Download Chemical Quantities

It matters how much matter matters.
Review
• Matter is made up of either elements or compounds.
• Elements are pure substances made of one kind of atom.
They cannot be broken down into simpler chemical
substances.
• Compounds are two or more elements chemically combined
into an unchangeable proportion. They can be broken down
into simpler substances.
• A Chemical formula indicates how many of each element is
in the compound
• The amount of matter is often measured in 3 ways
• Mass in grams (100 grams of potato chips)
• Volume in liters (2-liter bottle of soda)
• By count (12 eggs)
Chemical Quantities
• Measuring by Count:
• Atoms in a sugar molecule
• Demonstration: Sugar & Sulfuric Acid
(draw in your notes & make 3 observations)
• Sugar is made from carbon, hydrogen and oxygen.
• The addition of sulfuric acid causes bonds to break
(absorb energy), form new bonds (release energy) and
produces carbon and water
•
•
•
•
Formula for sugar: C12H22O11
12 Carbon
22 Hydrogen
11 Oxygen
Elements & Compounds
Which element is most abundant in a sugar molecule; carbon,
hydrogen, or oxygen?
• Hydrogen has the most atoms in the molecule, but only
makes up 6.49% in terms of mass
• Oxygen has the fewest but makes up 51.41% of the
mass of sugar.
How is that possible?
• Oxygen is much more massive. In fact, it has 16 times the
mass of hydrogen.
Elements & Compounds
• Atomic Mass: The weighted average of the masses of all the
isotopes of an element.
• Found on the Periodic Table
• Measured in amu (atomic mass units)
• Atomic masses on the P.T. are not exact masses but are
averages of all the isotopes multiplied by their abundance
in real life.
= 12.01 amu
Examples: Atomic mass of carbon
hydrogen = 1.01 amu
oxygen = 16.00 amu
Elements & Compounds
• Formula Mass: the sum of the atomic masses of all the atoms
in a chemical formula.
•
Examples:
C12H22O11
144.12amu
carbon: 12.01amu x 12 atoms =
22.22amu
hydrogen: 1.01amu x 22 atoms =
oxygen: 16.00amu x 11 atoms = + 176.00amu
342.34amu
• For sig. figs, assume the atomic masses in the P.T. are
measurements
•
•
•
•
•
•
•
Find the formula masses for:
H2O
CO2
Na2SO4
CH2O
Fe2O3
C6H12O6
Percent Composition

Percent composition: percent by mass of a element that is
contained in a compound
 total mass of the element divided by the total mass of the
compound, multiplied by 100.
 Example: What percent does hydrogen make up in Sucrose,
C12H22O11?
+144.12amu
Carbon: 12 atoms x 12.01amu =
+ 22.22amu } the part
Hydrogen: 22 atoms x 1.01amu =
Oxygen: 11 atoms x 16.00amu = +176.00amu
342.34amu } the whole
% comp = 22.22amu
x100
342.34amu
% comp = 6.49%
Percent Composition
 % composition formula works with mass in grams too
Example: 200.grams of sugar was decomposed into
84.5grams of carbon. What percent is carbon in the
sucrose?
% mass of carbon =
% mass of carbon =
84.5g carbon x 100
200.g sucrose
42.25%
Hydrates
• Hydrate – a substance that has a specific number of water
molecules bonded to each of its formula units.
• Most often a salt
• Example: sodium carbonate decahydrate: Na2CO3∙10H2O
• The dot represents a loose bond between Na2CO3 and H2O
• Coefficient shows how many water molecules are in the
formula
• What percent of water (H2O) is in the hydrate Na2CO3∙10H2O?
• Use % composition formula
45.98amu
Na: 22.99amu x 2 atoms :
12.01amu
C: 12.01amu x 1 atom :
48.00amu
O: 16.00amu x 3 atoms :
20.16amu
H: 1.008amu x 20 atoms:
+160.00amu the part
O: 16.00amu x 10 atoms:
286.15amu } the whole
% comp = (20.16amu + 160.00amu) x 100
286.15amu
% comp = 62.960%
}
Empirical Formula
Chemical (molecular) formula tells us the precise number of
atoms in a compound
● Empirical formula – the simplest ratio of atoms in a
compound
● Ex. glucose: C H
6 12O6
● ratio is 6:12:6, which can be reduced to 1:2:1
● therefore, the empirical formula for glucose is CH O
2
●
●
Ex:
N2H4
→
C3H6
→
Na2CO3 →
NH2
CH2
Na2CO3
Empirical Formula
Molecular Formula from Empirical Formula
· Determine the molecular formula for hydrazine if the empirical
formula is NH2 and the formula mass is 32.06amu.
Step 1: Find the empirical formula mass.
nitrogen: 1 atom
hydrogen: 2 atoms
x
x
14.01amu = 14.01amu
1.01amu = +2.02amu
16.03amu
Step 2: Divide the formula mass given by the empirical formula mass.
formula mass:
32.06amu = 2
empirical formula mass: 16.03amu
Step 3: Take the result and multiply it the number of atoms in the empirical
formula
N(1x2)H(2x2)= N2H4