Name: __________________________ Class: ____________________ Date: _____________
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Empirical Formula Introduction When a chemist is studying an unknown substance, it is possible to do a chemical analysis determining
the percent composition of each element that makes up the substance. This percent composition can then
be used to find the actual empirical formula of the compound. The empirical formula is the simplest
possible ratio of elements within a compound.
For example, let’s say that an unknown compound is analyzed and determined to have the following
components: 32.38% sodium, 22.65% sulfur, and 44.99% oxygen.
The first thing we should do is assume we have exactly 100g of the substance. This would mean that we
have 32.38g sodium, 22.65g sulfur, and 44.99g of oxygen. In order to figure out the empirical formula,
the first step is actually to convert the mass of each element into moles.
32.38g Na x
=
22.26g S x
=
44.99g O x
=
Divide each mole value by the smallest number of moles calculated. In this case, we have the least
number of moles of sulfur. Round to the nearest whole number.
____mol O
_____mol S⁄ =
____mol Na
_____mol S⁄ =
____mol S
_____mol S⁄ =
This is the mole ratio of the elements, and is used to write the actual empirical formula of the compound.
What is the empirical formula of this compound?
What is the name of this compound?
Practice Convert each of the following. Write out the full conversion factor.
1. An unknown compound is found to be 72.2% magnesium and 27.8% nitrogen by mass. Find the
empirical formula and name the compound.
2. Glucose is a simple sugar that is 40% carbon, 6.7% hydrogen, and 53.3% oxygen. What is its
empirical formula?
3. Find the empirical formula of a compound found to contain 26.56% potassium, 35.41% chromium,
and the remainder oxygen. Once you find the compound. Name the compound.
4. Analysis of 20.0 g of a compound shows that 16.0g of bromine and 4.00 g of calcium are present.
What is the empirical formula of the compound formed? Name the compound.
5. An acid extracted from a soft drink contains 0.3086g of hydrogen, 3.161g of phosphorus, and 6.531g
of oxygen.
Name: __________________________ Class: ____________________ Date: _____________
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..
Molecular Formula
Introduction Many compounds do not naturally exist as their simplest empirical formula. Glucose, for example, has an
empirical formula of CH2O. However, it actually exists as a multiple of this ratio, called the molecular
formula.
The molecular formula can be calculated by comparing the molar mass of the empirical formula to that of
the actual compound.
The molar mass of glucose’s empirical formula, CH2O, is…
The actual molar mass of glucose is 180g/mol. Divide this by the empirical molar mass, and you should
get a whole number.
180g/mol ÷ 30g/mol = 6
Multiply each element in the empirical formula by this number. Re-write the formula, and you have the
molecular formula.
The molecular formula of glucose is __________________.
Examples
A compound has an empirical formula of NO2 and a molar mass of 92g/mol.
A compound has an empirical formula of C2H3O and a molar mass of 172g/mol.
Ibuprofen has an empirical formula of C7H9O and a molar mass of 215g/mol.
Practice Find the empirical formula of each compound. Use this empirical formula to then calculate the molecular
formula.
1. Nicotine is 74.1% carbon, 17.3% nitrogen, and 8.6% hydrogen by mass. It has a molar mass of
160g/mol.
2. Adrenaline is a hormone in the body. It is 59.0% carbon, 7.1% hydrogen, 26.2% oxygen, and
7.7% nitrogen. It has a molar mass of 180g/mol.
3. A compound consists of 91.63 grams of carbon, 7.69 grams of hydrogen and 40.81 grams of
oxygen. Its molar mass is 220g/mol. Determine the empirical and molecular formulas.
4. A compound has a molar mass of approximately 90 g/mol. Analysis reveals that a sample
contains 16.10 g carbon, 3.36g hydrogen and 10.71 grams of oxygen. Determine the empirical
and molecular formula:
5. Hydrogenated fats are produced by passing hydrogen gas over long saturated carbon chains. A
biochemist starts with 211.4 grams of pure carbon. After hydrogenation, the sample weighs 249.4
grams. The molecular mass of the product is roughly 100 AMU. Determine the empirical and
molecular formulas.
Name: __________________________________ Period: ____________________ Date: _____________
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Chemistry Study Guide Unit 4 - Moles
Vocabulary These are new terms that you are likely to see on the test. Briefly define each in your own words.
1. Mole –
2. Avagadro’s Number –
3. Molar Mass –
4. Empirical Formula –
5. Molecular Formula –
Critical Thinking Be able to read, analyze, and give complete answers to questions like these.
1. Record each of these measurements in scientific notation.
a. 300,000,000m/s (Speed of light)
b. 2,500,000,000,000 rubles (Cost of World War 2 to the Soviet Union)
c. 2,028,000,000m (Length of the Great Barrier Reef)
d. 0.001s (Shutter speed of a camera)
e. 0.000000000000002g (mass of a bacterium)
2. Calculate the molar mass of each compound:
a. Oxygen gas
b. Aluminum bromide
c. Iron (III) oxide
d. Diphosphorus tetraoxide
e. Calcium nitrate
3. Complete each of the following one-step conversions. Show all work for full credit.
a. 30.0L of nitrogen gas to moles.
b. 105.0g of barium iodide
c. What would be the mass of 4.8mol of water?
d. 9.01x1023 molecules of ammonium chloride.
e. How many liters of helium gas are there in 0.78mol?
f. How many atoms of gold would be present in exactly 1.5mol?
4. Complete each of the following two-step conversions.
a. What would be the volume of 7.28x1024 atoms of neon gas at STP?
b. What would the mass be of a single atom of copper?
c. A bottle contains 75.1g of silver (I) chloride. How many molecules are present in the bottle?
d. A balloon contains 0.5L of sulfur hexafluoride gas. What is its mass?
5. Write the formula, molar mass, and percent composition of each element within these compounds:
6. An unknown compound was found to have a percent composition as follows: 47.0 % potassium, 14.5 %
carbon, and 38.5 % oxygen. What is its empirical formula? If the true molar mass of the compound is
166.22 g/mol, what is its molecular formula?
7. Find the molecular formula of each compound.
a. A compound has an empirical formula of C2OH4 and a molar mass of 88 grams per mole. What is the
molecular formula of this compound?
b. A compound has an empirical formula of C4H4O and a molar mass of 136 grams per mole. What is
the molecular formula of this compound?
c. A compound has an empirical formula of CFBrO and a molar mass of 254.7 grams per mole. What is
the molecular formula of this compound?
d. A compound has an empirical formula of C2H8N and a molar mass of 46 grams per mole. What is the
molecular formula of this compound?
e. A well-known reagent in analytical chemistry, dimethylglyoxime, has the empirical formula C2H4NO.
If its molar mass is 116.1 g/mol, what is the molecular formula of the compound?
