Chemistry 11
Molecular Formula
Molecular Formula
The molecular formula is the actual formula and is simply a multiple of the empirical formula.
Empirical
HO
CH
CH
NO
NO2
Molecular
H2O2
C2H2
C6H6
NO
N2O4
To find the molecular formula, you must figure out how many empirical formulas it takes to equal
the molar mass of the molecular formula.
Divide the molecular formula’s molar mass by the empirical formulas molar mass
Example 1.
A certain compound has an empirical formula of ClCH2 and a molecular weight of 98.96
g/mol. What is its molecular formula?
1. Find the mass of the empirical formula.
Mass of empirical formula = 35.45 + 12.00 + 2(1.008) = 49.47 g/mol
2. Figure out how many times bigger the molecular formula is
mass of molecular formula
mass of empirical formula
= 98.96 g/mol
49.47 g/mol
=2
3. Write the molecular formula.
It takes two empirical units to make a molecular unit, so, the molecular formula is:
Cl2C2H4
Example 2.
Given that the empirical formula of a compound is CH and the molar mass is 104 g/mol, calculate
the molecular formula.
mass of C = 12.0 g/mol
mas of H = 1.01 g/mol
empirical formula mass = 13.0 g/mol
to find the number of CH units in the compound:
# CH units = (104 g/mol)(1 mol/13.0 g) = 8.00
Molecular formula = 8(CH) or C8H8
Chemistry 11
Molecular Formula
Example 3.
A compound composed of hydrogen and oxygen are analyzed and a 10.0g sample yields 2.36 g of
hydrogen and 18.8g of oxygen. The molecular mass of this compound is 72.0g/mol. What is the
empirical an molecular formula for this compound?
2.36g x 1 mol/1.0g = 2.36mol
18.8g x 1mol/16.0g = 1.175mol
2.36/1.175 = 2.00
1.175 / 1.175 = 1.00
empirical formula is H2O , has a molar mass of 18.0g/mol
72.0 / 18.0 = 4
molecular formula = H8O4
Now it is time for the difficult questions. You will not be given the molar mass of
the molecular formula and you will be asked to find the molar mass given many
different pieces of information. ALLWAY REMEMBER THE UNITS FOR
MOLAR MASS IS g/mol
For example:
1. from moles and grams
Find the molar mass if 0.0250 moles of X has a mass of 1.775g
= 1.775g/0.0250mol = 71.0g/mol
2. from the density of a gas at STP
Find the molar mass if the density of gas X is 1.43 g/L at STP.
1.43 g/L x 22.4L/1mol = 32.0g/mol
3. from the mass and volume of a gas at STP
Find the molar mass if 0.0425L of gas X at STP has a mass of 0.135g
D = m/v 0.135g/0.0425 = 3.176g/L
3.176g/l x 22.4L/mol = 71.0g/mol
Chemistry 11
Molecular Formula
Molecular Formula
Student handout
The molecular formula is the actual formula and is simply a multiple of the empirical formula.
Empirical
HO
CH
CH
NO
NO2
Molecular
H2O2
C2H2
C6H6
NO
N2O4
To find the molecular formula, you must figure out how many empirical formulas it takes to equal
the molar mass of the molecular formula.
Divide the molecular formula’s molar mass by the empirical formulas molar mass
Example 1.
A certain compound has an empirical formula of ClCH2 and a molecular weight of 98.96
g/mol. What is its molecular formula?
1. Find the mass of the empirical formula.
Mass of empirical formula = 35.45 + 12.00 + 2(1.008) = 49.47 g/mol
2. Figure out how many times bigger the molecular formula is
mass of molecular formula
mass of empirical formula
= 98.96 g/mol
49.47 g/mol
=2
3. Write the molecular formula.
It takes two empirical units to make a molecular unit, so, the molecular formula is:
Cl2C2H4
Example 2.
Given that the empirical formula of a compound is CH and the molar mass is 104 g/mol, calculate
the molecular formula.
Chemistry 11
Molecular Formula
Example 3.
A compound composed of hydrogen and oxygen are analyzed and a 10.0g sample yields 2.36 g of
hydrogen and 18.8g of oxygen. The molecular mass of this compound is 72.0g/mol. What is the
empirical an molecular formula for this compound?
Now it is time for the difficult questions. You will not be given the molar mass of
the molecular formula and you will be asked to find the molar mass given many
different pieces of information. ALLWAY REMEMBER THE UNITS FOR
MOLAR MASS IS g/mol
For example:
4. from moles and grams
Find the molar mass if 0.0250 moles of X has a mass of 1.775g
5. from the density of a gas at STP
Find the molar mass if the density of gas X is 1.43 g/L at STP.
6. from the mass and volume of a gas at STP
Find the molar mass if 0.0425L of gas X at STP has a mass of 0.135g
Try questions pg 95 # 47, 48, 49, 52, 54