Emperical Formula explained with worked example

Biola Emmanuel - Sat, 16th March, 2019 @ 11:00: AM

Topics in Chemistry

Salts and Types of Salt in Chemistry Separation Techniques Separation Techniques part 2 Separation Techniques part 3 Physical Change, Chemical Change, Element, Compound and Mixture Electrolysis, Electrolyte, Electrode, Anode and Cathode Grahams law of diffusion Physical Properties of Metals Daltons Atomic Theory and its Modifications Periodic table and Classification of Elements

Academic Questions in Chemistry

The only metal that exist as liquid at room temperature is _____.

  1. Nickel
  2. Duralumin
  3. Mercury
  4. Lead
  5. Titanium
  6. Cesium

ANSWER is Mercury

Mercury is a liquid metal at room temperature. Its chemical symbol is Hg. The name mercury comes from its Greek name, hydrargyrum, which means "liquid silver" .  Mercury has an atomic number of 80. Please read on the periodic table of elements here

The element mercury is also considered to be named after the fastest-moving planet in the solar system. You can read on solar system here.


Empirical Formula:

Imagine that we have two atoms, say Hydrogen (H) and oxygen (O). If these atoms are combined to form a compound, for instance (HO), we will not be able to tell the ideal formular of the compound if this was an experimental sample, as it could be H2O, H2O​​​2 or something else. This is where empirical formula comes into play. It is the simplest formula of a compound because it is derived from experimental data or analysis. It is the empirical formula that will tell us the relative ratios of different atoms in a compound.


The empirical formula of a compound may be defined as  the formula that shows only the relative number of atoms of each element present in the compound.


Let's consider water molecule (H2O) as an example.

H2O = 2 atoms of hydrogen (H) and 1 atom of oxygen (O).

H2O = 1 mole of hydrogen molecule and 1 mole of oxygen. (Please remember when a number is not written in in front of a compound, then it means that the number "1" is actually in front of that compound. So, the H2O in our example is actually 1H2O (1 mole of water). The 1 is referred to as the Mole Ratio.


Therefore, if we know the amounts of mole for each element in a compound, the emperica formula can be determined.


Note: Whenever we are working with the percentages of each element contained within a compound, we will need to transform the percentages into the mole ratio of the elements in order to get the empirical formula.


Worked Example:

A compound composed of iron (Fe) and oxygen (O) was analyzed and found to contain 69.94% of iron and 30.06% oxygen. Find the empirical formula of the compound. ( Molar mass of Fe=55.85,  O=16)


Step 1: Identify the given parameter from the question.

Fe = 69.94%,   O = 30.06%.

Empirical formulae = Fe?O?


Step 2: Convert the percentages to gram. (just attribute grams to the %)

69.94% = 69.94g while 30.06% = 30.06g


Step 3: Convert the gram to moles using the formula (mole = mass/molarmass). Please merorize this formula

Mole of Fe: 69.94/55.85 = 1.252mol

Mole of O: 30.06/16 = 1.879mol


Step 4: Divide both sides by the smallest mole ratio

Iron has the smallest mole ratio in our case, therefore: 1.252/1.252 = 1,    1.879/1.252 = 1.5

We now have the formula = Fe1O1.5


Step 5: Multiply each of the moles by the smallest whole number that will convert each into a whole number. (In our case, the number "2" is the smallest whole number that will make 1.5 and 1 whole numbers when multiplied by it.

For iron (Fe), we will have 1 x 2 = 2

For oxygen (O), we will have 1.5 x 2 = 3


Step 6: Write the empirical formula

The empirical formula= Fe2O3


Amazing facts in Chemistry

Gold and copper are the only two non-silvery colored metals. Please read some interesting facts about gold here

Copper is the only metal that is naturally antibacterial. This is why you would see some children carry copper water bottles to school

Water actually freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy.
Please read on modification of Dalton's atomic theory here.

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre


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