water have a molarity if theres no diff b/w solute and solvent?

1. Sep 26, 2008

### jenzao

molarity is mols or a substance per Liter of a solution right?
that is, mols of the solute in litres of solution.

1. What is the solution?
2. if its water, then how can you say water has a molarity?

2. Sep 27, 2008

### Staff: Mentor

Don't think in terms of solution - ie separate solvent and solute. Concentration is just amount per liter, not per liter of water. Thus you have 55.55 moles of water per liter.

3. Sep 27, 2008

### gravenewworld

Because molarity is how much stuff you have in a certain amount of volume. It doesn't matter if it is water or an HCl solution.

4. Sep 27, 2008

### jenzao

So that means that compounds dont necessarily have set molarities right?
for ex, H20 always has mass of 16 + 1 + 1 = 18, and 1 mole is always 18g, but its molarity depends on how many liters of volume 1 mole is contaained in, is that right?

If yes, then why is water said to be 55.5M ??

5. Sep 27, 2008

### Redbelly98

Staff Emeritus
Not quite. Molarity is how many moles are in 1 liter. For water, it happens to be 55.6. 1 liter of water contains 55.6 moles of water.

6. Sep 28, 2008

### Staff: Mentor

To be exact, molarity of pure water is 1000/18.02 = 55.51

However, in the 1M solution of acetic acid there is only 946.9 g of water per liter, so its molarity in such solution will be 846.85/18.02 = 52.56

7. May 6, 2010

### harish8u

molarity=no.of moles per volume of soluton in liters
but u did 1000/18.02 how?

8. May 6, 2010

### Staff: Mentor

What is mass of 1L of water?

How many moles in that mass?

--
methods

9. Jul 15, 2011

### emiliostar

Let me breakdown:
Re: (1L = 1000 cm^3)
water has an approximate density of 1g/cm^3 (temperature dependent)
Therefore, 1000 cm of pure water has an approximate mass of 1000g
Mass of 1 mole of water = 18.02 g
Hence, no. of moles water in 1000 cm^3 of water = Total mass/mass of 1 mole = 1000 g/18.02g per mole = 55.55 moles