Bubble Gum Lab!
 Step 1 – Measure mass of empty cup. Record.
 Step 2 – Unwrap a piece of gum for each member of
your group and place in the cup. Measure mass of cup
again.
 Step 3 - Subtract masses to get mass of gum alone.
Record.
 Step 4 – Chew one piece of gum each.
The Law of Definite Proportions
The law states: “The elements in a chemical compound are
always in the same proportions by mass”
Eg. Humans use water in its many forms for recreation
(swimming, ice skating, skiing) What is the chemical
composition for water in all of these scenarios?
 H2O
How do you calculate percent?
How do you calculate the grade
you got on your test?
Percentage composition
 Indicates the relative amount of each element present
in a compound.
Calculating percentage
composition
 Step 1 : Calculate molar mass
Calculating percentage
composition
 Step 1 : Calculate molar mass
 Step 2 : Divide the subtotal for each element’s molar
mass by the total molar mass.
Calculating percentage
composition
 Step 1 : Calculate molar mass
 Step 2 : Divide the subtotal for each element’s molar
mass by the total molar mass.
 Step 3: Multiply by 100 to convert to a percentage.
Example 1
 Calculate the percentage composition of water, H2O.
Example 1
 Calculate the percentage composition of water, H2O.
 1. Calculate the molar mass of H2O.
 Step 1 – H- 2(1.01)= 2.02



O – 1(16.0)= 16.0
________
18.02 g/mol
 2. Divide the subtotal for each element’s molar mass by
the total molar mass.
 2. Divide the subtotal for each element’s molar mass by
the total molar mass.
 Step 2
Hydrogen  2.02/18.02 X 100 = 11.21%

Oxygen
 16.0 /18.02 X 100 = 88.79 %
 2. Divide the subtotal for each element’s molar mass by
the total molar mass.
 Step 2
Hydrogen  2.02/18.02 X 100 = 11.21%

Oxygen
 16.0 /18.02 X 100 = 88.79 %
 Water is composed of 11.21 % hydrogen and 88.79 %
oxygen.
 Note: 11.21 + 88.79 = 100% **(total is always 100)**
Example 2
 Calculate the percentage composition of sucrose ,
C12H22O11.
Example 3
 Find the percentage composition of hydrogen in
sulfuric acid. (H2SO4)
Bite the Bubble:
A Chemistry Mini-Lab
 Objective: To calculate the percentage of sugar in various types of gum.
(Assume chewing gum has 2 components; sugar and gum)
 Step 5 - Collect the gum back in the cup and measure the mass of the
chewed gum and the cup.
 Step 6 - Subtract the mass of the cup to get the mass of the chewed
gum. Record.
 Step 7 - Subtract the mass of the chewed gum from the mass of the unchewed gum to get the mass of the sugar that was removed.
 Step 8 - Calculate the percentage of sugar in the gum. Show your
calculations!
 *All measurements should be made to the nearest tenth of a gram.
 *One person from each group fill in chart on board.*
Questions:
 1) Would a dentist recommend chewing this gum?
Why or why not?
 Sources of error?
 2) Would changing the number of pieces of gum
change the results of the lab? Explain.