The mole is the SI base unit representing the chemical quantity of a substance. One of the major things that you must understand when doing mole calculations is Avogadro's constant. This is used when converting the number of moles that you have top a certain number of particles. This number is 6.02 x 1023. This will come up often and you must know when to use it and when not too. (see bottom of second page.)You will often see one mole referred to as the gram-formula mass, whose units are gram per mole.(g/mol).
Changing Moles to Grams
If you want to convert the number of moles that you have, to grams you must do the following: Multiply by the molar mass of the substance that is given to you..
Sample Problem
1. Convert 0.954 moles of CuSO4 to grams.
*To find the molar mass add up all the masses in the compound given.
Cu = 63.5
S = 32.1
O4 = 64.0
molar mass = 160
0.954 x 160 = 152.64 (since the original problem shows only two significant digits we can show only two significant digits in our answer.) the final answer = 150g
Changing Grams to Moles
In order to change the number of grams you have to moles you must: divide by the molar mass of the compound that is given to you.
Sample Problem
1. How many moles of particles are contained in 34g of NH3(formula
mass = 17g per mole)?
*To find the molar mass add up all the masses in the compound given.
N= 14.0
H3= 3.03
molar mass = 17.0
34/17 = 2.0 moles
Changing Moles to Number of Particles
In order to change the number of moles you have into a certain number of particles you need to:multiply by Avogadro’s number.
Sample Problem
1. How many molecules are there in 4.0 moles of H2O2?
* Remember: Avogadro's number is6.02 x 1023.
4.0 x 6.02 x 1023 = 24 x 1023 = 2.4 x 1024.
Need help with scientific notation, look here.
Changing Number of Particles to Moles
If you wish to change the number of particles that you have to moles you have to: Divide by Avagadro’s number.
Sample Problem
1. Change 7.14 x 1023 molecules to moles.
7.14 x 1023 / 6.02 x 1023 = 1.19 moles
Helpful Chart
Links
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Help with significant digits
Practice Problems
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