Question

A 2.335 gram sample of an organic compound containing C, H and O is analyzed by combustion analysis and 5.030 grams of CO2 and 2.060 grams of H2O are produced. In a separate experiment, the molar mass is found to be 102.1 g/mol. Determine the empirical formula and the molecular formula of the organic compound.

Enter the elements in the order C, H, O

empirical formula =

molecular formula =

Answer 1

All the Carbon in Hydrocarbon will completly converted to CO₂

So the number of moles of CO₂ = mass of CO₂ / Molar mass of CO₂

                                               = 5.030 g / 44 (g/mol )

                                               = 0.114 mol
Each CO₂ has 1 mole of C in it, therefore the number of moles of C = moles CO₂ = 0.114 mol

mass of C = number of moles x molar mass = 0.114 mol x 12 g/mol = 1.368 g

All the Hydrogen present in the given hydrocarbon will ends up into H₂O

So the number of moles of H₂O = mass of H₂O / Molar mass of H₂O

                                              = 2.060 g / 18(g/mol)

                                              = 0.114 mol

Each H₂O has 2 H atoms, so number of moles of H = 2 x number of moles of H₂O = 2x0.114 = 0.228 mol

So mass of H = 0.228 mol x 1 g/mol = 0.228 g

So mass of O = mass of sample - ( mass of C + Mass of H)

                      = 2.335 - ( 1.368 + 0.228)

                      = 0.739 g

Number of moles of O = mass/molar mass = 0.739 g / 16(g/mol) = 0.046 mol

The ratio of moles of C , H and O is = 0.114 mol : 0.228 mol : 0.046 mol

Divide each number in the ratio by the smallest number we get

                                       = (0.114/0.046) : (0.228/0.046) : (0.046/0.046)

                                       = 2.5 : 5 : 1

In order to make it a whole number , we have to multiply the ratio by the smallest number needed to get all whole numbers, in this case we have to multiply it by 2. then we get

                                      = 5 : 10 : 2

Therefore the emperical formula is C₅H₁₀O₂

Emperical formula mass = ( 5x12) + (10x1) + (2x16) = 102

Given molecular mass is 102.1 g/mol

So n = Molar mass/formula mass = 102.1 / 102 = 1

So molecular formula = n(Emperical formula)

                               = 1 (C₅H₁₀O₂)

                               = C₅H₁₀O₂