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Amount of substance calculation

Chemists use amount of substance for counting atoms. This uses moles to find out said substance by using relative atomic mass and the weight of the substance. This is represented by:

n=m/M                      n=Moles
                                  m=Mass
                                  M= Molar mass (relative atomic mass)

To find out molar mass of a substance you can look at a periodic table and looking at the mass number (larger number above the element) for example carbons mass number is 12 NOT 6 as this is atomic mass.

A simple example of this would be using carbon. First try this on your own using the formula above, if you have trouble follow this through after writing the question down.

How many moles are there in 48g of carbon?

Now let run through this piece by piece and we will break it down and increase difficulty as we run through different examples.

Returning to the original formula which is:

n=m/M

Looking at the periodic table of elements we can see that Carbon has a molar mass of 12 due to the mass of the element being 12. Now aware of this we can put both the molar mass and the amount of substance into this:

n=48/12

At this point you can most likely calculate this in your head and come up with 3 but in harder calculations feel free to use your calculator as it is allowed during tests! So now you can do simple calculations with singular elements, let try it with a substance now...

How many moles are there in 11g of carbon dioxide?

Keeping in mind balancing of equations which are pretty simple (If you are struggling check out my balancing equations post) we should know that carbon dioxide in symbol form is CO2. Being that we are now working with a substance we need to find out the molar mass of the whole substance.

Carbon as before is still 12 but now we have two lots of Oxygen which equals 32 (16 on its own) and to find the molar mass of the entire substance we add it all together resulting in  a Mr of 44. Now it's just a matter of plugging what you have into the same equation:

n=m/M
n=11/48=0.250 mol

Alternatively, you may be given a similar question but asked in a different format, to solve this you just alter the equation to suit the needs of your new question. For example:

Calculate the mass in grams in 3 moles of SiO2

Once again we find the molar mass by adding the mass of the different elements.

Si=28
O2=32

SiO2=60

Now we just need to rearrange the formula to fit to the data you have in which case we have th molar mass and the moles. The new equation would be:

m=n x M
m=60 x 3= 180g

This is all you need to know about the amount of substance formula!

Till Next tyme! :)


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