Atomic Theory

(ii)              Find the molecular formula of this compound given that its molecular mass is 90.  

(i) Following the steps indicated in example 1 above:

Step 1       C = 26.7  =  2.225             H = 2.2 = 2.200             O = 71.1 =  4.444 

                         12                                    1                                     16

Step 2           Smallest ratio = 2.200

                 C = 2.225 =  1.01               H = 2.200 = 1                O = 4.444 = 2.02             

                       2.200                                2.200                            2.200

                      So, the empirical formula of this compound must be: CHO2

(ii) The mass of the empirical formula found in (i) is 45a.m.u. But it is given that, the molecular mass is 90. This means that the molecular mass (90) is double the empirical formula mass (45). Therefore the molecular formula must be twice the empirical formula.