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 12.0000 by definition (six
neutrons and six protons).
One mole of anything, however, contains 6.0221367 x 1023
of that object. This is known as Avogadro's number.
Examples:
1 mole of carbon = 6.0221367 x 1023 carbon atoms
1 mole of bananas = 6.0221367 x 1023 bananas
Obviously it would be impossible to count out 6.0221367 x 1023
atoms. Remember, however that 1 mole of carbon-12 = 12 grams = 6.0221367
x 1023 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
6.0221367 x 1023 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 6.0221367 x 1023
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 = 6.0221367 x 1023 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 sulphuric acid, H2SO4, as an
example):
Make a list of each element and the number of atoms of each element
present in the substance.
H 2
S 1
O 4
Go to periodic table and determine the atomic mass average (atomic
weight) of each element.
H 1.00794
S 32.066
O 15.9994
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
Add up the results of step three:
2.015+32.066+63.998 = 98.079 = molar mass of sulphuric 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
Molar Mass Calculator
Formulas can have a max of two brackets open at the same time,
and the molecule of crystallization must be placed last.