Question

the compound dimethyl phthalate, used in insects repellants, is composed of carbon, hydrogen, and oxygen and has a molar mass of 194.2 g/mol. A 0.2572 g sample of the compound was burned in a combustion apparatus and 0.583 g of CO2 and 0.119 g of H2O were collected. What is the molecular formula of dimethyl phthalate?

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

= 0.583/44

= 0.01325

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

= 0.119/18

= 0.006611

Since 1 mol of CO2 has 1 mol of C

Number of moles of C in CO2= 0.01325

so, x = 0.01325

Since 1 mol of H2O has 2 mol of H

Number of moles of H = 2*0.006611 = 0.013222

mass o = total mass - mass of C and H

= 0.2572 - 0.01325*12 - 0.013222*1

= 0.084978

number of mol of O = mass of O / molar mass of O

= 0.084978/16

= 0.005311

so, z = 0.005311

Divide by smallest to get simplest whole number ratio:

C: 0.01325/0.005311 = 2.5

H: 0.013222/0.005311 = 2.5

O: 0.005311/0.005311 = 1

multiply all by 2:

C:5

H:5

O:2

So empirical formula is:C5H5O2

Molar mass of C5H5O2,

MM = 5*MM(C) + 5*MM(H) + 2*MM(O)

= 5*12.01 + 5*1.008 + 2*16.0

= 97.09 g/mol

Now we have:

Molar mass = 194.2 g/mol

Empirical formula mass = 97.09 g/mol

Multiplying factor = molar mass / empirical formula mass

= 194.2/97.09

= 2

Hence the molecular formula is : C10H10O4