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Chemistry 11
Ch 5.1-5.2 notes
Name: __________________
Blk: ______ Date: ________
Ch 5.1 & 5.2 Avogardo’s Hypothesis & Molar Mass
A) RELATIVE MASS:
It is very difficult to determine the mass of a single atom precisely and accurately, so scientists
determine the mass of elements by comparing their “RELATIVE MASS”, which is a mass ratio
between two elements.
Example 1: Reaction: H2 + Cl2  2HCl
The reaction between 2.74g of hydrogen gas and 97.26g of chlorine gas makes 100g of hydrogen
chloride. What is the relative mass of a chlorine atom to a hydrogen atom?
Note: When you compare X to Y, it means X over Y or __X (top)_
Y (bottom)
B) AVOGADRO’S HYPOTHESIS:
Equal volume of different gases at the same pressure and temperature contain the
same number of particles (including molecules and atoms).
Atom: the smallest indivisible unit of an element.
Molecule: a group of atoms chemically bonded together.
Diatomic molecule: these elements exists as a 2-atom molecule e.g. H2, N2, O2, F2, Cl2, Br2, I2
Particle: any small bit of matter, including ions, atoms and molecules.
Experiment: Nitrogen and oxygen gas were combined to form three different covalent
compounds. Apply Avogardo’s Hypothesis and the concept of relative mass to determine the
chemical formulas for the following compounds.
Volume of Nitrogen
Compound 1
Compound 2
Compound 3
(mL)
Volume of Oxygen
(mL)
100
100
100
49.5
102.9
201.7
Whole Number Ratio of
Nitrogen: Oxygen Atoms
Formula
C) What is a MOLE?
 A mole is a number equal to__________________________.
 It is also known as _________________________________.
 Atoms are so tiny that even the smallest speck of dust contains about 1016 atoms. Hence, it
is more convenient to count particles using MOLE, just like we count eggs per DOZEN.
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D) Molar Mass
 Molar mass means mass per mole. On the periodic table, the atomic mass (a.m.u.) is the
same as the molar mass (g), which means the mass for one mole or for 6.02 x 1023 atoms of
the element.
Examples & Equivalences:
1 mole of C = __________ atoms of C = _______a.m.u. (atomic mass unit) = ___________ g/mole
1 mole of O = __________ atoms of O = _______a.m.u. (atomic mass unit) = ___________g/mole
1 mole of O2 = _________ molecules of O2 = _______a.m.u. (atomic mass unit) = ________g/mole
E) Finding the Molar Mass of A Compound
Steps:
1. Look up the molar mass of each element from a periodic table.
2. Determine the number of atoms for each element (shown by the subscript).
3. Multiply the molar mass of each element by the number of atoms of that element in the
compound.
4. Add up the numbers produced in step 3.
5. Express the molar mass in proper unit: grams.
Notes: * Round the molar masses to one decimal place when you calculate the molar masses
using a Periodic Table.
Example1: Calculate the molar mass of C6H4Cl2
Type of atom
Total mass for each type of atom
Molar Mass for C6H4Cl2 = __________________________g
Example2: Calculate the molar mass of Mg3(PO4)2
Type of atom
Total mass for each type of atom
Molar Mass for Mg3(PO4)2 = _________________________g
Example 3: Calculate the molar mass of Cu(NO3)2 6H2O
Assignment: Hebden p.78 #2-5, p.80 #6a-e, 7a-d
HOLY MOLLY JOKES 
Q: What did Avogardo teach his students in math class?
A: Moletiplication
Q: What kind of fruit did Avogardo eat in the summer?
A: Watermolens
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