Science
Never Ignore a Mole - What is a Mole and Why it is so Important
In this episode of Chemistry Made Simple, host Matthew McAryo introduces the concept of the mole, a fundamental unit in chemistry. The discussion covers its significance, how it relates to atomic mass...
Never Ignore a Mole - What is a Mole and Why it is so Important
Science •
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Interactive Transcript
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In today's episode we're introducing the mole and this is perhaps the most important thing to
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learn at this stage and one of the most important concepts that you'll need through your chemistry.
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Hello and welcome to the chemistry made simple podcast. I'm your host Matthew McAryo and this is
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the podcast where you get chemistry confident and we take you from point A to grade A.
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Hello, welcome back and welcome to this episode. We're going to be talking about the mole today
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so firstly let's define what is the mole. Mole is the international standard unit for the amount
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of substance and you'll be using it in every calculation so it will be super important for you to
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learn and to understand and know how to use it. Why is the mole the standard unit? We discuss relative
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atomic mass in a recent episode about isotopes so why don't we use atomic mass units as our measure?
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Well it's because the atomic mass unit is an impossibly small unit you can't measure it out you
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can't weigh something in terms of atomic mass units so it would be completely practical to use that as a unit
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and to give you an idea of scale of an atomic mass unit one gram is approximately 10 to the 24
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bigger than an atomic mass unit. So relative to atomic mass and atomic mass units are important
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but using the mass of individual atoms or molecules isn't useful. So why don't we just use grams?
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Well we can't just compare the mass because reactions happen between substances in ratio of how many
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particles are reacting and not how heavy they are. What do I mean? Well a balanced chemical equation
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tells you how many particles of each of the substances react or are formed but substances
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have different masses, very different masses because the elements have different relative atomic
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masses. So using mass to compare quantities of substance would not be useful. It doesn't tell us
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how many particles until we include the mole concept and that's why we do.
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So what is the unit of a mole? A mole is a fixed number of particles and that number of
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particles is a very specific number. It's 6.02 times 10 to the 23 so it's a huge number
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and it's not a round number, it's not an easy to use number is it but it's this for a reason.
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That number is Avogadro's number and the reason it's that number is because that is the number
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of atoms of carbon in exactly 12 grams of carbon 12. Remember carbon 12 is our reference material
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for relative atomic mass. So how do we use the mole? Use it to calculate the mass of one mole
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of an element or a compound and we call that mass of one mole of the substance the molar mass of
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that substance. For molecules we add up the relative atomic masses of each item in the formula
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instead of quoting that total in atomic mass units that number becomes grams and that's because
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of the magic of using Avogadro's number that we can change it into grams. So let's think for
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an example. Water has of course two hydrogens and one oxygen. A hydrogen atom has a relative
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atomic mass of one atomic mass unit and oxygen atom has a relative atomic mass of 16 atomic mass
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units. Adding up the atoms present in the formula H2O gives us 18 and that means the molar mass of
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water is 18 grams. One mole of water weighs 18 grams. How do we use these numbers in calculations?
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Well we frequently need to know the number of moles of a substance used or created in a reaction.
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If we know how many grams then just divide that by the molar mass to find the number of moles of
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that substance. Considering our example of water if a reaction formed 3.6 grams of water we just
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divide that 3.6 the actual amount of water we got the actual number of grams by the molar mass of
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18 grams and that exists 0.2 moles so 3.6 grams of water is 0.2 moles of water.
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You can use the calculation the other way too. If you know how many moles and the molar
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mass of a substance then you can easily calculate the mass. Okay so let's summarize. We said that
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the mole is the international unit for quantity of a substance. We said we need it because atomic
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mass units are tiny and also because we just can't compare mass to mass for different substances
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because different atoms have widely varying relative atomic mass. We said that a mole is a very
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specific number of particles and that number is the alpha-gadron number 6.02 times 10 to the 23.
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Then we talked about how to use calculations. If we know two of these three things the number of
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moles, the molar mass and the mass of the substance present then we can calculate the one that we don't know.
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I hope this has been helpful and I'd love to hear any further questions you've got about moles.
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Best place to contact me if you've got questions is on Instagram and you'll find me at
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chemistry made simple. Thank you for listening to this episode. I hope you found it useful and if
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you have had value from it do consider visiting our Patreon community at patreon.com slash chemistry made
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simple where you'll be able to ask deeper questions about this topic and get more support for your
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studies too. So I'll look forward to speaking to you again in the next episode and until then
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do look after yourself and goodbye.