Stoichiometry Chemistry Worksheets With Answers
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11Stoichiometry
Stoichiometry is a way of describing the quantitative relationships among elements in compounds
and among substances as they undergo chemical changes. In plain English, if you have to calculate
just about anything relating to moles or other chemical quantities, the calculations will involve
stoichiometry.
The Mole
In the last chapter, we reviewed the process of balancing equations and based the rules for balancing
equations on the principle that matter is neither created nor destroyed in the course of a chemical
reaction. With this idea still in mind, let’s begin our discussion of moles and formula weights.
When you look at the periodic table, you see that one of the pieces of data given for each element is
its atomic weight. But what exactly is the atomic weight of a substance? It is the mass of one mole of
23
a substance. In turn, one mole of a substance is equal to 6.02
10
atoms or molecules of the
substance (depending on what it is), and finally, the number 6.02
10
23
is known as Avogadro’s
number. For example, carbon’s atomic weight is roughly 12 amu; this means that 6.02
10
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carbon atoms, in a pile, weigh 12 grams.
In order to find the formula weight of a substance, you simply add up the atomic masses of all of
the atoms in the molecular formula of a compound. But don’t forget to multiply the atomic mass of
each element by the subscript behind that element. Formula weights have the units amu, or atomic
mass units; for example, the formula weight of water, H
O, is about 18 amu. (O = 16 amu plus 2
2
times H = 1 amu = 18 amu.) Similarly, the molar mass of a molecule is the mass (in grams) of 1 mol
of a substance; so the molar mass of H
O is also roughly 18.
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Now try calculating some molar masses and formula weights on your own by filling in the following
chart.
Example
Substance
Molar mass
Number of moles
Mass in grams
Number of particles
Carbon dioxide, CO
3.0
2
Oxygen, O
64.0
2
Methane, CH
0.279
4
Nitrogen, N
2 5
2
9.50
10
Explanation
Three significant digits were used throughout, with the exception of molar masses, where two
decimal places were used. But don’t stress over significant figures for this test: it’s multiple choice,
and the answers will never be that precise. Here’s the table, filled in.
Substance
Molar mass
Number of moles
Mass in grams
Number of particles
Carbon dioxide, CO
44.01
3.00
132
2 4
2
1.81
10
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