applied science - unit 19 b part 3- chemistry -Determine empirical and molecular formulae- distinction level

Section 3: Determine empirical and molecular formulae from percentage elemental composition
data and mass spectra.

Introduction:

how to calculate empirical formula

firstly, you convert % to grams.

secondly you divide the grams by the AR of the element to find the mole.

thirdly, divided the moles of each element by the smallest mole.

Fourthly, multiply if needed to make it a whole number.



Describe the molecular ion peak on a mass spectrum

A scientist will use a technique called mass spectrometry if they want to study a molecule. In mass
spectrometry, the molecules are bombarded with special particles that give it a positive charge. This
charged molecule is then called a molecular ion. The molecular ion peak is the highest peak in the
graph.

explain how this can be used to work out molecular formula from empirical formula

you can obtain the molecular formula by dividing empirical formular from the molecular ion peak



molecular ion peak/ MR of empirical formula = molecular formula



Compound P

Percentage Elemental Composition:

Carbon: 92.3% Hydrogen: 7.7%

Empirical Formula calculation:

Step 1

92.3% = 92.3g

7.7% = 7.7g

Step 2

92. = 7.69

7.7/1 = 7.7

Step 3

7.69/7.69 = 1