Empirical Formula Of A Compound Worksheets Page 2
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6Moles of Al = (mass of Al) / (molar mass of Al) = (0.175 g) / (26.98 g/mol) = 0.00649 mol
Moles of O = (mass of O) / (molar mass of O) = (0.156 g) / (16.00 g/mol) = 0.00975 mol
At this stage, our formula is “
”.
Al
O
0.00649
0.00975
Now we need to convert the subscripts to the simplest whole numbers. Use the following steps to do this:
1) Start by dividing each number of moles by smallest number of moles in the formula. The smallest
number of moles is the moles of Al, 0.00649 mol.
Al
O
Al
O
=
0.00649
0.00975
1.00
1.50
0.00649
0.00649
2) Usually the subscripts at this stage are very near whole numbers, but in this case we still have a
fractional subscript (1.5). To correct this, we will multiply both subscripts by a whole number (2, 3, 4,
etc.) in order to obtain the simplest whole number subscripts. In this example, if we multiply 1.5 by 2, we
will get a whole number, 3 for the O subscript. Therefore, multiplying both subscripts by 2, we obtain,
Al
O
2.00
3.00
Al
O
which gives the empirical formula
.
2
3
You may find it convenient to organize your calculations by arranging both data and results in a table as
follows:
Element
Convert Grams to Moles
Divide Each by the Smallest
Convert to Whole
Number of Moles
Numbers
Al
0.175 g / 26.98 g/mol =
0.00649 mol / 0.00649 mol = 1.00
1.00
2 = 2.00 = 2
X
0.00649 mol
O
0.156 g / 16.00 g/mol =
0.00975 mol / 0.00649 mol = 1.50
1.50
2 = 3.00 = 3
X
0.00975 mol
In this experiment, you will react a known mass of magnesium (Mg) with hydrochloric acid, HCl (aq), to
form a compound containing only the elements Mg and Cl (magnesium chloride). The mass of Cl
reacting with the Mg will be found from the difference in the mass of the product and the mass of Mg
used. By following the same sequence of calculations used in the Al
O
example, you will be able to
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3
experimentally verify the empirical formula of magnesium chloride.
The reaction of magnesium with hydrochloric acid is an example of a single replacement reaction. Can
you write the balanced equation for the reaction using the known formula of magnesium chloride?
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