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HONORS CHEM. WORKSHEET: Chapter 10, Molecular Formulas – Combustion Problems
Nota Bene: These first two problems are not “complex” and resemble #4 on worksheet “Mind Stretch #1”
1. The formula of boron hydride, compound containing B and H, can be determined by burning a sample in
oxygen and collecting and weighing the combustion products, B2O3 and H2O. Assume a sample of boron
hydride is burned and 1.710 grams of B2O3 and 1.350 grams H2O are obtained. What is the empirical formula
of this compound?
BxHy + O2  B2O3 + H2O
2. Silicon and hydrogen form many compounds, SixHy, to find one you take a 6.22 g sample and burn it in
oxygen. The results produced 12.02 g of SiO2 and 5.40 g of H2O. If the experimental molecular mass is
62.2 g/mol, what is the empirical and molecular formula of this compound?
SixHy + O2  SiO2 + H20
3. Naphthalene is a hydrocarbon that is used for moth balls. A sample was burned in pure oxygen to produce
1.100 g of carbon dioxide and 0.1798 grams of water. If the molecular mass of the compound is 128 g/mole so
what is the empirical formula and molecular formula for this compound? (20 pts)
CxHy + O2  CO2
+ H2O
Nota Bene: These two problems are “complex”
4. Butyric acid, contains C, H, & O, a component of rancid butter, has a vile stench. Burning 0.440 g of butyric
acid in excess oxygen yields 0.882 g of CO2 and 0.360 g of H2O as the only products. What is the empirical
formula and the molecular formula of butyric acid? (molar mass = about 88 g/mol)
CxHyOz
+
O2

CO2
+
H2O
5. Caproic acid, which is responsible for the foul and ripe odor of dirty socks, is composed of C, H, & O atoms.
Combustion with excess oxygen of a 0.225 g sample of this compound produces 0.512 g CO2 and
0.209 g H2O. What is the empirical formula of caprioc acid and what is the molecular formula if the
molar mass is 116 g/mol?
One of the additives in unleaded gasoline that replaced tetraethyl lead in leaded gasoline is called
MTBE. When 15.078 g MTBE is burned completely, 37.640 g CO2 and 18.489 g H2O form. In a
separate experiment the molecular mass of MTBE is found to be 88.150. What is the molecular formula
for MTBE?
To illustrate how empirical and molecular formulas can be determined from data derived from
combustion analysis, lets consider a substance called trioxane. Formaldehyde, CH2O, is unstable as a
pure gas, readily forming a mixture of a substance called trioxane and a polymer called
paraformaldehyde. That is why formaldehyde is dissolved in a solvent, like water, before it is sold and
used. The molecular formula of trioxane, which contains carbon, hydrogen, and oxygen, can be
determined using the data from two different experiments. In the first experiment, 17.471 g of trioxane
is burned in the apparatus shown above, and 10.477 g H2O and 25.612 g CO2 are formed. In the second
experiment, the molecular mass of trioxane is found to be 90.079.
molecular formula
C3H6O3
Dianabol is one of the anabolic steroids that has been used by some athletes to increase the size and
strength of their muscles. It is similar to the male hormone testosterone. Some studies indicate that the
desired effects of the drug are minimal, and the side effects, which include sterility and increased risk of
liver cancer and heart disease, keep most people from using it. The molecular formula of Dianabol,
which consists of carbon, hydrogen, and oxygen, can be determined using the data from two different
experiments. In the first experiment, 14.765 g of Dianabol is burned, and 43.257 g CO2 and 12.395 g
H2O are formed. In the second experiment, the molecular mass of Dianabol is found to be 300.44. What
is the molecular formula for Dianabol?
molecular formula
C20H28O2