Simplest Formula or Empirical Formula

When 100 g of  H_xO_y à H     +   O

                                                                                    11.1g      88.9g

The ratio of   mass of C / mass of H =  88.9 g/ 11.1g =  8

doesn’t tell us anything about the formula. only that Oxygen by weight makes up 8 times the mass of Hydrogen.

mol of H  = 11.1g  ( 1mol/1.0g) = 11.1 mol H

mol of O =  88.9g ( 1mol/16.0g)=  5.56 mol H

 The ratio of H/O =  11.1/5.56=  2.00

 does tell us the simplest or empirical formula  must be H₂O .

but the real formula (molecular formula) could be

                                                                          H₄O₂

                                                                          H₆O₃

                                                                          H₈O₄ 

↓

We need to know the molecular weight of the unknown  compound to determine its true MOLECULAR FORMULA:

What if the molecular weight was 54 g/mol.

Which one would it be?

H₆O₃

Really it is 18.0 g/mol … so the unknown formula is H₂O  !

PRACTICE:

- Simplest Formula Worksheet

- Heath Text p. 123  Q 17- 27