Friday, October 18, 2013

CBSE 9th [Atoms and molecules]Empirical formula and molecular formula

Empirical formula :The empirical formula is the simplest formula for a compound in which atoms of different elements are present in simple ratio. It shows the relative number of atoms of each element. For example CH2O is the empirical formula of Glucose C6H12O6

Calculating Empirical Formula from Percentage Composition

Assume 100g of sample

Convert all percentages to a mass in grams, eg, 21% = 21g, 9% = 9g

Find the relative atomic mass (r.a.m) of each element present using the Periodic Table

Calculate the moles of each element present: n = mass ÷ r.a.m

Divide the moles of each element by the smallest of these to get a mole ratio

If the numbers in the mole ratio are all whole numbers (integers) convert this to an empirical formula

If the numbers in the mole ratio are NOT whole numbers, you will need to further manipulate these until the 
mole ratio is a ratio of whole numbers (integers)

A compound is found to contain 47.25% copper and 52.75% chlorine. Find the empirical formula for this 

mass in grams
moles = mass ÷ r.a.m
47.25 ÷ 63.6 = 0.74
52.75 ÷ 35.5 = 1.49
divide throughout by lowest number
0.74 ÷ 0.74 = 1
1.49 ÷ 0.74 = 2.01 = 2

Empirical formula for this compound is CuCl2

Molecular formula  : It is the formula in which the actual number of atoms of  different element are present. For example, if the empirical formula of benzene is CH where as molecular formula  is C6H6 , etc.

An empirical formula is often calculated from elemental composition data. The weight percentage of each of the elements present in the compound is given by this elemental composition. 
Let's determine the empirical formula for a compound with the following elemental composition:40.00% C, 6.66% H, 53.34% O....Read full Post
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