Chapter 10 – Measuring Chemical Quantities
Matter is measured in different ways:
- Volume
- Number
- Mass
Sometimes definitions are used to measure matter
1 dozen = 12 of something
For chemical quantities we use a definition called a mole
-
One mole is equal to 6.02 x 1023 representative particles
o Avagadro’s Number
A representative particle can be an atom, molecule or compound. It must be
specified in the problem.
A mole of any substance is equal to 6.02 x 1023 units.
When converting moles to particles we use one of two conversion factors.
1. 1 mole/6.02 x 1023 particles
or
2. 6.02 x 1023 particles/ 1 mole
If converting particles to moles, use conversion factor number 1.
If converting moles to particles, use conversion factor number 2.
Mass- Moles
In order to convert mass to moles, you must find the molar mass of the compound or
element. This information in located on the periodic table. The atomic mass of the
elements is converted to grams to obtain the molar mass.
EX. NaCl
Na has an atomic mass of 23 g. and Cl has an atomic mass of 35.5g
23 + 35.5 = 58.5 g of NaCl in l mole
The Molar Mass is different for all compounds.
EX. CaCO3
Ca = 40 g
C = 12 g
O = 3(16) = 48 g
Total = 100g / mole
There are two conversion factors for converting mass to moles.
3. 1 mole/ molar mass
or
4. Molar mass/ 1 mole
If converting mass to moles, use conversion factor 3.
If converting moles to mass, use conversion factor 4.
Moles to volume
1 mole of any gas occupies 22.4 L
This calculation only works at standard temperature and pressure (STP)
Standard Pressure is 101.3 kPa or 1 atm
Standard Temperature is 00C
Avagadro’s hypothesis
- equal volumes of gas at the same temperature have the same number of
particles.
There are two conversion factors for converting moles to volume.
5. 1 mole / 22.4 L
or
6. 22.4 L / 1 mole
To convert volume to moles, use conversion factor 5.
To convert moles to volume, use conversion factor 6.
EX. 34 Liters of O2 gas converted to moles
34 L of O2 x 1 mole/ 22.4 L = 1.5 moles of O2 gas
3.4 moles of N2 gas to liters
3.4 moles N2 x 22.4 L/ 1mole = 76 L of N2 gas
YOU DO NOT TAKE INTO ACCOUNT MASS OR PARTICLES WHEN
CALCULATING VOLUME. 34 L OF ANY GAS = 1.5 MOLES!
Calculating the density of a gas
Density = molar mass/ molar volume
Density of O2 gas
Molar mass of O2 = 32 g
Molar volume of O2 = 22.4 L
Density = 32g/22.4L = 1.4 g/L of O2
Percent Composition
Calculate how much of the whole each
element contributes.
1. Find the mass of each element in
the compound.
2. Divide that mass by the total
mass.
3. Multiply by 100.
Ex. Find the percent composition of
CO2.
Total mass of the molecule is 44g.
Carbon is 12g / 44g = 0.27 x 100 = 27%
2 Oxygen are 32g / 44g = 0.73 x 100 = 73%
Find the % composition of
 NO2
 CaCO3
 Al2S3
We can use % composition to determine
empirical formula.
The empirical formula is the lowest
whole number ratio in a compound.
The molecular formula is the actual
ration of the elements in a compound.
CH2 is an empirical formula
C2H4 is the molecular formula
C3H6 is the molecular formula
H2O is both molecular and empirical
The empirical formula also tells us the
ratio of moles of atoms.
CO2 has 1 mole of Carbon for every 2
moles of oxygen.
We can use percent composition to
determine empirical formulas.
 Assume you have 100g
 Percentages then become grams
 Convert grams to moles
 Find the lowest whole number
ratio by dividing by the smallest
number
We have a compound that is 38.67% C,
16.22% H, and 45.11% N.
Assume 100g
So we have a sample with:
38.27g of C x 1 mole C / 12.01g of C =
3.220 moles of C
16.22g of H x 1 mole H / 1.01g of H =
16.09 moles of H
45.11g of N x 1 mole N / 14.01g of N =
3.219 mole of N
Nitrogen is the smallest amount of
moles.
3.220/3.219 = 1
16.09/3.219 = 5
Formula is C1H5N1
A compound is 43.64% P and 56.36% O.
What is the empirical formula?
Caffeine is 49.48% C, 5.15% H, 28.87%
N, and 16.49% O. What is the empirical
formula?