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COUNTING
ATOMS
Target 1.2D: Determine the number
of atoms in a compound
COEFFICIENTS
2. COEFFICIENTS apply to the entire
compound. You MULTIPLY the
coefficients and SUBSCRIPTS.
RULES FOR COUNTING
ATOMS
1. SUBSCRIPTS only refer to the
atom that they are BEHIND. For
example
H2S
There are TWO atoms of
HYDROGEN and only ONE atom
of SULFUR.
COEFFICIENTS
2. COEFFICIENTS apply to the entire
compound. You MULTIPLY the
coefficients and SUBSCRIPTS.
2 H2S
2 H2S
ATOMS OF HYDROGEN: 4
ATOMS OF HYDROGEN: 4
ATOMS OF SULFUR: 2
IF THERE ISN’T A SUBSCRIPT BEHIND AN
ELEMENT, ASSUME THERE IS ONLY ONE ATOM OF
THAT ELEMENT!
COEFFICIENTS
• Think algebra and the distributive property
– 2(2x + y) = 4x + 2y
– 2(H2S) = 4H and 2S
ATOMS OF SULFUR: 2
Think algebra: 2(x+y) = 2x + 2y
PARENTHESES
3. If elements or compounds are inside of
PARENTHESES, then the SUBSCRIPT
behind the parentheses applies to
everything inside.
Ba(OH)2
ATOMS OF BARIUM:
1
ATOMS OF OXYGEN:
2
Thinks of distributive property
again, but with multiplier in
the back
ATOMS OF HYDROGEN: 2
1
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LET’S PRACTICE!
PRACTICE
MgCl2
Atoms of Magnesium: 1
Atoms of Chlorine: 2
H2SO4
Atoms of Hydrogen: 2
Atoms of Sulfur: 1
Atoms of Oxygen: 4
Al2S3
Atoms of Aluminum: 2
Atoms of Sulfur: 3
CH3OH
Atoms of Carbon: 1
Atoms of Hydrogen: 4
Atoms of Oxygen: 1
THIS COULD BE A LITTLE
TRICKY4
Ca3(PO4)2
Atoms of Calcium: 3
Atoms of Phosphorus: 2
Atoms of Oxygen: 8
What about this BAD
BOY???
2Ca3(PO4)2
Atoms of Calcium: 6
Atoms of Phosphorus: 4
Atoms of Oxygen: 16
Al2(SO4)3
Atoms of Aluminum: 2
Atoms of Sulfur: 3
Atoms of Oxygen: 12
Molar Mass
• Target 1.2E: Calculate the molar mass of a
compound
Molar Mass of H2S
• There are two hydrogen atoms
– Each one with a mass of one (1)
• So sayeth the periodic table…
• The mass of a compound can be determined
using information of the periodic table
– Just find the atomic mass or atomic weight of
each atom and add them together!
– Total mass of all the hydrogen = 2
• There is one (1) sulfur atom
– It has a mass of 32
• Mass of ALL hydrogen + Mass of ALL
sulfur = 34
– That’s molar mass!
2
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LET’S PRACTICE!
MgCl2
Atoms of Magnesium: 1
Mass of each Magnesium: 24
Total Mass of Magnesium: 24
Atoms of Chlorine: 2
Mass of each Chlorine: 35
Total mass of Chlorine: 70
Total Mass of MgCl2:
24 + 70 = 94
Molar Mass of Al2S3
• Mass of Al
– Number of Al x mass of Al
– 2 x 27 = 54
• Mass of S
– Number of S x mass of S
– 3 x 32 = 64
• Mass of Al2S3 = 54 + 64 = 118
LET’S PRACTICE!
Al2S3
Atoms of Aluminum: 2
Mass of each Aluminum: 27
Total Mass of Aluminum: 54
Atoms of Sulfur: 3
Mass of each Sulfur: 32
Total mass of Sulfur: 96
Total Mass of Al2S3:
54 + 96 = 150
Molar Mass of CH3OH
• Mass of C
– 1 x 12 = 12
• Mass of H
–4x1=4
• Mass of O
– 1 x 16
• Mass of CH3OH: 12 + 4 + 16 = 32
3