Name: _____________________________ ( ) Class: _________ Date: __________

Empirical Formula:- Simplest formula of a compound

- Shows the types of _______________ present in the compound

- Shows the simplest _______________ of the different types of atoms in it

Worked Example 1:A sample of an oxide of copper contains 8g of copper combined with 1g of oxygen. Find the empirical formula of the compound.

Element Cu OMass of element

Ar No. of Moles

(Mass/ Ar)Molar ratio

(Divide by smallest number)

Empirical formula of the compound is Cu2O1 or (usually written as this Cu2O)

Worked Example 2:A compound has the following percentage composition: Sodium 32.4 %, Sulphur 22.6%, Oxygen 45.0 %(Note: The percentage of each element is directly proportional to its mass in grams. Thus, the mass in each element in 100g of compound is the same as its percentage in the compound.)

Element Na S O% composition by mass

Ar No. of Moles

Molar ratio ( divide by smallest number)

Empirical formula: Na2SO4

Molecular Formula

• shows the ___________________________________________________________ in a molecule

• sometimes the same as the empirical formula, i.e. in actual the compound exists in the simplest ratio For example: water and ammonia

• possible for different compounds to have the same empirical formula For example: Ethane has a molecular formula of C2H4; Propane has a molecular formula of C3H6, Therefore, they have the same empirical formula which is CH2.

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Secondary 3 Express Chemistry (with SPA)Chapter 9: The MoleNotes 4 (Finding the formula of a compound)

If the compound’s molecular formula and empirical formula are different, the molecular formula is just a multiple of the empirical formula

To find the molecular formula of a compound we use this method, n is the multiple to the empirical formula

Once you find n, Molecular Formula = (Empirical formula ) n

Worked example 3Propane has the empirical formula CH2. The relative molecular mass of propane is 42. Find the molecular formula of propane

n = relative molecular mass of the compound Mr of the empirical formulan = 42 (12 X 1) + 2 = 3

Molecular formula = (Empirical formula ) n

= (CH2)3

= C3H6 Hence the molecular formula for propane is C3H6

Worked example 4Compound X contains 40.0% carbon, 6.6% hydrogen and 53.3% oxygen. Its relative molecular mass is 180. What is the molecular formula of X?

Element C H O% composition by mass

Ar No. of Moles

Molar ratio ( divide by smallest number)

Empirical formula is:

n =

=

=

Molecular formula of X = (CH2O)6 = C6H12O6

Practice Questions1. Find the empirical formula of the following:

(a) A compound consisting of 3.5g of nitrogen combined with 8.0g of oxygen

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n =

Element N OMass of element

Ar No. of Moles

Molar ratio

Empirical formula:

(b) A compound consisting of 2.4g of carbon , 6.4g of oxygen and 0.2g of hydrogen

Empirical formula:

(c) A compound with composition of 75% carbon and 53.3% hydrogen by massElement C H

% composition by mass Ar

No. of Moles

Molar ratio

Empirical formula:

(d) A compound with composition 11.3% carbon, 43.4% sodium and 45.3% oxygen by mass

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2. 17.4g of a compound of chlorine and oxygen contains 3.2g of oxygen. Find the formula of the compound.

3. The empirical formula of a compound is H2CO. Its relative molecular mass is 90. Find the molecular formula

4. The empirical formula of a compound is COH3. Its relative molecular mass is 62. Find the molecular formula.

5. An unknown compound contains 42.5% chlorine and 57.5% oxygen. Its molecular formula mass is 167. What is its molecular formula?

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