Lab 5
Chemical formulas
Concepts
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–
–
–
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Formula vs molecular weight
% composition from formula
The mole
Empirical formula
Atomic weight
Formula Weight
Formula weight – the mass of the collection of
atoms represented by a chemical formula. For
Ex. water contains two hydrogen atoms and
one oxygen atom.
1 x 16.00
(mass of O) =
16.00
2 x 1.01
(mass of H) =
+ 2.02
Formula Weight 
18.02
The formula weight tells us the mass of one
mole of our substance.
Gram Formula Weight
This refers to the mass of an
ionic compound
ion or
Ex. Mass of a Cl-
Gram Atomic Weight
This refers to the mass of a
Ex. Mass of a Cl atom
atom
Gram Molecular Weight
molecule
or molecular compound
This refers to the mass of a
Ex. Mass of one molecule of H2O
What is the Mole?
• A counting number (like a dozen)
• Avogadro’s number (NA)
• 1 mol = 6.02  1023 items
A
large amount!!!!
In chemistry everything needs to go through moles
Molar Mass
• Mass of 1 mole of an element or compound.
• Atomic mass tells the...
– atomic mass units per atom (amu)
– grams per mole (g/mol)
• Round to 2 decimal places
Elemental Molar Mass Examples
• carbon
12.01 g/mol
• aluminum
26.98 g/mol
• zinc
65.39 g/mol
Formula Molar Mass Examples
• water
– H2O
– 2(1.01) + 16.00 = 18.02 g/mol
• sodium chloride
– NaCl
– 22.99 + 35.45 = 58.44 g/mol
Molar Conversion Examples
grams to moles
• How many moles of carbon are in 26
g of carbon?
26 g C
1 mol C
12.01 g C
= 2.2 mol C
Molar Conversion Examples
moles to molecules
• How many molecules are in 2.50
moles of C12H22O11?
2.50 mol
6.02  1023
molecules = 1.51  1024
molecules
1 mol
C12H22O11
B. Empirical Formula
• Smallest whole number ratio of
atoms in a compound
C 2H 6
reduce subscripts
CH3
B. Empirical Formula
1. Find mass of each element.
2. Find moles of each element.
3. Divide moles by the smallest # to find
subscripts.
4. When necessary, multiply subscripts
by 2, 3, or 4 to get whole #’s.
B. Empirical Formula
• Find the empirical formula for a
sample of 25.9g N and 74.1g O.
25.9 g 1 mol
= 1.85 mol N
=1N
1.85 mol
14.01 g
74.1 g 1 mol
= 4.63 mol O
= 2.5 O
16.00 g
1.85 mol
B. Empirical Formula
N1O2.5
Need to make the subscripts whole
 multiply by 2
numbers
N2O5
Part A - omit
• Fill in data given by teacher
– Mass of zinc = .45g
– Mass of zinc chloride = .94 grams
• Based on data given determine
– Mass of chlorine
– Empirical formula for zinc chloride
– Balanced chemical equation for zinc and HCl
• Hint: the rxn releases H2 gas
Part B
• Clean and mass a evaporating dish based on procedures
from pg 52
• Get a 2.00g of copper and place into the crucible
** Sulfur must be treated in the hood at all
times**
• Add sufficent sulfur to cover the copper
• Determine mass of copper sulfide
• Determine mass of sulfur
How to Determine Empirical Formula from data
in 3 easy steps
1. Determine the # of moles present for each reactant based on the
initial grams present
Example data
Cu = 2.0 g
S = 1.0 grams
CuS =3.0 grams
For my trial
2.0 grams Cu =
1.0 grams S =
.031 moles
.032 moles
How to Determine Empirical Formula from data
2. Divide both by the
smaller # of moles
For my trial
S: .032 mols of S = 1.03
.031 mols of Cu
For my trial
Cu: . 031 mols of Cu = 1.00
. 031 mols of Cu
How to Determine Empirical Formula from data
3. Determine Empirical Formula based on mole to
mole ratio
Rule # 1 – round to 1 sig fig
Ex. 1.03 of S = 1
1.00 of Cu =1
Final answer is CuS
How to Determine Empirical Formula from data
Rule # 2 – if mole comes out as a fraction
multiply both numbers to next whole number
Ex. 1.5 x 2 of S = 3
1.0x 2 of Cu =2
Final answer is Cu2S3
Due next week
• Pgs 55 – 57
• Quiz on lab 6