Percent

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Percent Composition
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Its just like finding out your test score!!!

• 97 correct out of 100 questions 97 x 100
  97
• 100
• Total of something x 100
• TOTAL

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The chemical composition can be expressed as the
mass percent of each element in the compound.
Carry out mass to the tenths place.
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Example Determine the percent composition of
C3H8.

• Assume you have one mole of the substance.
• 3 C 3(12.01) 36.03 g
• 8 H 8(1.01) 8.08 g
• Total MW 44.11 g

Carry out mass to the tenths place.
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Example Determine the percent composition of
iron(III) sulfate.

• Iron(III) sulfate Fe2(SO4)3
• 2 Fe 2(55.85) 111.70 g
• 3 S 3(32.07) 96.21 g
• 12 O 12(16.00) 192.00 g
• Total MW 399.91 g

Carry out mass to the tenths place.
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Hydrated Compounds

• Some compounds exist in a hydrated state.

• Some specific number of water molecules are
  present for each molecule of the compound.

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Example oxalic acid (COOH)2 can be obtained in
the laboratory as (COOH)22H2O.

• Note the dot in (COOH)22H2O shows that the
  crystals of oxalic acid contain 2 water molecules
  per (COOH)2 molecule.

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Example oxalic acid (COOH)2 can be obtained in
the laboratory as (COOH)22H2O.

• The molar mass of (COOH)2 90.04 g/mol
• The molar mass of (COOH)22H2O 126.08 g/mol
• Water can be driven out of a hydrated compound by
  heating it to leave an anhydrous (without
  water) compound.

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Example A 7.0 g sample of calcium nitrate,
Ca(NO3) 24H2O, is heated to constant mass. How
much anhydrous salt remains?

• mass anhydrous salt
• Mass of anhydrous salt remaining

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Hydration Number
Some molecules attach themselves to water
molecules. This is done in set numbers,
depending on the molecule. For example,
Magnesium sulfate attaches to 7 water molecules.
We say its hydration number is
7 MgSO4?7H2O (Magnesium Sulfate Heptahydrate)
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Anhydrides
A compound that is normally a hydrate and has
lost its hydration water is said to be anhydrous
and is called an anhydride. BaCl2?2H2O Barium
Chloride Dihydrate BaCl2 Barium Chloride
Anhydride -or- Anhydrous Barium
Chloride
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Finding the Hydration Number
The hydration number can be conveniently found
by heating the compound and measuring its mass
loss. This mass loss is usually due to the
hydration water molecules being driven off. For
example A 15.35 g sample of Strontium nitrate,
Sr(NO3)2?nH2O, is heated to a constant mass of
11.45 g. Calculate the hydration number.
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Sample Data
Mass Hydrate 15.35g Mass Anhydride
11.45g Mass of Water (mass loss) 3.90g
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Calculations
Moles Anhydride Moles Water Molar ratio
Hydration Number is 4, Sr(NO3)2?4H2O (Strontium
Nitrate Tetrahydrate)