Counting Atoms
Chapter 9
MOLE??
Moles of Particles
In one mole of a substance, there are
6 x 1023 particles
Calculating the number of moles
from the number of particles
Number of moles =
Number of particles
6 × 1023
TB P147
1. Calculate the number of moles of particles in
a. 2.4 x 1023 atoms of hydrogen, H
b. 4.2 x 1024 molecules of carbon dioxide, CO2
c. 3 x 1022 copper ions, Cu2+
Calculating the number of moles
from the number of particles
Number of moles =
TB P147
Number of particles
6 × 1023
2. Calculate the number of particles in
a. 3 moles of carbon atoms, C
b. 0.75 moles of ammonia molecules, NH3
c. 25 moles of sodium chloride, NaCl
Extension Question!
How many atoms are there in 0.75 moles of
ammonia molecules?
Molar Mass
The mass of one mole of any substance is
called the molar mass.
For Substances Consisting of Atoms
The molar mass is the relative atomic mass of the element
in grams.
For Substances Consisting of Molecules
The molar mass is the relative molecular mass of the
substance in grams.
For Substances Consisting of Ions
The molar mass is the relative formula mass of the
substance in grams.
Calculating the number of moles
from the molar mass
Number of moles =
Mass (in g)
Molar mass (in g)
TB P150
1. (a) How many moles of atoms are contained
in 32 g of oxygen atoms?
4. (c) Calculate the mass of 4 mol of oxygen
molecules, O2?
Different Kinds of Formulae
 The molecular formula of a compound shows the
actual number and kinds of atoms present.
 The empirical formula of a compound shows the
simplest whole number ratio of the atoms present.
 The structural formula of a compound shows
how the atoms are joined in the molecules.
Calculating the Empirical Formula
The empirical formula can be calculated from the masses of
elements that combine together.
Follow these steps…* It is important to present the answers in a table!
Element
Mass
Molar mass
Number of moles
Simplest ratio
The empirical formula can also be
found from its percentage
composition.
We assume that 100 g of the
compound was analysed.
Let’s try!
TB P154
1. (a) Find the empirical
formula of a compound
consisting of 3.5 g of
nitrogen combined with
8.0 g of oxygen.
2. (a) Find the empirical
formula of a compound
with composition 75%
carbon and 25% hydrogen
by mass.
Element
Mass
Molar mass
Number of
moles
Simplest ratio
From Empirical Formula to
Molecular Formula
You will need the relative molecular mass of the compound!
TB P154
4. The empirical formula of a compound is COH3. Its relative
molecular mass is 62. Find the molecular formula.
Let the molecular formula be CnOnH3n
Mr of CnOnH3n = n(12) + n(16) + 3n(1) = 31n
31n = 62
n = 62/31 = 2
Therefore, the molecular formula is C2O2H6.
From Empirical Formula to
Molecular Formula
TB P154
5. The empirical formula of a compound is H2CO. Its relative
molecular mass is 90. Find the molecular formula.
Let the molecular formula be _____________
Mr of _____________ =
Therefore, the molecular formula is _________.
Molar Volume of GASES
Avogadro’s Law
Equal volumes of gases,
at the same temperature and pressure, contain
equal numbers of molecules.
Molar volume at r.t.p.
 it is the volume occupied by one mole of the gas
 at room temperature, one mole of gas has a volume of
24 dm3 or 24 000 cm3
 the molar volume is the same for all gases
Calculating the number of moles of
a gas from its volume
Volume of gas
Number of moles of gas =
Molar volume of gas
TB P157
1. (b) Calculate the mass of 4.8 dm3 of carbon dioxide,
CO2 at r.t.p..
2. (b) Calculate the volume of 2 g of hydrogen, H2 at r.t.p..
Concentration of Solutions
Compare the concentrations of the following solutions.
Solution A and C has the same concentration.
Solution A has half the concentration of Solution B.
The Idea of Concentration
The concentration of a solution tells us the amount
of solute in a unit volume of a solution.
Two ways to measure concentration:
1. the mass (in grams) of solute in 1 dm3 of solution
g/dm3
2. the number of moles of solute in 1 dm3 of solution
mol/dm3
Calculations involving
Concentration of Solution
3
Concentration in g/dm =
3
Concentration in mol/dm =
Number of grams of solute
3
Volume of solute (in dm )
Number of moles of solute
3
Volume of solute (in dm )
Let’s Try!
TB P160
1. Calculate the concentrations of the following
solutions, in (i) mol/dm3, and (ii) g/dm3.
a. 0.4 g of H2SO4 in 2 dm3 of solution
b. 0.5 mol of HCl in 250 cm3 of solution
5. What mass of potassium hydroxide, KOH, must be
dissolved in 500 cm3 of solution to prepare a 0.4
mol/dm3 solution?