Transcript C 3.2 Masses of atoms and moles Pages 144 -145

Mass Numbers & Masses of
atoms and moles
Learning Outcomes
1) Masses of atoms and molecules of
compounds can be compared using atomic
mass.
2) A mole of an element or a compound is the
relative formula mass expressed in grams.
What do we know by looking at a chemical formula
equation?
Mg(s) +
2HCl(aq)

MgCl2(aq)
+
H2(g)
1. Elements and compounds involved as reactants and
products
2. State of each
3. Ratio of elements in a compound
4. Ratios of each substance involved
Chemical Equations
• Step 1: Write down the word equation.
• Step 2: Replace words with the chemical formula .
• Step 3: Check that there are equal numbers of each type
of atom on both sides of the equation. If not, then balance
the equation by using more than one.
• Step 4: Write in the state symbols (s), (l), (g), (aq).
Reactants
Products
magnesium + oxygen  magnesium oxide
Mg +
O2
MgO

Oxygen doesn’t balance.Need 2 MgO and so need 2 Mg
2Mg +
2Mg(s)
O2

+O2(g)

2MgO
2MgO(s)
Try balancing these equations
Ca
+
Cl2

CaCl2
Na
+
Cl2

NaCl
Na
+
O2

Na2O
Na
+
H2O

NaOH +
H2
CH4
+
O2

CO2
H2O
+
Check your answers very carefully
Ca
+
Cl2

CaCl2 Already done
2Na
+
Cl2

2NaCl
4Na
+
O2

2Na2O
2Na
+
2H2O 
CH4
+
2O2

2NaOH
+
CO2
2H2O
+
H2
Relative Atomic mass RAM
• The mass of an atom compared with 12C.
This is usually the same as or similar to
the mass number of the element
• It takes into account the isotopes of that
element
Relative atomic mass.
• Think about carbon. It has 6 protons and
6 neutrons so we say it has a mass of 12
units. We compare every other atom to
carbon.
• Therefore the mass of magnesium is 24 –
12 protons and 12 neutrons.
Relative formula mass Mr
• The mass of a chemical compound based
on the relative atomic masses of the
elements in it.
The mole
The relative
atomic or
formula mass
in grams is one
mole
How many atoms in a mole?
In one mole of a substance
there is always
6020000000000000000000
00 atoms, ions or
molecules.
This is called Avogadro’s
number.
Don’t worry you don’t need
to learn it, just remember
there is always the same
number of ‘bits’ if you have
equal moles
Practice
Moles 1: Formula mass
Answer questions 1 and 3 from
the gdoc in your personal Drive
folder