Unit 3 Chemistry
 Gravimetric
analysis – discovering the mass
of a chemical that is isolated from a mixture
or precipitated in a reaction
 What
is a precipitate?
 Common
examples:
 Percentage of water in bread
 Concentration of salt in baby food
 Determination of chemical formula of a
compound
 Accuracy
– pretty good
due to high quality
measuring balances
(very sensitive)
 Measuring
water
content:
 Heat sample at 110◦C
until no change in mass
 This is called heating to
constant mass
Food
Water content (% by mass)
Lemonade
90
Bread
40
Cake
20
Egg
75
Sardines
49
Apple
65
Orange
65
Rice Bubbles
3
Steak
59
Honey
23
Lettuce
96
Initial mass of soup
223.1g
Second mass after heating
33.6
Third mass after heating
24.3
Fourth mass after heating
24.3
How would you calculate
the % mass of water in the
soup sample?
Where has the water gone?
 Mole
= amount of substance
 1 mol = exact amount of particles in 12g of
carbon-12
 In 1 mol = 6.02x1023 particles
 Make sure you qualify what you are
measuring in mol (mole)
 n(Na) or n(NaCl)
 n=m

M
Calculate the amount of H2O
in 9g of water......
 The
mass of a precipitate formed in a
precipitation reaction can be used to
determine the purity or composition of some
substances. The solid precipitate is heated to
constant mass to ensure that it is dry.
 We
need to consider the degree of solubility
of a compound when performing gravimetric
analysis
Steps in the analysis of
the chloride content in
peanut butter.
 Step
1: Write a balanced equation
 Step 2: Calculate the amount, in mol, of the
compound that’s mass has been provided
 Step 3: Using your equation, use ratios to
work out the amount in mol of the compound
who's mass you are trying to discover
 Step 4: Now you have the compound amount
in mol, work out the mass present (n=m/M)
 If a percentage composition is required,
divide the obtained mass by the initial mass
x100 – DONT FORGET TO CHECK SIG FIGURES!