In this video I’m going to go over how

to calculate molecular weight and molar mass and so the first thing is that

molecular weight and molar mass are found the same way the only difference

is the units you use at the end of the problem and so the formula for both of

these is you could take the atomic weight of an element and then you

multiply that by how many you have in that molecule and we have to do this for

every element and then we add them all together and so let’s start off with a

basic example of just h2o of water and so we’re gonna need the atomic weight of

both of these so hydrogen and oxygen so we get that from the periodic table

and so just pulling in these two squares and remembering that the atomic weight

is our number down here and just a quick overview of atomic weight this is

basically the average of what this element weighs in the universe so we’re

not saying that every oxygen weighs fifteen point nine ninety nine but we’re

saying that on average that’s what oxygen weighs and that’s because there’s

different isotopes there’s oxygens out there that way a little bit more a

little bit less than 15.99 and they all average out to this alright and so now

here’s how I like to set up my calculations so basically I have my

formula for each element and now I’m going to do them all and add them

together so the atomic weight of hydrogen we just take that from here one

point zero seven eight and then we use our subscript this two is referring to

hydrogen’s so it’s saying that water has two hydrogen’s and so multiplying

together we get two point zero one five six alright so now for oxygen atomic

weight fifteen point nine nine nine we don’t see any subscript here which means

it’s an implied one so our subtotal here is just fifteen point nine nine nine and

now we just add them together so we get eighteen point oh one five and it’s up

to you how many sig figs you have to use that really depends on a per problem

basis and so now we get into the difference between molecular weight and

molar mass so the number four both of them is going to be 18 point oh

one five but for molecular weight we’d say our answer is amu and this stands

for atomic mass unit and for molar mass we’d say our units is grams per mole so

grams a little Division sign and then mole and so in your class your teacher

is gonna go over both these concepts and when you need to use which one but the

number will always be the same the only difference is the units and the units of

course mean different things but in terms of calculating it which is what

this video is focused on it’s the same number you find it the same way all

right now let’s do another example it’s a little bit more complicated so what’s

the molecular weight / molar mass of aluminum sulfate and so for this we have

to have the molecular formula of aluminum sulfate which is al2 so4 3 and

so now we have to really figure out okay how many of each of our element do we

have that’s gonna be really important for all these problems this one’s a

little bit more complicated so our subscript of 2 for aluminum is here so

we just have these two aluminum’s and so for a sulfur we have to look at these

parenthesis and what these parenthesis mean is that we actually have three so4

units so when I look at it I see this I see okay that parenthesis means that we

have three little chunks of so4 and so now after we’ve drawn it out we can see

okay I have one two three sulfurs the other way to do that is just to do the

multiplying subscripts route so we have our secret invisible 1 subscript of

sulfur which is right there and then we have some have to multiply that by the 3

so overall we could have also gotten to 3 by just saying 1 times 3 because those

were the two subscripts that apply it to sulfur all right and so for oxygen again

looking out and saying well we have this 4 here but we also have this 3 outside

the parenthesis so what we really have is back to our three little chunks of

so4 we have 4 plus 4 plus 4 so that’s going to be 2 12 and the other way we

can do that is again multiplying the subscripts so I could have just said

well 4 applies to oxygen right here and three also applies to oxygen right

here so I can just multiply four times three and also get to twelve all right

so now that I have that I’m going to need the atomic weights so just pulling

those three periodic table boxes right here and now I’m ready to go ahead and

do my math and add everything up so aluminum twenty six point nine eight two

and I have two of them multiplying them together I get fifty three point nine

six four and so next sulfur thirty-two point O six five I have three of them so

multiplying together I have ninety six point one nine five and last but not

least oxygen fifteen point nine nine nine I have twelve of them multiplying

together I have 191 point nine eight eight so now just adding those all

together I chose to use five sig figs I have three hundred and forty two point

one five and now let’s again do our breakdown of molecular weight slash

molar mass so it’s gonna be the same number for both of them if we’re saying

molecular weight that’s going to be an atomic mass units am use and if we’re

saying molar mass we’re gonna do grams over moles all right so that’s how you

can calculate any molecular weight or any molar mass that’s all this same

formula hope this was helpful and happy studying hey I hope you liked that video

please feel free to subscribe click around and watch my other tutorials and

as always happy studying

Thank you!!

Is avogadro's number the same for a compound such as Na2Cr2O7 Sodium dichromate?

Very WEll!

Your voice is similar to the Ted Ed

Thanks by the way