"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..."
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More info..
How to calculate molar mass
Molar mass calculator
The mole and molar mass