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Problem Set 12.1
Percent composition of a hydrocarbon is 85.6% C and 14.5% H. Calculate the empirical
formula.
A 1.00 g sample of the hydrocarbon at a temperature of 273 K and a pressure of 1.0x10 5
Pa (1.00 atm) has a volume of 0.399 dm3. Calculate the molar mass
Using 1 and 2, deduce the molecular formula.
Explain why incomplete combustion is harmful to humans
Give the structural formulas for the isomers of C4H10 and state the name of each one.
Name the compound C2H4 and draw its structural formula
What is the IUPAC name for CH3CH2CH(CH3)2 ?
Which statement about neighboring members of all homologous series is correct?
a. They have the same empirical formula.
b. They differ by a CH2group.
c. They possess different functional groups.
d. They differ in their degree of unsaturation.
When one mole of ethene reacts with two moles of oxygen gas
a. ΔH is positive.
b. the oxidation number of carbon is unchanged.
c. an alcohol is formed.
d. carbon monoxide is produced.
Which formula is a correct representation of pentane?
a. CH3CH2CH2CH2CH3
b. CH3(CH2)2CH3
c. CH3(CH2)3CH3
d. CH3(CH3)3CH3
Which is a correct definition of the term empirical formula?
a. formula showing the numbers of atoms present in a compound
b. formula showing the numbers of elements present in a compound
c. formula showing the actual numbers of atoms of each element in a compound
d. formula showing the simplest ratio of numbers of atoms of each element in a
compound
The reaction of ethanal and oxygen can be represented by the unbalanced equation
below.
__ CH3CHO __ O2 + → __ CO2 + __ H2O
When the equation is balanced using the smallest possible integers, what is the
coefficient for O2 ?
The equation for the complete combustion of butane is
2C4H10 +13O2→8CO2 +10H2O
What is the amount (in mol) of carbon dioxide formed by the complete combustion of
three moles of butane?
How many hydrogen atoms are contained in one mole of ethanol,C2H5OH?
The percentage by mass of the elements in a compound is 72% Carbon, 12%H and 16%
Oxygen. What is the mole ratio of C:H in the empirical formula of this compound.
Write a balanced combustion reaction for propane
Calculate the heat of combustion for propane using bond enthalpies and compare to
the heat of combustion listed in your data booklet.
Is the combustion of propane exothermic or endothermic?
A compound that contains only carbon, hydrogen, and oxygen has the following
percentage by mass: carbon 60%, hydrogen 8%, oxygen 32%. What is the empirical
formula?
State and explain the trend in boiling point for the alkanes homologous series.
Problem Set 12.1
1. Percent composition of a hydrocarbon is 85.6% C and 14.5% H. Calculate the empirical
formula.
 Assume you have a 100 g sample of the hydrocarbon. Therefore you have
 85.6 g of C and 14.5 g of Hydrogen.
 Find the moles 85.6/12.01 = 7 mol C and 14.5/1.01 = 14 mol H
 Mole ratio of C:H is 7:14 or 1:2
 Formula is CH2
2. A 1.00 g sample of the hydrocarbon at a temperature of 273 K and a pressure of 1.0x10 5
Pa (1.00 atm) has a volume of 0.399 dm3. Calculate the molar mass
Use PV = nRT  (1.00 atm) (0.399 dm3) = (1.00g/X)(0.821 L-atm/mol-K)(273)
X = 56 g/mol
3. Using 1 and 2, deduce the molecular formula.
The molar mass of CH2 = 14g/mol so the molecular formula is 56/14 = 4 times as big
Molecular formula = C4H8
4. Explain why incomplete combustion is harmful to humans
a. Incomplete combustion forms carbon, carbon monoxide, and other
hydrocarbons that didn’t completely burn
b. CO is toxic for humans b/c it binds to hemoglobin in blood like oxygen – but
unlike oxygen, it is never released and can cause suffocation if enough sites on
the hemoglobin molecule are blocked.
c. Unburned hydrocarbons react with other molecules in the air (like NO2) and
make ozone which is also toxic for humans to breath.
d. Particulates in the air from unburned hydrocarbons can also impair breathing
ability.
5. Give the structural formulas for the isomers of C4H10 and state the name of each one.
Butane and 2-methylpropane
6. Name the compound C2H4 and draw its structural formula
Ethane CH2=CH2
7. What is the IUPAC name for CH3CH2CH(CH3)2 ?
2-methylbutane
8. Which statement about neighboring members of all homologous series is correct?
a. They have the same empirical formula.
b. They differ by a CH2 group.
c. They possess different functional groups.
d. They differ in their degree of unsaturation.
9. When one mole of ethene reacts with two moles of oxygen gas
a. ΔH is positive.
b. the oxidation number of carbon is unchanged.
c. an alcohol is formed.
d. carbon monoxide is produced.
In combustion reactions, the oxidation number always changes, heat is always given off
(exothermic, ΔH is negative), and CO, CO2, and H2O are possible products – not alcohols which
are very flammable and would therefore burn! CO is produced only during incomplete
combustion and that is what is happening here because ethane is only reacting with 2 moles of
O2 instead of 3. How do you know ethane is supposed to react with 3 moles of O2? Write a
balanced reaction…
Complete combustion C2H4 + O2  CO2 + H2O …to balance, the coefficient on O2 must be 3
Incomplete combustion C2H4 + O2  CO + H2O …to balance, the coefficient on O2 is 2
10. Which formula is a correct representation of pentane?
a. CH3CH2CH2CH2CH3
b. CH3(CH2)2CH3
c. CH3(CH2)3CH3
d. CH3(CH3)3CH3
11. Which is a correct definition of the term empirical formula?
a. formula showing the numbers of atoms present in a compound
b. formula showing the numbers of elements present in a compound
c. formula showing the actual numbers of atoms of each element in a compound
d. formula showing the simplest ratio of numbers of atoms of each element in a
compound
12. The reaction of ethanal and oxygen can be represented by the unbalanced equation
below.
_2_ CH3CHO _5_ O2 + → _4_ CO2 + _4_ H2O
When the equation is balanced using the smallest possible integers, what is the
coefficient for O2 ?
13. The equation for the complete combustion of butane is
2C4H10 +13O2→8CO2 +10H2O
What is the amount (in mol) of carbon dioxide formed by the complete combustion of
three moles of butane? 12 moles
14. How many hydrogen atoms are contained in one mole of ethanol,C2H5OH?
6 H atoms * 6.022 x 1023 atoms/mol = 3.6 x 1024 atoms
15. The percentage by mass of the elements in a compound is 72% Carbon, 12%H and 16%
Oxygen. What is the mole ratio of C:H in the empirical formula of this compound.
See number 1 for method 72/12 = 6, 12/1 = 12 and 16/16=1 so ratio is 1C: 2H: 1O
Answer: 1:2
16. Write a balanced combustion reaction for propane
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
Don’t forget state symbols!
17. Calculate the heat of combustion for propane using bond enthalpies and compare to
the heat of combustion listed in your data booklet.
Using previous reaction
Bonds Broken: 8 (C-H) + 2 (C-C) + 5 (O=O) = 6472 kJ
Bonds Made: 6 (C=O) + 8 (O-H) = 8162 kJ
6472-8162 = -1690 kJ which is pretty far off from data booklet’s 2200 kJ. Perhaps this is
because CO2 actually has a higher bond enthalpy of 799 kJ instead of 743 kJ. If you use 799 kJ,
the heat of combustion comes out to be -2026 kJ which is closer. Remember that bond
enthalpies are averages so the heat of reactions are only going to be valid to one significant
figure.
18. Is the combustion of propane exothermic or endothermic?
Since change in heat is negative, reaction is exothermic.
All combustion reactions are exothermic
19. A compound that contains only carbon, hydrogen, and oxygen has the following
percentage by mass: carbon 60%, hydrogen 8%, oxygen 32%. What is the empirical
formula?
See number 1 for method… 60/12 = 5 mol C 8/1 = 8 mol H 32/16 = 2 mol H
Empirical formula: C5H8O
20. State and explain the trend in boiling point for the alkanes homologous series.
The boiling point increases as the alkane chain gets longer because the attraction between
molecules increases with size due to increased dispersion forces. Also, boiling point
increases with mass.