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How to Calculate Molar Mass

Updated on April 21, 2017

Molar mass is one of the most important aspects of chemistry. You need to know molar mass to figure out stoichiometry, (The ratio of the substances in a reaction) , Molarity (Concentrations and Dilutions), and so much more. Figuring out the molar mass of a molecule is one of the basic backbones of chemistry. Molar mass tells you how many grams of an element or compound there is in one mole of that element or compound. A mole is an amount that is the same as 12g of Carbon 12. This is around 6.022x1023 . In other words, a really really huge number. This number is also known as Avogadro's number. It may not make much sense now, but using the mole as a counting unit really comes in handy later on, when you need to compare different elements and molecules. The mole is the basic counting unit in chemistry, and as noted above, is quite important in the rest of chemistry. Below, I'll show how to find the molar mass of a compound.


Just Elemental!

Finding the molar mass is actually very easy. Let's take a look at sulfur- S. On the periodic table, the atomic weight of sulfur is 32.065. So, for S, the molar mass is 32.065g/mol. For S2 you need to double the atomic weight because the subscript is 2. The subscript is kind of saying that you have 2 atoms of sulfur. So, logically, if you have twice the amount of atoms, the molar mass will be bigger! So the molar mass for S2 is 32.065*2 = 64.13 g/mol. The same goes for any freestanding element. All you need to do is look at the atomic weight on the periodic table, and multiply by whatever the subscript is. C12 The atomic weight of Carbon is 12.011. So we take 12.011 and multiply by the subscript, 12. The molar mass of C12 is then 144.132g/mol.

Compounding the Issue

Now we need to move on to compounds. Lets start with something simple, Hydrochloric acid, or HCl. Now the atomic mass of Hydrogen is 1.008g. The atomic mass of Chlorine is 35.453g. To find the molar mass of HCl is just adding the two together!

1.008g+35.453g= 36.461g. Easy, right? Let's try something a bit harder, H2SO4 . We have two Hydrogen, one Sulfur, and four Oxygen. So we need the atomic weights, times the subscripts, and added together. So,



O-15.999*4=63.996g. Now we add them all up, 2.016+32.065+63.996= 98.077g/mol! Simple? Great!

Finding The Mole.

Now, how can we use the molar mass? Well, we can use it to find out how many moles of a certain substance we might have. Lets take another look at HCl. If you recall, we calculated that the molar mass of HCl was 36.461g/mol. Assume that we are preforming an experiment with 10g of HCl. How would we find out how many moles are in 10g of HCl? We use a conversion factor. Here is how you set it up.

10.00g HCl x  1mol HCL
             36.461g HCL

What this conversion factor is telling us to do, is to divide 10g by 36.461g/mol. If we divide 10 by 36.461, you get .274. This is how many moles of HCl are in 10g of HCl. So in essence, you just divide the amount that you have by the molar mass that you just found! Lets try another one. H2SO4. If you did the quiz above, then you know the molar mass is 98.079. Let's assume we have 37g of H2SO4. So you would set it up as

37g H2SO4 x 1 Mole

We divide 37 by 98.079 to get .377 mol. Using these conversion factors to find the amount of moles you have in a gram is exactly like using them to find how many centimeters go into an amount of meters! Just make sure you set up your conversion factors so that all the units cancel out. The more that you practice at these problems, the easier they get. Take it from me, practice really does help! If you are unsure on how to use the conversion factors, I would recommend getting practice with those before doing molar calculations.

Wrapping Up!

And that is all there is to finding molar mass! Just make sure that you are multiplying just the atomic number and the subscript. If you happen to run across a coefficient, or a number that comes before the elemental symbol, that is only telling you how many moles of that element or molecule you happen to have. 2HCl is only telling you, "Hey, look here, you have two of this!" In regards to the molar mass, just ignore the coefficient. Hopefully, reading this helped you figure out molar mass! If not, just leave a comment below, and let me know what needs to be fixed! Happy calculations!


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