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The Mole & Molar Mass
In order to grasp the concept of molar mass calculations it is important to
understand the molar unit. The mole also called mol is the basic unit of
measurement in chemistry. By definition, in modern chemistry, one mole
represents the number of carbon atoms in exactly 12 grams of carbon 12
isotope. Remember that carbon-12 has an atomic mass of 12 (six neutrons
and six protons).
One mole of anything, however, contains 6.0221367E23 of that object. This
is known as Avogadro's number.
Examples:
1 mole of carbon = 6.0221367E23 carbon atoms
1 mole of bananas = 6.0221367E23 bananas
(Aren't you glad they don't sell bananas by the mole?)
obviously it would be impossible to count out 6.0221367E23 atoms.
Remember, however that 1 mole of carbon-12 = 12 grams = 6.0221367E23
atoms. It has been established that 1 mole of any element = the atomic mass
of that element expressed in grams. Since magnesium has an atomic mass of
24, one mole of magnesium weighs 24 grams and contains 6.0221367E23
atoms of magnesium. A mole of any molecule = the molecular mass of that
molecule expressed in grams. In order to determine the weight of one mole
of bananas, one would have to get an average weight of a banana and
multiply that by 6.0221367E23 then we could weigh out that weight of
bananas and presto, we would have a mole of bananas. Of course, nobody
would ever do that. It just demonstrates that mole of anything =
6.0221367E23 and we can measure out a mole of something by counting it or
by weighing it out. Since atoms are too small to count, we must weigh out a
mole of atoms.
Molar massis a unit that enables scientists to calculate the weight of any
chemical substance, be it an element or a compound. Molar mass is the sum
of all of the atomic masses in a formula. Once one determines the molar
mass of a substance, it will be easy to measure out one mole of that
substance.
The molar mass calculation of a substance is complete the following steps
(We will use sulfuric acid, H2SO4, as an example):
1. Make a list of each element and the number of atoms of each element
present in the substance.
H2
S1
O4
2. Go to periodic table and determine the atomic mass average (atomic
weight) of each element.
H 1.00794
S 32.066
O 15.9994
3. Multiply each atomic mass by the number of atoms in the formula.
H 1.00794 * 2 = 2.015
S 32.066 * 1 = 32.066
O 15.9994 * 4 = 63.998
4. Add up the results of step three:
2.015+32.066+63.998 = 98.079 = molar mass of sulfuric acid
In this example the results have been rounded off to the correct number of
decimal places. (Since the atomic mass average of sulfur given above only
has 3 decimal places, accuracy can not be determined beyond that point).
These calculations will be necessary before one can determine the molarity
or normality of a solution and many other formulas in stoichiometry (the
quantitative relationships between chemical substances in a chemical
equation).
Try these formulas and then compare your results to those you get on the
calculator below:
H2O
NH3
C2H4O2
K2Cr2O7
Formulas can have a max of two brackets open at the same time, and the
molecule of crystallization must be placed last.
Examples:
3BeO.Al2O3.6(SiO2) = (BeO)3.Al2O3.6(SiO2)
(BaH2O)2Mn5O10 = (BaH2O)2Mn5O10
(KMgCl3).6H2O = (KMgCl3).6H2O
What is molar mass?
Molar mass is the weight of one mole (or 6.02 x 1023 molecules) of any
chemical compounds. Molar masses of common chemical compounds that
you might find in the chemistry laboratory can range between 18
grams/mole for compounds like water to hundreds of grams per mole for
more complex chemical compounds.
The lightest possible chemical that one can have under normal conditions is
hydrogen gas, or H2. There is no limit to how heavy a chemical compound
can be - it is not uncommon for macromolecules (large organic or
bioorganic compounds such as DNA) to weigh thousands of grams per mole.
How can I find the molar mass of an element?
The molar mass of elements is found by looking at the atomic mass of the
element on the periodic table. For example, if you want to find the molar
mass of carbon, you would find the atomic mass of carbon on the periodic
table, and this is equal to the molar mass in grams per mole. So, in our
example, carbon has a molar mass of 12.01 grams per mole.
There are a few exceptions to this rule. In some cases, the element is usually
found in a different form than just one unbonded atom. In the case of
hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine, the
element is diatomic, meaning that each molecule of the element has two
atoms of that element stuck together. As a result, the formula of hydrogen is
H2, nitrogen is N2, etc.
This gets weirder for a couple of cases... phosphorus is normally found in
clumps of four atoms, P4, and sulfur is found in clumps of eight atoms, or S8.
Still, aside from the exceptions above, all elements have the same molar
mass as the atomic masses on the periodic table.
How can I find the molar mass of a chemical compound?
For any chemical compound that's not an element, we need to find the molar
mass from the chemical formula. To do this, we need to remember a few
rules:
1. Molar masses of chemical compounds are equal to the sums of the
molar masses of all the atoms in one molecule of that compound. If we
have a chemical compound like NaCl, the molar mass will be equal to the
molar mass of one atom of sodium plus the molar mass of one atom of
chlorine. If we write this as a calculation, it looks like this:
(1 atom x 23 grams/mole Na) + (1 atom x 35.5 grams/mole Cl) = 58.5
grams/mole NaCl
2. If you have a subscript in a chemical formula, then you multiply the
number of atoms of anything next to that subscript by the number of
the subscript. For most compounds, this is easy. For example, in iron (II)
chloride, or FeCl2, you have one atom of iron and two atoms of chlorine.
The molar mass will be equal to (1 atom x 56 grams/mole Fe) + (2 atoms x
35.5 grams/mole of chlorine) = 127 grams/mole of iron (II) chloride.
For other compounds, this might get a little bit more complicated. For
example, take the example of zinc nitrate, or Zn(NO3)2. In this compound,
we have one atom of zinc, two atoms of nitrogen (one atom inside the
brackets multiplied by the subscript two) and six atoms of oxygen (three
atoms in the brackets multiplied by the subscript two). The molar mass of
zinc nitrate will be equal to (1 atom x 65 grams/mole of zinc) + (two atoms x
14 grams/mole of nitrogen) + (six atoms x 16 grams/mole of oxygen) = 189
grams/mole of zinc nitrate.
For all other compounds, the general idea is the same. Basically, you should
know how to find the molar masses of any chemical compound now. In the
next and final section, I'll give you some practice problems, followed by a
solution key...
Some sample problems:
These are the kind of molar mass calculation probems I might ask you on a
quiz. The solutions are given at the end. Give the molar masses of the
following compounds:
1. sodium fluoride
2. potassium hydroxide
3. copper (I) chloride
4. manganese (IV) oxide
5. calcium sulfate
6. magnesium phosphate
Solutions:
1. In sodium fluoride, there is one atom of sodium and one atom of fluorine.
The molar mass will then be:
(1 atom x 23 grams/mole of sodium) + (1 atom x 19 grams/mole of fluorine)
= 42 grams/mole of sodium fluoride
2. In potassium hydroxide, there is one atom of potassium, one atom of
hydrogen, and one atom of oxygen. The molar mass will then be (1 x 39
grams) + (1 x 1 gram) + (1 x 16 grams) = 56 grams/mole of potassium
hydroxide
3. In copper (I) chloride, there is one atom of copper and one atom of
chlorine. The molar mass is then (1 x 63.5 grams) + (1 x 35.5 grams) = 99
grams/mole of copper (I) chloride
4. In manganese (IV) oxide, there is one atom of manganese and two atoms
of oxygen. The molar mass is then (1 x 55 grams) + (2 x 16 grams) = 87
grams/mole of manganese (IV) oxide
5. In calcium sulfate, there is one atom of calcium, one atom of sulfur, and
four atoms of oxygen. The molar mass is then (1 x 40 grams) + (1 x 32
grams) + (4 x 16 grams) = 136 grams/mole of calcium sulfate
6. In magnesium phosphate, there are three atoms of magnesium, two atoms
of phosphorus, and eight atoms of oxygen. (The formula is Mg3(PO4)2). The
molar mass is then (3 x 24 grams) + (2 x 31 grams) + (8 x 16 grams) = 262
grams/mole of magnesium phosphate