Quantitative
Composition of
Compounds
•Define the MOLE
•Determine molar mass of compounds
•Calculate percent composition of compounds
•Distinguish the differences between (and be able
to calculate) empirical formulas and molecular
formulas
Composition of Compounds
• Ratios of ingredients determines end result
–
• Ratios in chemical compounds
– Determine identity of the substance
–
– Changing changes the entire compound
– Example
•
Chemical Measurements
• How can we measure masses of atoms?
–
–
• Counting by weighing
–
Chemical Measurements
• Atomic mass:
–
• Mole (mol):
–
Chemical Measurements
• Avogadro’s number:
–
–
• Molar mass:
–
–
The Mole
• Mole – unit for counting atoms
–
The Mole
• Molar Mass
– Grams of element (or compound) that
contain Avogadro’s number of atoms
–
The Mole
• The atomic mass expressed in grams is
the molar mass of an element
–
–
–
• One mole of any element contains
Avagadro’s number of atoms
–
The Mole
• Can now convert between:
mass
moles
molecules
• 25.0 g of iron is how many moles?
The Mole
• How many grams of Na are in 2.55 moles?
• How many atoms of Carbon are in 2.35
grams?
Molar Mass of Compounds
• One mole of a compound contains
6.022 x 10-23 formula units of that
compound
–
–
Molar Mass of Compounds
• Molar mass – add molar masses of all
atoms
– What is the molar mass of H2O?
•
– What is the molar mass of Ca(OH)2?
•
Molar Mass of Compounds
• How many molecules are there in a
3.46 g sample of HCl?
= 5.71 x 1022 molecules
Molar Mass of Compounds
• If you burned 6.10 x 1024 molecules of
ethane (C2H6) what mass of ethane did
you burn?
= 304.7 g C2H6
Moles and Gases
• At the same temperature and pressure,
equal volumes of gases contain the
same number of gas particles
• STP = Standard Temperature (00 C) and
Pressure (1 atmosphere)
•
Moles and Gases
• A student fills a 1.0 L flask with CO2 at
STP. How many molecules of gas are in
the flask?
Moles and Gases
• A container with a volume of 893 L contains how
many moles of air at STP?
• A chemical reaction produces 0.37 moles of N2
gas. What volume will that gas occupy at STP?
Percentage Composition of
Compounds
• Mass of one element in a compound
compared to the total mass of the
whole compound
• Two ways to determine…
–
–
Percentage Composition of
Compounds – from Formula
1) Calculate the molar mass
2) Divide the total mass of each element
in the formula by the molar mass and
multiply by 100.
Total mass of element x 100 = % of element
molar mass
Percentage Composition of
Compounds – from Formula
• What is the percent composition of
each element in table salt (NaCl)?
Should
equal
100%
Percentage Composition of
Compounds – from
Experimental Data
1) Calculate the mass of the compound
formed
2) Divide the mass of each element by
the total mass of the compound and
multiply by 100
Percentage Composition of
Compounds
• A sample of 2.45 g of aluminum oxide
decomposes into 1.30 g of aluminum and 1.15
g of oxygen. What is the percent composition
of each element?
Percentage Composition of
Compounds
• Find the percentage composition of a
compound that contains 1.94 g of
carbon, 0.480 g of hydrogen, and 2.58
g of sulfur in a 5.00 g sample of that
compound.
Empirical Formula
• A formula that gives the SIMPLEST
whole-number ratio of elements in that
compound
Ratio of masses
(percent composition)
Ratio of atoms
(formula)
Empirical Formula
• What is the empirical formula of a compound containing
40% carbon, 53.3% oxygen, and 6.7% hydrogen?
=1
=1
=2
Divide by
smallest
number
Empirical Formula
• Determine the empirical formula of a
compound containing 5.75 g Na, 3.5 g
N, and 12.0 g O
–
Empirical Formula
• Determine the empirical formula of a
compound containing 2.644g of gold
and 0.476 g of chlorine.
Molecular Formula
• Formula that gives the actual number of
atoms in a given compound
• Some compounds may have same empirical
formula but actually be very different
• Sometimes empirical formula and molecular
formula are the same
• Compare molar mass with empirical formula
mass
Molecular Formula
• Ribose has a molar mass of 150 g/mol
and a chemical composition of 40.0%
carbon, 6.67% hydrogen, and 53.3%
oxygen. What is the molecular formula
for ribose?
–
Molecular Formula
• Find the molecular formula of a compound
that contains 42.56 g of palladium and 0.80 g
of hydrogen. The molar mass of the
compound is 216.8 g/mol.
• Octane, a compound of hydrogen and carbon,
has a molar mass of 114.26 g/mol. If the
compound contains 18.17 g/mol hydrogen,
what is its molecular formula?
Homework
• Questions #1, 6-10
• Paired Exercises #11-31 odd (a, b, c only);
33; 37-41 odd
• Additional Exercises #43, 47, 50 & 51
• NO CLASS TUESDAY – Study help???
– Morning, afternoon, evening?
• TEST THURSDAY – Chapters 11, 6 & 7