2.50g of a certain Compound X, known to be made of carbon, hydrogen and perhaps oxygen, and to have a molecular molar mass of 128. g/mol, is burned completely in excess oxygen, and the mass of the products carefully measured:

product mass
carbon dioxide 8.60g
water 1.41g
Use this information to find the molecular formula of X.

                      Molecular Formula  =  C₁₀H₈

Explanation:

Step 1: Calculating Moles for each Element:

C  =  Mass of CO₂ × (1 mol of CO₂÷ M.Mass of CO₂) × (1 mol of C ÷ 1 mol of CO₂)

C  =  8.60 g × (1 mol of CO₂÷ 44.011 g.mol⁻¹) × (1 mol of C ÷ 1 mol of CO₂)

C =  0.1954 mol

H  =  Mass of H₂O × (1 mol of H₂O ÷ M.Mass of H₂O) × (2 mol of H ÷ 1 mol of H₂O)

H  = 1.41 g × (1 mol of H₂O ÷ 18.106 g.mol⁻¹) × (2 mol of H ÷ 1 mol of H₂O)

H =  0.1557 mol

Step 2: Calculate the Smallest whole number ratio as,

                                 C                                                        H

                              0.1954                                               0.1557

                      0.1954/0.1557                                  0.1557/0.1557

                               1.25                                                       1

Multiply ratio by 4,

                                 5                                                         4

Result:

          Empirical Formula of Propane is C₅H₄

Step 3: Calculate Molecular Formula:

Molecular formula is calculated by using following formula,

                    Molecular Formula  =  n × Empirical Formula   (1)

Also, n is given as,

                     n  =  Molecular Weight / Empirical Formula Weight

Molecular Weight  =  128.0 g.mol⁻¹

Empirical Formula Weight  =  5 (C) + 4 (H) =  64 g.mol⁻¹

So,

                     n  =  128.0 g.mol⁻¹ ÷ 64 g.mol⁻¹

                     n  =  2

Putting Empirical Formula and value of "n" in equation 1,

                    Molecular Formula  = 2 × C₅H₄

                    Molecular Formula  =  C₁₀H₈