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This is a brief tutorial about converting from moles of a substance to mass in grams.
Just a reminder of some common steps that we'll use. First of all, you will need to
find the formula mass for the element or compound in question, and please remember that this
is equal to one mole of that substance. We will use dimensional analysis again to convert
from moles to grams. The last is a reminder to round to the correct significant figures
and don't forget to label your answer. Some examples of converting moles to grams. In
this first equation, we're given 2.75 moles of sodium chloride, and I need to know how
many grams of sodium chloride that is. So from previous calculations, we know that the
formula mass of (I'll abbreviate that with an "FM") for NaCl is equal to 58.44 grams.
So now my new starting point for my dimensional analysis this time is 2.75 moles. So that
will go at the beginning point of my conversion. Now the mole will need to go opposite, so
on the bottom in the denominator of my large fraction. So one mole will go there so that
it can cancel, and the formula mass will go on top because that is the new unit I'm trying
to convert to. So my 58.44 grams will go in the numerator. Same math as before. Multiplying
my 2.75 moles times my 58.44 grams is 160.71 divided by one, gives me an answer of 160.71
grams. Take a look at significant figures: My 2.75 starts with 3 sig figs, so my answer
needs to be rounded to three significant figures, meaning the 160 would actually be rounded
to 161 because of the seven so the answer is 161 grams of NaCl. Down to problem #2:
15.40 moles of oxygen is my starting point, and I'm wondering how many grams of oxygen
then that is equal to. Formula mass (F.M.) remember is 02, is equal to 32.00 grams. So
let's go ahead and start this dimensional analysis problem with my beginning, which
is 15.40 moles of 02. My one mole will go on the bottom, and the 32.00 grams will go on the top because
that is the formula mass which is equal to one mole. And that's where my unit of equality
goes, so please remember how to use those. So the 15.40 times 32.00 will give me 494.08.
Divide that by anything that's in the denominator, so 494.08 divided by 1 is 494.08. Looking
back at the 15.44 that I started with, gives me 4 significant figures, so my answer should
be rounded to four , which means we'll keep the 494, the 8 will round the 0 up to 1. So
my final answer is 494.1 grams of O2. Okay, once again, good luck with your practice problems.
Please ask if you have any questions.