Lesson 2
Relationship between mass and moles
In the last lesson you saw that the relationship between the number of moles (amount of a substance), molar mass (the mass of one mole) and mass
can be represented using the formula:
Mass, m (g)
Moles, n (mol)
(amount of the substance)
=
Molar mass, M (g mol-1)
The molar mass is equal to the relative molecular mass of a molecule or the relative atomic mass of an atom. Why relative? Well, atoms come in
more than one form called isotopes. These different forms have the same number of protons but different masses. For example, there are two
isotopes of silver. Both of these isotopes would have 47 protons because the atomic number is 47 and 47 electrons because they are neutral atoms.
However, the first isotope, silver-107 has 107 - 47 = 60 neutrons and the second, silver-109 has 109 - 47 = 62 neutrons.
Protons
Electrons
Neutrons
Atomic Number (Z)
Mass Number (A)
Symbol notation
silver-107
47
47
107 – 47 = 60
47
107
107 Ag or 107Ag
47
silver-109
47
47
109 – 47 = 62
47
109
107 Ag or 109Ag
47
To generate the relative scale of atomic masses chemists chose the most abundant isotope of the element carbon, the ‘carbon-12’ isotope ( 126C ) and
assigned it a relative mass of exactly 12 units. The element carbon was chosen for a number of important reasons:
•
carbon is very cheap and is widely available
•
it is relatively easy to isolate and purify this isotope
•
carbon is not toxic in any way
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Lesson 2
The relative atomic mass of an element, Ar, is defined as the weighed mean mass of all the naturally occurring isotopes of an element relative to the
mass of a carbon-12 atom. The relative molecular mass of a molecule, Mr is the sum of the relative atomic masses of the elements in a molecule
relative to the mass of a carbon-12 atom.
For example, chlorine has a relative atomic mass of 35.45 because the element chlorine contains some chlorine atoms which have eighteen neutrons
in their nucleus and some which have twenty neutrons in their nucleus.
Chlorine
Atomic mass, A = 35 (17 protons and 18 neutrons)
Relative atomic mass, Ar = 35.45
If a sample of the element chlorine was made up of 100 chlorine atoms 80 would be the chlorine-35 atom and 20 the chlorine-37 atom.
Number of
chlorine atoms
80
20
Mass number for each
chlorine atom
35
37
Relative atomic mass
= (35 x 80) + (37 x 20) ÷ 100 = 35.4
The relative atomic mass can be calculated using the following formula.
Ar
=  (mass of each isotope x abundance of each isotope) ÷ 100
This explains why the relative atomic masses on the periodic table is not exactly whole numbers. In other words they are non integer numbers.
Relative atomic masses have no unit because they are comparative value.
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Lesson 2
Activity
Determine the mass of one mole of sodium chloride (NaCl) and sucrose (C12H22O11). Weigh out each mole and compare them. What do you notice?
Questions
Points are awarded for showing the step by step reasoning you used to solve the problem. Steps:
1. If an equation is given check that it is balanced. Otherwise write a balanced equation if required.
2. Identify the variables provided in the problem and their units.
3. Identify the unknown – what you are solving for and its units.
4. Write the relevant formula with the subject. e.g n(NaCl), m(I2).
5. Substitute the known variables into the formula, including units.
6. Calculate the answer.
7. Make sure the answer has the correct number of significant figures and units.
1. Determine the Relative molecular mass of:
a)
HI
b)
NaClO3
c)
Cr2O3
d)
(NH4)2HPO4
e)
Fe2SO4
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Lesson 2
2. Calculate the mass of 0.70 moles of Li2SO4 if its molar mass is 110 gmol-1
3. 0.20 moles of a substance has a mass of 2.7g. Calculate the molar mass.
4. One drop of water weighs 0.040g. How many molecules are there in one drop, if the molar mass of water is 18.00 gmol-1?
5. Calculate the mass of:
a)
3.0 moles of ammonia
b)
0.25 moles of lithium oxide
c)
0.050 mole of aluminum nitrate
d)
3.01 x 1023 molecules of phosphorus trichloride
e)
2.6 x 1022 molecules of dinitrogen monoxide
6. Calculate how many moles are there in:
a)
31g of phosphorus
b)
303g of potassium nitrate
c)
4.0g of nickel (II) sulphate
d)
87.3g of methane, CH4
7. Vitamin C, ascorbic acid has the formula, C6H8O6
a) The recommended daily dose of Vitamin C is 0.60 grams. Calculate how many moles of Vitamin C are you consuming if you ingest 0.60
grams of the vitamin?
b) A typical tablet contains 1.0 g of vitamin C. Determine how many oxygen atoms are you eating?
8. Chlorofluorocarbons are molecules used as the propellant in hair sprays and other aerosol cans. They have been banned from use because
they deplete the ozone layer. If there is 255.0 g of chloroflorocarbon, CCl 2F2 in a can calculate:
a) How many molecules of CCl2F2 are released to the air when the can is empty?
b) How many chlorine atoms are released to the air when the can is empty?
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Lesson 2
9. Determine:
a)
Molar mass of the hydrate salt copper sulphate pentahydrate, CuSO4 . 5H2O
b)
Moles of water in 1 mol of CuSO4 . 5H2O
c)
Mass water in 1 mol of CuSO4 . 5H2O
d)
% water in CuSO4 . 5H2O
10. In the following reaction:
2NaN3 (s)
a)
b)
 2Na (s)
+
3N2 (g)
If 500.0 grams of NaN3 decomposes to form 323.2 grams of nitrogen gas, N2 determine how many grams of sodium is produced.
What law does this represent?
11. Extra for Experts. Determine the mass of 100 atoms of copper metal.
Decoding the “m” words
Complete the following summary table
M word
Symbol
Mass number
A
Atomic number
Z
Unit of
measurement
Definition
5
How to determine (words or equation)
Lesson 2
g
Ar
Mass of one mole of a substance
Mole
Relative
molecular mass
N
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Lesson 2
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