# The Naked Scientists Forum

### Author Topic: How do I calculate the number of atoms in a given amount of something?  (Read 45210 times)

#### jaiii

• Sr. Member
• Posts: 114
##### How do I calculate the number of atoms in a given amount of something?
« on: 09/12/2011 11:53:16 »
Hello.

How to calculate how many atoms a particular element is located in one liter?

Thank you
goodbye
« Last Edit: 15/12/2011 08:10:32 by chris »

#### bumblebee

• First timers
• Posts: 5
##### Re: How do I calculate the number of atoms in a given amount of something?
« Reply #1 on: 11/12/2011 18:43:31 »
You can use Avogadro's constant (6.0221415 × 10^23 mol-1) to work out the number of atoms there are :)

#### widereader

• Jr. Member
• Posts: 25
##### Re: How do I calculate the number of atoms in a given amount of something?
« Reply #2 on: 21/12/2011 13:01:26 »
Convert liter to mole then mole to number of particles using Avogadro's number.

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: How do I calculate the number of atoms in a given amount of something?
« Reply #3 on: 21/12/2011 17:47:52 »
You'll have to know what you're dealing with.

If it is a pure substance, then you can convert liters to grams using the density.  Then grams to moles using the atomic mass of the item.  Then, as mentioned above, moles to atoms or molecules.

If it is a mixture and you are only interested in a single solute, often it is already expressed in either grams per liter, or moles per liter.

An example:
1L of Ethanol has a density of 0.789 g/cm3.
So, 1L Ethanol * 1000 cm3/L * 0.789 g/cm3 = 789 g

Now, if you go to the Periodic Table for the atomic weights.
H = 1
C = 12
O = 16

So the molecular weight of Ethanol (CH3CH2OH) = 12 * 2 + 1*6 + 16*1= 46

So, you get:
789g /46 g/mol = 17.2 mol

17.2 mol * 6.022 x 1023 molecules / mol = 1.03 * 1025 molecules.

Your question was about atoms.  Ethanol has 9 atoms / molecule.

And you would get: 1.03*1025 molecules * 9 atoms/molecule = 9.3*1025 atoms.

#### The Naked Scientists Forum

##### Re: How do I calculate the number of atoms in a given amount of something?
« Reply #3 on: 21/12/2011 17:47:52 »

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