Download File

MASS, MOLES & FORMULA
Noadswood Science, 2012
Wednesday, May 24, 2017
Mass, Moles & Formula
 To know how to calculate formula mass and work out the percentage of
an element within a compound
Mass Number
 Each atom consists of a nucleus containing protons and neutrons,
with electrons arranged around it.
 Protons and neutrons have a relative mass unit of 1
 Electrons have a very small mass compared to protons and neutrons
with a mass unit of 1/2000th
 Most of the mass of an atom is found in its nucleus:  The mass number of an atom is the total number of protons and
neutrons it contains
 The atomic number (also called the proton number) is the
number of protons it contains
Chemical Symbol
 The full chemical symbol for an element shows its mass number at
the top, and atomic number at the bottom, e.g. carbon
 It tells us that a carbon atom has six protons (it will also have six
electrons, because the number of protons and electrons in an atom
is the same)
 The symbol also tells us that the total number of protons and
neutrons in a carbon atom is 12
 *You can work out the number of neutrons from the mass number
and atomic number – in this example, it is 12 - 6 = 6 neutrons
Relative Atomic Mass (Ar)
 The atoms of each element have a different mass – the relative
atomic mass is a way of saying how heavy atoms are in comparison
to others
 Carbon is given a relative atomic mass (RAM) of 12
 The RAM of other atoms compares them with carbon
Element
Symbol
x Heavier Than Carbon (C)
RAM
Helium
He
1/3
4
Beryllium
Be
3/4
9
Molybdenum
Mo
8
96
Krypton
Kr
7
84
Oxygen
O
1 & 1/3
16
Silver
Ag
9
108
Calcium
Ca
3 & 1/3
40
Formula
 Formulas tell you how many atoms are used but not the mass of
each of the elements
 For example in the reaction: -
Mg + 2HCl → MgCl2 +H2O
 This shows us we need twice as many hydrogen and chlorine atoms,
but not the masses needed (because different atoms have different
masses)
 Even though we are using more hydrogen atoms the mass of 2
atoms will be less than the mass of 1 magnesium
Relative Formula Mass (Mr)
 Relative formula mass is the sum of all the relative atomic masses
added together
 For example, MgCl2 has a relative formula mass of 95: -
MgCl2
Mg = 24
Cl = 35.5 x 2
 *Relative atomic mass for any element can be found using the
periodic table – relative formula mass is also ‘molar mass’…
Relative Formula Mass
 Work out the following relative formula masses using the atomic
masses given (N = 14; H = 1; Na = 23; O = 16; Mg = 24; Ca = 40):  Ammonia NH3
 Sodium oxide Na2O
 Magnesium hydroxide Mg(OH)2
 Calcium nitrate Ca(NO3)2
 Ammonia = 14 + (3 x 1) = 17
 Sodium oxide = (2 x 23) + 16 = 62
 Magnesium hydroxide = 24 + 2 x (16 + 1) = 58
 Calcium nitrate = 40 + 2x (14 + (3 x 16)) = 164
% Mass
 Percentage composition is just a way to describe what proportions
of the different elements there are in a compound
 If you have the formula of a compound, you are able to work out the
percentage by mass of an element in it using relative atomic mass
and relative formula mass…
% mass of an element in a compound = Ar x number of atoms (of element) x 100
Mr (of whole compound)
% Mass Example – CO2
 % of oxygen in carbon dioxide
 First find the atomic masses – C = 12 and O = 16
 Formula = CO2
 Number oxygen atoms = 2
 Atomic mass of O = 16
 Formula mass CO2 = 12 + (16 x 2) = 44

% oxygen is therefore…
16 x 2 x 100
44
= 72.7%
% Mass Example – Na2CO3
 % of sodium in sodium carbonate
 First find the atomic masses – Na = 23, C = 12 and O = 16
 Formula = Na2CO3
 Number sodium atoms = 2
 Atomic mass of Na = 23
 Formula mass Na2CO3 = (23 x 2) + 12 + (16 x 3) = 106

% sodium is therefore…
23 x 2 x 100
106
= 43.4%
Moles
 The mole is a name given to a Avogadro's number: 6.023 x 1023 or
602,300,000,000,000,000,000,000
 When you have precisely this number of atoms or molecules (of any
element or compound) they weigh exactly the same number of
grams as the relative atomic mass of the element or compound
 1 mole of atoms of molecules of any substance will have a mass in
grams equal to the relative formula mass (Ar or Mr) for that
substance
Moles
 1 mole of atoms of molecules of any substance will have a mass in
grams equal to the relative formula mass (Ar or Mr) for that
substance
 For example:  Carbon has an Ar of 12 – 1 mole of carbon weighs 12g
 Iron has an Ar of 56 – 1 mole of carbon weighs 56g
 Carbon dioxide has a Mr of 44 – 1 mole of carbon dioxide weighs
44g
 This means that 12g of carbon, 56g of iron, 44g of carbon dioxide
etc… all contain the same number of atoms – 1 mole!
Moles Within A Mass
 The number of moles within a given mass is worked out using the
following equation: Number of moles = Mass (g) of element or compound
Mr of element or compound
 For example, to calculate the number of moles in 42g of carbon…
Number of moles = 42g
12
= 3.5 moles
Molar Solutions
 A 1 molar solution contains 1 mole per litre
Number of moles = volume (litres) x molarity solution
 For example – how many moles are in 185cm3 of a 2M solution?
0.185 x 2
= 0.37 moles