# Litres to moles?

1. May 20, 2005

### ChemRookie

How would you go about changing liters, like 1L into moles?

Thanks

2. May 20, 2005

### dextercioby

You can't.Liters measure capacity or volume of fluids especially,while moles measure amount of substance.

Concentration is sometimes given in moles/liter.

Daniel.

3. May 20, 2005

### SpaceTiger

Staff Emeritus
Are you given the name of a substance in the liquid or solid phase? What about a density? If so, you can convert by way of:

$$n_m=\frac{V\rho}{N_Am}$$

where V is the volume in liters, $$\rho$$ is the density in kg/m3, NA is Avogadro's number, and m is the mass of an individual atom or molecule of your substance.

Without this information, however, dexter is right that the two can't be converted.

4. May 20, 2005

### The Bob

Again, I am only adding to what dextercioby and SpaceTiger have already said. It is not possible to convert a volume to a mole unit without another factor. Dextercioby said one of them (concentration) and SpaceTiger said the another way to work it out.

Going on what dextercioby said:

$$Moles \ (mol) = Volume \ (dm^3) \ \times \ Concentration \ (mol \ dm^3)$$

With concentration you can work out the number of moles. (Take note that the dm3 can be changed for any volume value if you know the conversion factors).

5. May 20, 2005

### dextercioby

$mol \ dm^{-3}$,okay ?Or u can use the fraction line.

Daniel.

6. May 20, 2005

### Jelfish

I'm not sure what sort of question you're looking to answer. The above posts are completely correct in that it doesn't make sense to convert moles to liters directly.

However, if you're learning about basic chemistry or thermodynamics and you're looking to change an IDEAL GAS from moles to liters, then you have the following equation:

$$22.4\text{L}\approx 1\text{mol}$$ for ideal gases at STP

This "constant" is due to the fact that ideal gases are presumed to be point masses and statically neutral at STP and therefore exhibit similar properties despite being different molecules. If it's not an ideal gas, you'll need the concentration, as others have said.

7. May 20, 2005

### ChemRookie

the info I have is:

a 1L container with 5mL of water helt at 20degcel

I need to find the vapour pressure. I'm supposed to use PV=nrt right?
how do I get n though?

8. May 20, 2005

### dextercioby

U got the volume of water,u need the # of moles.

18 ml of water<------>18 g of water<------>1 mol

Daniel.

9. May 22, 2005

### ChemRookie

huh?

I dont get it.

10. May 22, 2005

### The Bob

One gram of water is the same as 1ml of water.

You know that you have 5 ml but lets pretend you have 18ml (you will see why in a minute).

You can convert 18ml into 18g because it is water. Other substances will needed a different change factor.

Now you know the molecular formula of water (with is H2O). The molar mass of this is 18g ml-1.

To convert from mass to moles you have to use the equation:

$$moles \ (mol) = \frac{mass \ (g)}{molar \ mass \ (g \ mol^{-1})}$$

Therefore, 18g divided by 18g ml-1 is equal to 1 mole.

Use this equation find how many moles of water there are in 5 ml.

11. May 22, 2005

### brewnog

Can I just make sure that ChemRookie realises that 'ml' means millilitres, not moles? I'm not convinced that there isn't some confusion going on here!

12. May 22, 2005

### ChemRookie

thanks..now I get it.

13. May 23, 2005

### The Bob

's alright.

14. May 23, 2005

### Silverbackman

It isn't that hard, I think you just divide the element by the STP which is 22.4, and then you got your moles . You have to know a little chemistry though to know some of this but it really isn't that hard once you learn it.

15. May 23, 2005

### ArielGenesis

by the way, do ChemRookie really understand the concept of moles, as he asked that question is the first place. if he don't, latter on he might use 18 to measure other things.

16. May 23, 2005

### Bystander

Wrong.
You do NOT need it.

17. May 23, 2005

### ChemRookie

how come? explain..what formula do I use then?

18. May 23, 2005

### Bystander

IF "5 ml" is 5 gm, rather than releasing 5ml water vapor in the liter container, you are dealing with a two phase system, water vapor in equilibrium with liquid water. The vapor pressure is determined by temperature alone.

Formula? You come up with one, and the Nobel committee will hand deliver your prize. You get vapor pressure from tables in which the blood, sweat, and tears of lab boffins from the last 150 years have been compiled, fit, reviewed, interpolated, cubic-splined, and converted from this system of units to that.