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This is a brief how-to video that specifically covers mass to mole calculations. So in these
types of calculations you'll be asked to convert the mass of the substance in grams to moles.
This first slide shows a few steps to remember. First of all, you'll need to know the formula
mass for the element or compound that you're dealing with, and then remember that this
equals one mole of any substance. Next, we'll use dimensional analysis to convert from mass
to moles. Remember with dimensional analysis the set up is like a railroad track. So we
start with what we have, and then same units on the bottom, the new unit on top, and that
will equal the new unit in question. So my units that I start with will cancel. And then
remember we multiply through the numerator of our big fraction and then divide by anything
in the denominator. Lastly, then would be to round to the correct number of significant
digits and then label your answer. So on this slide have two example problems. We're sticking
with sodium chloride and oxygen because those are the substances that we dealt with in our
first video. The first question is asking you to change 12.5 g of sodium chloride to
moles. So that equals how many moles? So if you remember, the first step is to find the
formula mass for sodium chloride. Sodium, having a mass of 22.99. Chlorine, having a
mass of 35.45. Which gives us a formula mass for sodium chloride of 58.44 grams. So now
using the dimensional analysis, I know that I have 12.5 grams of sodium chloride, so that
goes in my starting position. I'll set up my brackets. Grams, because that's the unit
I'm changing from, will go opposite. So my formula mass will go on the bottom, and that's
equal to one mole. Since I'm being asked to change to moles, that will go on top. Go ahead
and cancel grams out because I'm dividing by my denominator. So it ends up being 12.5
times 1, which equals 12.5, divided by 58.44 equals 0.2139 label would be moles. Last step
says to round to significant figures. If you look at my 12.5, that gives me three significant
figures to work with. I'll write that up there so that we know. So my answer, then, could
be rounded to .214 moles. If you wanted to label the NaCl that's a good habit to get
into for some future multi-step problems that we're going to work with. Next problem: 25.45
grams of oxygen equals how many moles? Well, same as before. We need our formula mass,
so remember oxygen has a mass of 16.00 grams, but it is diatomic, so we need to multiply
that by 2 because oxygen exists in pairs. The formula is actually 02. So the formula
mass for oxygen is 32.00 grams. Starting this problem with what I know, with the 25.45 grams
of oxygen, we'll begin my dimensional analysis bracket. Go ahead and put our little railroad
tracks in there, grams will go opposite. So my formula mass (32.00 grams of oxygen) equal
to one mole on top because that is the unit that I am changing to. So 25.45 times 1 equals
25.45 divided by the denominator, 32.00, equals 0.7953. Taking a look at the initial 25.45, I see I have
four sig figs to work with, so I can just leave this as .7953 moles of O2 as my final
answer. Okay, good luck with your practice problems. Please let me know if you have any
questions. We can work through them in class.