# How do you calculate the number of moles from volume?

##### 2 Answers

It depends on the substance whose volume you know.

#### Explanation:

If you have a pure liquid or a solid, you use its density to calculate its mass and then divide the mass by the molar mass.

If you have a solution, you multiply the molarity by the volume in litres.

**MOLES FROM VOLUME OF PURE LIQUID OR SOLID**

There are two steps:

- Multiply the volume by the density to get the mass.
- Divide the mass by the molar mass to get the number of moles.

**Example**

How many moles are in 10.00 mL of acetic anhydride? The molar mass of acetic anhydride is 102.1 g/mol and its density is 1.080 g/mL?

**Solution**

The formula for density is

#color(blue)(ul(bar(|color(white)(a/a) ρ = m/V color(white)(a/a)|)))" "#

where

We can rearrange the formula to get

#m = V × ρ#

**MOLES FROM VOLUME OF SOLUTION**

**Molarity** is the number of moles of a substance in one litre of solution.

#color(blue)(bar(ul(|color(white)(a/a) M = n/Vcolor(white)(a/a)|)))" "#

where

We can rearrange this equation to get the number of moles:

**Example**

How many moles of

**Solution**

Here are a couple of examples of mole-volume conversions.

The first part shows you how to convert from 0.30mol of CO to liters of CO.

The second part shows the conversion from 18.2L Ne to moles of Ne.

The problems were completed assuming both gases are at STP (standard temperature and pressure conditions).

video from: Noel Pauller

Hope this helps.