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March 29, 2024, 05:10:25 am

Author Topic: Understanding and using moles  (Read 1233 times)

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Bri MT

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Understanding and using moles
« on: June 14, 2019, 05:09:21 pm »
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Being able to convert between moles and other units is crucial for success in chemistry (units 1&2, units 3&4, uni…) so if you don’t ‘get’ moles you should work on this know – before it prevents you from understanding later concepts.

Note: like how we can say "the mass is 6 grams" or m = 6 g, we can also say "there are 7 moles" or n = 7 mol

Why do we use moles?
When you look at an equation like: \(\ce{C6H12O6(aq) + 6O2(g) -> 6H2O(l) + 6CO2(g)}\)
you know that for every 1 glucose that’s reacted 6 water is produced. However, we need a unit to talk about this. Imagine if someone asked you to give them 6 water – you’d have no idea if they wanted 6 drops of water or 6 litres – or something else entirely! You might think that a good solution is to talk in terms of grams, but because different atoms and molecules weigh different amounts this would get very complicated very quickly.

By using moles, we can easily discuss the amount of reactants & products involved, convert that to a mass, or calculate the number of atoms involved. A mole is just a numerical amount, like how when we say “a dozen” we just mean 12, but it’s a very useful one.


How does it work?
Going back to this equation, \(\ce{C6H12O6(aq) + 6O2(g) -> 6H2O(l) + 6CO2(g)}\), we can see that 1 mol of glucose + 6 mol of oxygen combine to make 6 mol of water and 6 mol of carbon dioxide – all we have to do is read the number at the front of the product/reactant to figure this out. If we want to know how many mols of hydrogen atoms are produced we need to consider that for each mol/amount of water there are 2 hydrogen atoms: 6(the amount of water) x 2(the amount of hydrogen/amount of water) = 12 (the amount of hydrogen atoms).

The ratio of any particular reactant or product to another is known as the molar ratio – eg the molar ratio of carbon dioxide to glucose is 6/1 .

How do I convert between mols and mass?
Often a question might need you to find if you have _____ g of [thing] how many mol of  [thing] do you have?

The first step is to make sure you do have the mass in grams. I recommend writing it in your answer section as m(thing) = ___ g  as a way of double checking and making your answers clear. Next, you want to figure out how many grams it takes to make 1 mol of the thing – this is known as the molar mass (M). To find M, you count how many atoms of each element are in the molecule and add the atomic masses together.

For example, for glucose we know that:
 M = 6*(atomic mass  of carbon) + 12*(atomic mass of hydrogen) + 6*(atomic mass of oxygen) g/mol
= 6*12 + 12*1 + 6*16 g/mol
= 180 g/mol   
Note: Use the number of decimal places provided in the formula data booklet

Now that we have m and M, we simply use the equation n = m/M  to calculate the number of mols.

With the equation n=m/M, it can feel like you’re taking a 50% gamble on whether you’re remembering the m & M the right way around, so I like to think of it in terms of the units: M(g/mol)*n(mol)  = m(g). This is a useful reminder that M*n=m, which you can then rearrange if you need to find n or M.


How do I convert between mols and number of atoms/molecules/particles?
You might remember that I said a mole is a numerical amount. To be more exact, “a mole is a precisely defined quantity of matter equal to Avogadro’s number of particles” – QCAA Chemistry General Senior Syllabus 2019.

If you wanted to know how many bread rolls there are if you bought 2 dozen bread rolls, you would take 2, multiply it by how much is in a dozen (12) and calculate: 2 * 12 = 24, there are 24 bread rolls.

If you wanted to know how many bread rolls there are if you bought 2 mol bread rolls, you would take 2, multiply it by Avogadro’s number (it’s in the formula and data booklet) and calculate 2 * 6.022*10^23 = 12*10^23, there are 12*10^23 bread rolls.

You probably aren’t going to have many questions asking about bread rolls, but regardless of if you’re finding the number of atoms, molecules or something else the process is the same.


If you have any questions or ways of thinking about this that help you please feel free to comment below :)
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