Empirical and Molecular Formulas Chemistry

Empirical Formula – the lowest whole-number ratio of the atoms of elements in a compound. For example, C2H2 and C8H8 are both gases. The empirical formula for both is CH.

The empirical formula for: C2H4O2 is CH2O N2O4 is NO2 P4O10 is P2O5

You can calculate an empirical formula from % composition You can calculate an empirical formula from % composition. For a compound with 25.9% nitrogen and 74.1% oxygen, what is its empirical formula?

If you had 100 g of compound, you would have 25. 9g N and 74. 1 g O If you had 100 g of compound, you would have 25.9g N and 74.1 g O. Change this to moles 25.9g N mol N = 1.85 mol N 14 g 74.1 g O mol O = 4.63 mol O 16 g

Find the ratio (divide largest by smallest) 4. 63 = 2. 5 Ratio is 2 Find the ratio (divide largest by smallest) 4.63 = 2.5 Ratio is 2.5 : 1 1.85 O : N What is the next whole number from 2.5? 5 So multiply both by 2. 2.5 X 2 = 5 O5 1 X 2 = 2 N2

Find the empirical formula of a compound of 94. 1% Oxygen, 5 Find the empirical formula of a compound of 94.1% Oxygen, 5.9% Hydrogen

Change to moles 94. 1 g O mol = 5. 9 mol 16 g 5. 9 g H mol = 5 Change to moles 94.1 g O mol = 5.9 mol 16 g 5.9 g H mol = 5.9 mol 1 g Ratio is 1:1 so empirical formula is OH

What is the empirical formula of a compound with 67. 6% Hg, 10 What is the empirical formula of a compound with 67.6% Hg, 10.8% S, and 21.6% O?

67. 6 g Hg mol Hg =. 33 mol Hg 201 g 10. 8 g S mol S = 67.6 g Hg mol Hg = .33 mol Hg 201 g 10.8 g S mol S = .34 mol S 32 g 21.6 g O mol O = 1.35 mol O 16 g 1.35 = 4 1 : 1 : 4 .34 Hg S O HgSO4

Molecular Formulas – is either the same as the empirical formula, or is a whole number multiple. If you know (or calculate) the empirical formula, you can calculate the molecular if you know the compound’s molar mass.

What is the molecular formula for a compound with the empirical formula of CH4N and a molar mass of 60.0 g/mol?

The molar mass of CH4N is 30 g/mol 60 = 2 So 2 x CH4N = C2H8N2 30

What is the molecular formula of a compound with empirical formula CH3O and 62g/mol molar mass?

Molar mass of CH3N = 31 g/mol 62 = 2 So, 2 X CH3N = C2H6N2 31