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In Class Exercise #3
Empirical Formulas, Molecular Formulas, and %
Composition
Atomic Masses:
C 12.011 amu
S 32.066 amu
H 1.0079 amu
As 74.9216 amu
O 15.9994 amu
Percent composition by mass = The percentage of the mass of a compound or
solution represented by each of its constituent element.
Empirical Formula = A formula showing the simplest integral ratio of atoms of
each element in a compound.
Molecular Formula = A formula that expresses the precise number of atoms of
each element in a molecule.
1.
Orpiment is a mineral found near volcanic environments. It is mainly
composed of arsenic and sulfur. It is described as a bright yellow (lemon or yellow
canary) solid. Egyptians used it in artistry and cosmetics despite its toxicity.
Heating 150.0 g of the pure compound to decomposition yields 91.4 g of pure
arsenic; the rest is lost as gaseous sulfur.
a)
How much gaseous sulfur is produced in the decomposition of the 150.0 g of
the arsenic sulfide?
b)
Determine the percentage composition of arsenic in the arsenic sulfide.
c)
Determine the percentage composition of sulfur in the arsenic sulfide.
d)
The empirical formula represents the smallest integer ratio of atoms in a
compound. Determine the empirical formula of the sulfide, AsxSy.
2.
Sucrose is a carbohydrate that is used in beverages. Since the 1970’s it often
has been replaced by fructose which is sweeter. Elemental analysis by burning
100.0 g of sucrose in excess oxygen gas shows the following (unbalanced chemical
equation):
Sucrose + O2(g)  CO2(g) + H2O(g)
Burning 100.0 g sucrose produces 154.3 g of CO2 and 57.9 g of water.
a)
Calculate the number of moles of CO2 produced by the combustion of
sucrose.
b)
Calculate the number of moles of C atoms from the moles of CO2.
c)
Calculate the moles of H2O produced by the combustion of sucrose.
d)
Calculate the moles of hydrogen atom from the moles of H2O.
e)
Determine the moles of oxygen present in the 100.0 g of sucrose. (Note:
The mass of sucrose equals the mass of carbon, hydrogen and oxygen.)
f)
Determine the empirical formula of sucrose, CxHyOz.
g)
Soft-ionization mass spectrometry of sucrose reveals a large peak at 342
amu. What is the molecular formula of sucrose?
h)
Write a balanced equation for the combustion of sucrose.
2
Limiting Reagent = The reactant in a chemical reaction that limits the maximum
amount of product that can be formed.
i)
If one reacted 1.00 moles of sucrose to completion with O2, how many
moles of O2 would be required?
j)
If one reacted 1.00 moles of O2 to completion with sucrose, how many
moles of sucrose would be required?
k)
If one reacted 1.00 moles of O2 and 1.00 moles of sucrose, would there be
any unreacted material left? Which one?
l)
From part (k) what is the limiting reagent?
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