How To Calculate Molecular Weight and Molar Mass!
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How To Calculate Molecular Weight and Molar Mass!


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

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