Hw ISSUES:
P165 # 5-8
RECALL
percentage composition
-the percentage, by mass, of each element in the compound
- will always be the same, regardless of the actual mass of sample
ex. mH = mH/mcompound x 100%
ex. Calculate the % composition of CO2
Molar mass of each element/molar mass of compound x 100 %
Mc/Mco2
or
Mo2 / Mco2
ex. Determine the % composition by mass of caffeine, C8H10N4O2
DETERMINING THE MOLECULAR FORMULA
molecular formula
-a formula that indicates the actual numbers of atoms in one molecule of a
compound ex. HO empirical: H2O2 molecular
mass spectrometer --> gives molar mass
- measures the molar mass of a compound
o vaporizes the sample
o knocks out electrons to ionize the sample
o uses a magnetic field to deflect the cations
o since deflection depends on mass and charge allows for
determination of molar mass
example 1
-given that the empirical formula of a compound is CH3O and its molar mass
(mass spec) is 93.12 g/mol what is the molecular formula of the compound?
Mcompound = 93.12 g/mol
Mempirical formula = 31.04 g/mol
find: ratio molecular M to empirical formula M
M compound/ Mempirical formula = 3
molecular formula = CH3O x 3
=C3H9O3 (some type of an ester)
example 2
A compound's percentage composition by mass is 49.5% of C, 5.2 % H, 28.8%N
and 16.5% O. What is its molecular formula if its molar mass is 194.2 g/ mol?
HW.
p.165 #5, (emp)
# 6, 7, 8 (molecular formula)
Calculate the % of water in popcorn
Could we do this with hydrates?
Ex. Calculate the formula of a hydrate: a formal lab
Hydrates:
- compounds that contain molecules of water within the crystal structure
- when you heat them a vapour comes off: when you
condense it it ends up being pure water
- ex. Magnesium sulphate heptahydrate: cook off the water to get
magnesium sulphate
ex. A 1.2698 g sample of an unknown organic compound containing only carbon,
hydrogen and oxygen was burned completely to produce 3.206 g of carbon
dioxide and 0.5620g of water.
Find the percentage by mass of carbon, hydrogen and oxygen in the compound.
1 mol CO2 : 44.01g CO2
? mol CO2 : 3.206g CO2
3.206
44.01
n(CO2) = 1 x
= 0.072847 mol
n(C)
= 0.072847 mol
1 mol C : 12.01 g C
0.072847 mol C :
m(C) = 12.01 x
?gC
0.072847
1
= 0.87489 g
0.87489 g C : 1.2698 g compound
?gC:
m(C) =0.87489 x
100 g compound
100
1.2698
=68.9000 g
1 mol H2O : 18.016 g H2O
? mol H2O : 0.5620 g H2O
n(H2O) = 1 x
0.5620
18.016
= 0.031194 mol
n(H) = 2 x 0.031194
= 0.062389 mol
1 mol H : 1.008 g H
0.062389 mol H :
m(H) =1.008 x
?gH
0.062389
1
= 0.062888 g
0.062888 g H : 1.2698 g compound
?gH:
100 g compound
m(H) = 0.062888 x
100
1.2698
= 4.9526 g
m(O) = 100 g - 68.9000 g - 4.9526 g
= 26.1474 g
In 100 g of the compound there are 68.90 g C, 4.953 g H and 26.15 g O.
There is 68.90 % C, 4.953 % H and 26.15 % O
PORTFOLIO:
1. Tests show that aspirin has 4.48%H, 35.52%O and the remainder carbon. If the molar
mass of aspirin is 180.2gmol-1 find the empirical and molecular formulae of aspirin.
2. Ascorbic acid has a molar mass 176.1gmol-1 and it is found that it contains 4.58%H,
54.50%O with the remainder being carbon. Find the empirical and molecular formulae
ascorbic acid.
3. It has been experimentally determined that succinic acid has a molar mass 118.1gmol-1
and that it is 40.68% carbon, 5.12% hydrogen with the remainder being oxygen. What
are the empirical and molecular formulae succinic acid?
4. Capsaicin has been shown to have a molar mass of 305.4gmol-1 and to consist of
70.79%C, 8.91%H, 4.59%N and 15.71%O. Find the empirical and molecular formulae of
capsaicin.
5. It has been found experimentally that bismuth subsalicylate is
23.22%C, 1.39%H, 17.67%O and the remainder is bismuth. If the
molar mass of bismuth subsalicylate is 1086.3gmol-1 find the empirical and molecular
formulae of bismuth subsalicylate.
Empirical and molecular formulae - Answers
1.
2.
3.
4.
5.
C9H8O4, C9H8O4
C3H4O3, C6H8O6
C2H3O2, C4H6O4
C18H27NO3, C18H27NO3
C7H5BiO4, C21H15Bi3O12
Do
1. A 8.48 g sample of an unknown organic compound containing only carbon, hydrogen and
oxygen was burned completely to produce 12.42 g of carbon dioxide and 5.08 g of
water.
Find the percentage of carbon, hydrogen and oxygen in the compound.
2. A 11.61 g sample of an unknown organic compound containing only carbon, hydrogen and
oxygen was burned completely to produce 17.40 g of carbon dioxide and 4.74g of
water.
Find the percentage of carbon, hydrogen and oxygen in the compound.
3. A 15.96 g sample of an unknown organic compound containing only carbon, hydrogen and
oxygen was burned completely to produce 44.01 g of carbon dioxide and 9.03 g of
water.
Find the percentage of carbon, hydrogen and oxygen in the compound.
4. A 12.76 g sample of an unknown organic compound containing only carbon, hydrogen and
oxygen was burned completely to produce 18.12 g of carbon dioxide and 11.16 g of
water.
Find the percentage of carbon, hydrogen and oxygen in the compound.
5. A 3.15 g sample of an unknown organic compound containing only carbon, hydrogen and
oxygen was burned completely to produce 7.71 g of carbon dioxide and 1.05 g of water.
Find the percentage of carbon, hydrogen and oxygen in the compound.
Percentage composition - Answers
1.
2.
3.
4.
5.
40.0 % C, 6.70 % H, 53.3 % O
40.9 % C, 4.57 % H, 54.5 % O
76.5 % C, 6.44 % H, 17.0 % O
38.75 % C, 9.787 % H, 51.46 % O
66.8 % C, 3.73 % H, 29.5 % O