UNIT 1.1 MOLES AND MOLAR MASS
Chemistry is the study of matter and interactions. Chemistry overlaps with many other sciences.
Below is the contents for this sub unit. This will allow you to jump to any section you desire. If you have taken Chemistry 1, this should all be review but it is always good to brush up on the basics (significant figures especially).
MOLES AND AVOGADRO'S NUMBER
So a mole is a unit in chemistry and abbreviated mol (I know it's not much but that's the real abbreviation).
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In chemistry we can measure grams using a balance but not count individual atoms or molecules (they are too small and too many of them). So scientists have come up with a way to convert using dimensional analysis.
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What is a mol? A mol is like a dozen. You know a dozen is 12. Well a mol is just like that but a much larger number.
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Avogadro was a scientist who worked with gasses and learned a lot about the amounts of molecules/atoms. Avogadro's number is the way to convert from moles to particles (atoms or molecules). 6.022 x 10^23 atoms = 1 mol
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MEASURING AND CALCULATING MOLES
To go from grams to mol and mol to grams you are going to need a periodic table. You can find them online easily.
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The mass number for each element tells you the grams/mol. For example hydrogen is 1.008 g/mol
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So how many mol are in 5.0 gram of hydrogen? (use dimensional analysis)
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n = m/M
Number of moles = mass / molar mass
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So we will be taking the mass (grams) and dividing by molar mass but in dimensional analysis that means we just put it on the bottom of the fraction.
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5.0 gram x 1 mol = 4.960 or 5.0 mol with sig figs
1.008 g
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What about the other way? How many grams are in 2.3 mol of oxygen?
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Rearranging the equation is one of the best skills you can have. This time we want to solve for m.
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n = m/M so m = n*M so molar mass goes on top this time
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2.3 mol x 15.999 g = 0.1438 or 0.14 g with sig figs
1 mol
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What about for compounds? How many grams are in 2.0 mol of water?
Water is H2O. This means we add 2 hydrogens and 1 oxygen to get the molecular mass.
So 2 x 1.008 g/mol + 1 x 15.999 g/mol = 18.015 g/mol
2.0 mol x 18.015 g = 36 g with sig figs
1 mol
SCIENTIFIC NOTATION AND CALCULATING PARTICLE
To go from particles to mol and mol to particles. To go from mol to particles you multiply by Avogadro's number. To go from particles to mol you divide by Avogadro's number.
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Here is a good spot to talk about Scientific Notation. So for very large or small numbers it is hard to write them out so we use scientific notation. So Avogadro's number is:
602,200,000,000,000,000,000,000
So we write: 6.022 x 10^23
If you divide Avogadro's number by 10, 23 times you get 6.022
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In scientific notation you will have 1 number before the decimal and all other significant figures behind it.
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What is 25000 in scientific notation?​
So 25000. has the decimal at the end but only the 2 and 5 are significant. Since the decimal has to go after the 2 we will end up by 2.5. So to get the decimal over there we had to divide by 10, 4 times. So 2.5 x 10^4 is the answer. If you plug that back into your calculator, you get the same answer.
Answer: 2.5 x 10^4
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What is 0.000055 in scientific notation?
5.5 x 10^-5
Notice that for small numbers we end up with a negative exponent on the ten. This means we divided by 10, 5 times.
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How many molecules are in 2.5 mol of H2O?​
So we will multiply by Avogadro's number.
2.5 mol x 6.022 x 10^23 molecules/mol = 1.5 x 10^24
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How many moles are in 3.55 x 10^22 atoms of iron?
We divide by Avogadro's number.
3.55 x 10^22 atoms / 6.022 x 10^23 atoms/mol = 0.0590 mol
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Notice that Avogadro's number doesn't change with atoms are mol. It's the same no matter what. Quiz to come tomorrow.
ATOMIC MASS UNIT (AMU)
The atomic mass unit is basically how much mass per mol an atom has. Basically it is the molar mass. So for carbon:
Molar mass = 12.011 g/mol
A carbon atom = 12.011 amu
It's just a unit to make life a little easier for physicists who deal with atoms a lot. You will see this unit throughout the website so don't be afraid of it.
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