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Chemistry – Unit 5 – WS 5
Empirical / Molecular Formulas and Percentage Composition
On a separate sheet of paper , show all your work for the following problems. Box your answer with correct
significant figures. Include UNITS and CHEMICAL FORMULAS in the answer and the work.
Percent Composition Review:
1. Find the percentage composition of a compound that contains 17.6 g of iron and 10.3 g of sulfur. The
total mass of the compound is 27.9 g.
2. Find the percentage composition of a compound that contains 1.94 g of carbon, 0.48 g of hydrogen, and
2.58 g of sulfur in a 5.00 g sample of the compound.
3. Find the percent by mass of oxygen (O) in calcium sulfate, CaSO4, (hint: consider a 1 mole sample of
calcium sulfate) .
4. Find the percentage composition of the compound sodium perchlorate, NaClO4.
Using Composition Data for Empirical and Molecular Formulas:
5. Analysis of a certain compound yielded the following percentages of elements by mass: 43.5% calcium,
26.1% carbon, 30.4% nitrogen. What is the empirical formula for this compound? Draw a particle
diagram representing a formula unit of this compound.
6. Terphthalic acid has a percentage composition of 57.83% carbon, 3.61% hydrogen, and 38.56% oxygen.
The molar mass of this acid is 166 grams. Determine the empirical and molecular formulas.
7. A 1.2569 g sample of a new compound has been analyzed and found to contain the following masses of
elements: carbon, 0.7238 g; hydrogen, 0.07088 g; nitrogen, 0.1407 g; oxygen, 0.3214 g.
a. Calculate the percentage composition for this compound
b. Determine the empirical formula of the compound.
8. The molar mass for caffeine is 194 grams. An analysis of caffeine reveals that a 5.00 gram sample of the
compound contains 2.474 g carbon, 0.2575 g hydrogen, 1.4435 g nitrogen, and 0.825 g oxygen.
Determine the empirical and molecular formula of caffeine.
9. When white phosphorus, P4, is burned in an excess of oxygen, it forms a white powdery compound. A
10.00 gram sample of this oxide was analyzed and found to contain 43.7 % by mass phosphorus. What is
the empirical formula for this compound? If the molar mass for this particular compound is 284 grams,
what is the molecular formula?
10. When metallic chromium is heated in the presence of chlorine gas, a chromium chloride salt results.
Using the following lab data, determine the empirical formula of the compound.
mass of beaker
mass of chromium and beaker before reaction
mass of chromium chloride and beaker after reaction
102.45 g
103.45 g
105.50 g
Bonus: Certain ionic compounds like the copper(II) chloride incorporate water into their crystalline lattice
structure. Another example of this is magnesium sulfate heptahydrate, MgSO4·7H2O, in which each
formula unit of magnesium sulfate is hydrated with seven water molecules. A strikingly beautiful
hydrated copper compound with the common name “blue vitriol” has the following elemental
composition: 25.45% Cu, 12.84% S, 4.036% H, and 57.67% O. Determine the empirical formula of this
hydrated compound.
Modeling Chemistry
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U5 ws 2 v2.0