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Molar Mass - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

The molar mass of a substance is the mass of one mole of that substance. Because of the nature of the mole, the atomic mass of an element in atomic mass units is equal to the molar mass of that substance in grams. Molar mass is useful in finding the number of moles of a substance within a given sample.

We're going to talk about of molar mass and what that means? The molar mass is the mass of 1 mol of part- of particular substance so how does the mass of one atom relate to the mass of mol of atoms? Well they're the same but not really. So if we look at the periodic table, okay and we know these numbers underneath here represent the atomic mass of that particular atom so for example if I'm talking of potassium I know the mass of potassium is the average mass of potassium atom is 39.0983 atomic mass units but Avogadro actually discovered that if he had a mol of that unit of that atom meaning 6.02 times 10 the twenty third particles of potassium, they actually weigh 39.0983 grams that number is actually extremely useful and that that's the mass of one mol of that particular substance aka molar mass.

Alright let's use in calculation for this we see how we can apply this. First there we have 525 grams of calcium atoms, well many mols of calcium atoms do I have? Well lets start out with the given which is 525 grams of calcium okay? Well we're to find the mass of a mol of calcium so let's get back to our periodic table and we say we know that we have 40.078 we're just going to round off to 40 grams for every mol so we have 40 grams for every one mol of calcium. I put this in the bottom so I cross this unit out 525 divided by 40 is going to give us 13.1 mol of calcium okay so I know now that 525 grams of calcium equals 13.1 mols of calcium and that might not seem helpful now but will definitely be helpful when you learn about future things dealing with mols.

Let's do another problem 25.0 grams of silver sorry not silver gold atoms, how many gold atoms is that? Well let's start with what we're given, we have 25.0 grams of gold oops gold atoms, how many atoms is that? Well first we have to figure out the molar mass of gold, looking back at our periodic table and we know that it is 196.9 approximately 197 grams for every one mol of gold and cross this out so 25 divided by 197, actually we're not quite done yet because this actually is in mols we want to get to atoms, we're not actually quite done. Let's continue and say that we have for every one mol, we have 6.02 times 10 to twenty third atoms. So now we have to do 25 divided by 197 times 6.02 times the twenty third atom, let's cross this out, how many gold atoms is that? Well we have 7.65 times 10 to the twenty second atom of gold. That molar mass is very helpful.

Alright let's say instead of atoms we talk about molar masses of compounds because they're actually more typically used in the molar mass of atoms so let's say we have 2.5 mols of glucose or in other words C6H12O6, how much does that weigh? Well we have to figure out the mass of one mol of glucose, how do we do that? So we have to break it up into it's atoms. We have 6 carbon atoms, carbon has a mass of approximately 12 grams per mol from our periodic table we can note that, we have 12 hydrogen atoms hydrogen has approximately a mass of 1 gram for every mol of hydrogen, I'm rounding a lot here just bear with me and then I have 6 oxygen atoms and oxygen has a molar mass of approximately 16 grams per every mol. Okay so 6 times 12 we are going to multiply it out together 6 times 12 is 72 so 72 grams of carbon for every mol of carbon for every mol of glucose I have 12 grams of hydrogen and I have 6 times 16 which is, hold on just a second 96 grams. Alright since they're all together in one compound I'm going to add them all up 72+12+96 is 180 grams per mol so I know now this right here is my molar mass of glucose the whole compound so how can I use this, this is the mass of one mol of glucose but our question asks me 2.5 mols of glucose so what do I do? I do 2.5 mols of glucose C6H12O6, one mol of glucose we just found out is 180 grams, so crossing this out, 2.5 times 180 is 450 grams of glucose so we know that 2.5 mols of glucose is 450 grams of glucose using a molar mass since molar mass is actually very helpful when converting between units.