February 28, 2014
• Final Polyatomic Ion Quiz
• Due: HW 7B
• Percentage Composition
• Lab Report due Wednesday March 5
• Today’s video: Oldest known piece of earth
– How old?!
– What do you think their “technique” is?
– Zirconium silicate - ZrSiO4
Percent Composition
• Percent composition: Percentage by mass of
each element in a compound.
• What percent of gum is sweetener?
Percent Composition
• Mass percentage of an element in a
compound is the same regardless of the
sample size. This is because the ratio of one
element to another in a given compound is
the same regardless of sample size.
• H2O = 2 parts H : 1 part O
To calculate % Composition
• % element in compound =
n(Molar mass of element)
Molar mass of compound
x 100
n = how many of that atom in the compound
What are hydrates?


Ionic Compounds that include water in the
crystal lattice
ZnSO47H2O

For every one formula unit of zinc sulfate, there
are seven water molecules.

If heated, the water will leave
CuSO45H2O v. CuSO4
Example: Carbon dioxide
• Find the percentage composition of:
• Carbon dioxide, CO2
– Molar mass of CO2: 12.01 + 2(16.00) = 44.01 g/mol
– % C: 12.01 g/mol x 100 = 27.29% C
44.01 g/mol
– % O: 2(16.00 g/mol) x 100 = 72.71% O
44.01 g/mol
Check by adding the %s: 27.29 + 72.71 = 100.00%
Example: Sodium nitrate
• Find the percentage composition of:
• Sodium nitrate
– Molar mass of NaNO3: 22.99 + 14.01 + 3(16.00) =
85.00 g/mol
– % Na: 22.99 g/mol x 100 = 27.05% Na
85.00 g/mol
– % N: 14.01 g/mol x 100 = 16.48% N
–
85.00 g/mol
– % O: 3(16.00 g/mol) x 100 = 56.47% O
85.00 g/mol
Check by adding the %s: 27.05 + 16.48 + 56.74 = 100.00%
Practice: PbCl2
• Find the percentage composition of:
• Lead (II) Chloride
– Molar mass of PbCl2: 207.2 + 2(35.453) = 278.106 g/mol
– % Pb: 207.2 g/mol x 100 = 74.50% Pb
278.106 g/mol
– % Cl: 2(35.453 g/mol) x 100 = 25.50% Cl
278.106 g/mol
Check by adding the %s: 74.50 + 25.50 = 100.00%
Practice: Ba(ClO3)2
• Find the percentage composition of:
• Barium chlorate
– Molar mass of Ba(ClO3)2 :
– 137.33 + 2(35.453) + 6(15.999) = 304.23 g/mol
– % Ba: 137.33 g/mol x 100 = 45.14% Ba
304.23 g/mol
– % Cl: 2(35.453 g/mol) x 100 = 23.31% Cl
304.23 g/mol
– % O: 6(15.999 g/mol) x 100 = 31.55% O
304.23 g/mol
Check by adding the %s: 47.14 + 23.31 + 31.55 = 100.00%
Practice: CH4
• Find the percentage composition of hydrogen in
methane
– Molar mass of CH4
– 12.011 + 4(1.0079) = 16.0426 g/mol
– % H: 4(1.0079) g/mol x 100 = 25.13 % H
16.0426 g/mol
Practice: ZnSO47H2O
• Find the percentage composition of water in zinc
sulfate heptahydrate.
– Molar mass of ZnSO47H2O
– 65.39 + 32.066 + 4(15.999) + 14(1.0079) + 7(15.999) =
287.561 g/mol
– % H2O: 14(1.0079) + 7(15.999) g/mol x 100
287.561 g/mol
= 43.85 % H2O
Wait - What are hydrates?





Ionic compounds that include water in the
crystal lattice.
ZnSO47H2O
For every one formula unit of zinc sulfate, there
are seven water molecules.
The dot indicates ratio, not “multiply”
If heated, the water will leave the compound and
the color will change.