1. C2H6 CH3 B. Empirical Formula
Smallest whole number ratio of atoms in a compound
C2H6
reduce subscripts
CH3

2. B. Empirical Formula 1. Find mass (or %) 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.

3. B. Empirical Formula
Find the empirical formula for a sample of 25.9% N and 74.1% O.
25.9 g
1 mol
14.01 g
= 1.85 mol N
= 1 N
1.85 mol
74.1 g
1 mol
16.00 g
= 4.63 mol O
= 2.5 O

4. N2O5 N1O2.5 B. Empirical Formula
Need to make the subscripts whole numbers
 multiply by 2
N2O5

5. STP STP = Standard temperature and pressure Where, For ANY gas at STP,
Temperature = 0°C = 273 K
Pressure = kPa = 1 atmosphere (atm)
For ANY gas at STP,
1 mol = volume of 22.4 L (molar volume) = 6.02 x 1023 particles of that gas

6. Unit of density of a gas = g/L
Can be used to calculate the gram formula mass of that gas
Example 1:
Determine the volume of mol of sulfur dioxide gas at STP

7. More Examples Determine the # of moles in 52.5 L of He gas at STP
Find the Number of Particles in 30 L of sulfur gas at STP
Find the mass of 43 L of nitrogen gas at STP