Chemistry - Rawda Harb 2013
Review notes
1Atomic mass is equal to the total weight of an atom. Because most of the weight of an atom is due to the protons and neutrons (electrons don't weigh much), atomic mass is usually found by adding the number of protons and neutrons. Atomic mass is the weighted average mass of an atom of an element based on the relative natural abundance of that element's isotopes.
2Atomic number is just the number of protons. The atomic number is used to define the element being examined. The atomic number is also known as the proton number. It may be represented by the capital letter Z .
3Mass number is a count of the total number of protons and neutrons in an atom's nucleus.
Example : Hydrogen has three natural isotopes: 1 H, 2 H, and 3 H. Each isotope has a different mass number.
1
H has 1 proton. Its mass number is 1.
2
H has 1 proton and 1 neutron. Its mass number is 2. has 1 proton and 2 neutrons. Its mass number is 3. 99.98% of all hydrogen is
3
H
1
H 0.018% of all hydrogen is
2
H 0.002% of all hydrogen is hydrogen equal to 1.0079 g/mol.
3
H. Together, they give a value of atomic mass of
4Molar mass and Moles
Molar mass: (M) is the mass of 1 mole of a chemical substance. The unit for molar mass is grams per mole or g·mol
−1
.
Moles: The mole is a counting unit just like hours or days. We can easily count one second or one minute or one hour. If we want bigger units of time, we refer to days, months and years. Even longer time periods are centuries and millennia. The mole is even bigger than these numbers. The mole is 602 204 500 000 000 000 000 000 or 6.022
× 10
23
particles. This is a very big number! We call this number Avogadro's number.
Two formulae to keep in mind:
1Number of moles(mol)=mass(g)/Mmass(g/mol)
2Number of atoms, molecules or ions= moles(mol) x N
A
(6.022 × 10
23
)

Chemistry - Rawda Harb 2013
Questions:
1Calculate the number of moles of 4.2g of sodium bicarbonate. Sodium bicarbonate is
NaHCO
3
.
2Calculate the grams in 0.75mol Octane. Octane is C
8
H
18
.
3How many atoms are there in the following? a.
1 mole of a substance b.
2 moles of calcium c.
5 moles of phosphorus d.
24.3 g of magnesium e.
24.0 g of carbon
4Complete the following table:
Element
Relative atomic mass (u)
Hydrogen 1.01
Magnesium 24.3
Carbon 12.0
Chlorine 35.45
Sample mass (g)
Number of moles in the sample
1.01
24.3
24.0
70.9
Nitrogen 14.0 42.0
5Calculate the number of moles of iron (Fe) in an 11.7 g sample.
6You have a sample that contains 5 moles of zinc. a.
What is the mass of the zinc in the sample? b.
How many atoms of zinc are in the sample?
7Give the molar mass of each of the following elements: a.
hydrogen gas b.
nitrogen gas c.
bromine gas
8Calculate the number of moles in each of the following samples: a.
21.6 g of boron ( ) b.
54.9 g of manganese ( ) c.
100.3 g of mercury ( ) d.
50 g of barium ( ) e.
40 g of lead ( )
9Calculate the number of moles of copper there are in a sample that with a mass of
127g.

Chemistry - Rawda Harb 2013
10Calculate the number of atoms there are in a sample of aluminium that with a mass of 81 g.
11Calculate the number of moles in each of the following samples: a.
5.6 g of calcium b.
0.02 g of manganese c.
40 g of aluminium
12A lead sinker has a mass of 5 g. a.
Calculate the number of moles of lead the sinker contains. b.
How many lead atoms are in the sinker?
13Calculate the mass of each of the following samples: a.
2.5 mol magnesium b.
12 mol lithium c.
atoms of silicon
14Calculate the molar mass of .
15Calculate the number of moles in 1 kg of .
16What are the units of the mole? Hint: Check the definition of the mole.
17How many moles are in: a.
10 g of b.
34 g of c.
2.45 × 10 23 molecules of ?
18You have a sample containing 3 mol of calcium chloride. a.
What is the chemical formula of calcium chloride? b.
How many calcium atoms are in the sample?
19Calculate the mass of:
1.
3 moles of
2.
4,2 moles of