Question

7.00 g of a certain Compound X, known to be made of carbon, hydrogen and perhaps oxygen, and to have a molecular molar mass of 26. g/mol, is burned completely in excess oxygen, and the mass of the products carefully measured: mass product carbon dioxide water 23.70 g 4.85 g Use this information to find the molecular formula of X. x 6 ?

Answer 1

let in compound number of moles of C, H and O be x, y and z respectively

Number of moles of CO2 = mass of CO2 / molar mass CO2

= 23.7/44

= 0.5386

Number of moles of H2O = mass of H2O / molar mass H2O

= 4.85/18

= 0.2694

Since 1 mol of CO2 has 1 mol of C

Number of moles of C in CO2= 0.5386

so, x = 0.5386

Since 1 mol of H2O has 2 mol of H

Number of moles of H = 2*0.2694 = 0.5389

Molar mass of O = 16 g/mol

mass O = total mass - mass of C and H

= 7.0 - 0.5386*12 - 0.5389*1

= -2.525*10^-3

Since this is negative, there is no oxygen present.

Divide by smallest to get simplest whole number ratio:

C: 0.5386/0.5386 = 1

H: 0.5389/0.5386 = 1

So empirical formula is:CH

Molar mass of CH,

MM = 1*MM(C) + 1*MM(H)

= 1*12.01 + 1*1.008

= 13.018 g/mol

Now we have:

Molar mass = 26.0 g/mol

Empirical formula mass = 13.018 g/mol

Multiplying factor = molar mass / empirical formula mass

= 26.0/13.018

= 2

So molecular formula is:C2H2

Answer: C2H2