PERCENTAGE CALCULATIONS
% = ( portion / total ) 100
What is the percent of salt in a
saltwater solution if 246.99 g of
sodium chloride was added to 4.00 L
of pure DI water?

How
much palladium is present in a
59.00 g sample of white gold
(composition 90% Au & 10% Pd)?
LAW OF DEFINITE PROPORTIONS
In a compound, the ratios by
mass of the elements in that
compound are fixed independent
of the origins or preparation of
that compound. A compound is
unique because of the specific
arrangement and weights of the
elements which make up that
compound. That is, elements
combine in whole numbers. Also it is
not possible to have a compound with
portion an atom.
LAW OF DEFINITE PROPORTIONS

Elements combine in specific ratios to form
compounds
 Use the Generic equation for percent:
% = ( portion / total ) 100
1. What is the experimental percent of oxygen
in CO if 36.0 g of carbon reacted completely
with 48.0 g of oxygen?
2. What is the theoretical percent of sulfur in
silver sulfide (Ag2S)?
3. What is the percent composition of
potassium bromide?
LAW OF DEFINITE PROPORTIONS

Elements combine in specific ratios to form compounds

Use the Generic equation for percent:
% = ( portion / total ) 100
1. What is the experimental percent of oxygen in CO2 if
42.0 g of carbon reacted completely with 112.0 g of
oxygen?
% O = (mass of O / mass of CO2) 100
% O= [112.0 g O / (42.0 g + 112.0 g) CO2] 100 = 72.7% O
2. What is the theoretical percent of aluminum in
aluminum oxide?
% Al = (Atomic mass of Al / Formula mass of Al2O3) 100
% Al = (54amu / 102 amu) 100 = 52.9%
3. What is the percent composition of sodium chloride?
% Na = 39.3%
% Cl = 60.7%
LAW OF DEFINITE PROPORTIONS
In a pure substance, the elements are always
present in the same definite proportions by mass.

Carbon monoxide, CO, is a component of car exhaust.
If a 10.00 g sample of CO is found to contain 42.86% C
and 57.14% O,
a) what would be the percent composition of a
7.4822 g sample? The same!!
b) what is the mass of carbon in the 10.00 g
sample?
4.286 g
c) what is the maximum amount of CO formed
from 50.00 g C and 57.14 g O?
100.00 g of CO since 57.14 g of O is 57.14% of 100.00 g
therefore only 42.86 g of C was used. 7.14 g of carbon
was leftover (not used in the reaction).
LAW OF DEFINITE PROPORTIONS

Use the Generic equation for percent:
% = ( portion / total ) 100
1. How many grams of copper will combine
with 7.97 g of oxygen to produce CuO?
2. How many grams of mercury will
combine with 126.9 g of oxygen to form
HgO?
LAW OF DEFINITE PROPORTIONS

Use the Generic equation for percent:
% = ( portion / total ) 100
1. How many grams of copper will combine with 62.75 g
of oxygen to produce CuO?
% O = (Atomic mass of O / Formula mass of CuO) 100
% O = (16 amu / 79.6 amu) 100 = 20.1% O
thus 100 % total - 20.1% O = %Cu = 79.9% Cu
so 20.1 % O = (62.75 g / total mass) 100
total mass = 62.65 g / 0.201 = 311.69 g CuO
next: 79.9% Cu = (mass Cu / 311.69 g) 100
mass Cu = 311.69 g (0.799) = 249.0 g
2. How many grams of mercury will combine with 62.75 g
of oxygen to form HgO? 786.5 g Hg
LAW OF MULTIPLE PROPORTIONS
When two elements form a series
of compounds, the masses of the
one element that combine with a
fixed mass of the other element
stand to one another in the ratio of
small integers.
Iron oxide exists in different ratios
with different properties
FeO and Fe2O3
LAW OF CONSERVATION OF MASS

In every chemical operation an equal
quantity of matter exists before and after
the operation. That is, the amount of
matter before a reaction must equal the
amount of matter after a reaction. No
matter is lost.
The total mass of reactants = total mass
of products
LAW OF CONSERVATION OF MASS

When 1.445g of silver was heated in sulfur,
1.661 g of silver sulfide (Ag2S) was produced.
a) what is the mass of sulfur needed to
produce 1.661 g of silver sulfide (Ag2S) ?
b) If a student started with 43.776 g of
sulfur, how much silver would be needed to
use up all of the sulfur?
LAW OF CONSERVATION OF MASS

When 0.0976 g of magnesium was heated in air,
0.1618 g of magnesium oxide (MgO) was
produced.
a) what is the mass of oxygen needed to
produce 0.1618 g MgO?
Using the LCM:
Total mass reactants = total mass products
mass of Mg + mass O = mass of MgO
0.0976 g Mg + mass O = 0.1618 g MgO
mass O = 0.1618 g - 0.0976 = 0.0642 g O
Law of conservation of mass & Law of definite
proportions

When 0.0976 g of magnesium was heated in air,
0.1618 g of magnesium oxide (MgO) was
produced.
b) what is the percent of Mg in MgO?
% Mg = (mass Mg / Mass MgO) 100
= (0.0976g / 0.1618 g) 100 = 60.3 %
c) Using only LDP, what mass of oxygen was
needed to combine with the magnesium?
% O = 100% MgO - 60.3% Mg = 39.7% O
%O
= (mass O / mass MgO) 100
39.7 % = (mass O / 0.1618 g) 100
mass O = 0.397 ( 0.1618 g) = 0.0642 g O
Same as using the LCM!!
Law of conservation of mass & Law of definite
proportions

When 0.0976 g of magnesium was heated in air,
0.1618 g of magnesium oxide (MgO) was
produced.
d) What is the mass of oxygen in 10.00 g MgO?
% O = 39.7% O & % Mg = 60.3% this does not change!
Because the mass relationship (& thus percent) of Mg to O in MgO
never changes; a porportionality ratio can be used.
Mass MgO = Mass O
% MgO
%O
10.00 g MgO = Mass O
100% MgO
39.7 %
Mass O = 3.97 g
PRACTICE PROBLEMS #6
When 6.00g of iron and 6.00g of sulfur were mixed and reacted to
give the compound ferrous sulfide (FeS), 2.57 g of sulfur remained
unreacted.
a) Write a chemical equation describing this reaction.
Fe (s) + S (s)  FeS (s)
b) What percentage of this compound is sulfur?
36.4 % S
c) How many grams of sulfur will combine with 500.0 g of
iron?
286.2 g S
d) How much FeS will be produced from 347.65g Fe and
398.8 g S?
546.6 g FeS
e) Which substance will be leftover (in d) and by how much?
199.9 g S
f) Is this the only possible compound that can be formed
from iron and sulfur? According to which law?
No, the law of multiple proportions
GROUP STUDY PROBLEM #6
Practicing percents:
________1. Pure gold is too soft a metal for many uses, so it is alloyed to
give it more mechanical strength. One particular alloy is made by mixing
29.17 g of gold, 3.81 grams of silver, and 5.91 g of copper. What carat
gold is this alloy if pure gold is considered to be 24 carat?
________2. If 9.0 ounces of a meat sample contains 21.9 g of fat, what
percentage of fat is present?
Using the LAWS:
________3. How many grams of CuO can be obtained from 1.80 g of copper?
4. When aluminum combines with bromine gas, they produce the substance
aluminum bromide, AlBr3.
a)Write a chemical equation describing this reaction.
_______ b) If 56.88 g of aluminum bromide is formed from 5.75 g of
aluminum, how many grams of bromine was needed?
_______ c) Since 56.88 g of aluminum bromide is formed from 5.75 g of
aluminum, how many grams of bromine will combine with 16.50 g of
aluminum?