4.4 Percent Composition
3 Steps to Determining Chemical Formulas
1. ____________________________________________________
2. Convert this information into an empirical formula
3. Find the true number of atoms/ elements in the compound (Molecular Formula)
1. Percent Composition
•
Compounds are ______________ composed of elements in ____________________. This is often
referred to as the ________________________________________.
•
This law states that elements are always present in the ______________________________ in a
compound. I.e. H2O is always 2H:1O any other proportions would not form water. H2O2 is hydrogen
peroxide
•
Since compounds always form in fixed proportions, we can calculate the
______________________________________ of an element in a compound.
% Composition =
Problems
1. Calculatethe percentage composition of C3H7NO2
Step 1: Findthe molar mass of C3H7NO2
Step 2: Findthe % of eachelement by dividing the mass of eachelement by the total mass
2. What is the percentagecomposition of N2O?
Step 1: Findthe molar mass of N2O
Step 2: Findthe % of eachelement by dividing the mass of eachelement by the total mass
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CHEMISTRY 20
4.4 Percent Composition
3. Calculatethe mass of aluminum in 28g of aluminum oxide.
Step 1: Findthe molar mass of aluminum oxide
Step 2: Calculate the % mass of aluminum in aluminum oxide
Step 3: Findthe mass of aluminum in28g of aluminum oxide
Assignment
1. Calculate the percent composition of all of the elements in the following compounds:
a. cupric bromide [Cu=28.45%, Br=71.55%]
b. sodium hydroxide[Na = 57.5%, O= 40%, H= 2.5%]
c. ammonium sulphide [N= 41.1%, H= 11.9%, S= 47%]
d. dinitrogen disulphide[N=30.4%, S=69.6%]
e. hydrobromic acid [H=1.2%, Br=98.8%]
f.
ammonium bromide [N=14.3%, H=4.1%, Br=81.6%]
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4.4 Percent Composition
2. What is the percent composition in a sample that contains 4.5g of magnesium and 6.7g of sodium.
[Mg=40.2%, Na=59.8%]
3. What is the percent composition in a sample that contains 25g of oxygen and 50g of hydrogen.
[O=33.3%, H=66.7%]
4. What is the percentage of carbon in sucrose, C12H22O11? [42.1%]
5. What is the percentage of strontium in SrCl2? [55.3%]
6. How many grams of hydrogen can be obtained from 35mL of water? [3.92g]
7. How much iron can be obtained from 100.0g of Fe3O4? [72.4g]
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4.4 Percent Composition
8. A piece of magnesium reacts with 16.0g of oxygen to form 40.3g of a compound.
a. Calculate the % of magnesium in the compound. [60.3%]
b. Calculate the mass of magnesium that would be present in 10,000g of the compound.
[6.03 x 103g]
9. a. Calculate the % composition of iron(III) oxide.
[69.94% Fe, 30.06% O]
b. Calculate the mass of iron that could be obtained from 20,000g of this compound. [1.399 x 104g]
10. A sample of a compound (with K, Cl and O in it) was placed in a test tube and decomposed by
heating into a residue and oxygen gas. From the given data,
a. calculate the % of oxygen in the compound. [39.2%]
1. mass of empty test tube
2. mass of test tube and compound
3. mass of test tube and residue
= 18.00g
= 24.13g
= 21.73g
11. Indigo, C16H10O2, is the common name of the dye that gives blue jeans their characteristic
colour. Calculate the mass of oxygen in 25.0 g of indigo. [3.42 g]
12. Potassium perchlorate, KClO4, is used extensively in explosives. Calculate the mass of
oxygen in a 24.5 g sample of potassium chlorate. [11.3g]
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CHEMISTRY 20