Question

Terephthalic acid contains C, 0 and H only. Combustion of 19.81 mg of this compound produces 41.98 mg of C02 and 6.45 mg of H20. If the molar mass is between 115 and 125 g determine the molecular formula for terephthalic acid.

Answer 1

Let, Terephthalic acid be C_xH_yO_z

Balanced equation:

C_xH_yO_z + (z-2x-y/2)O₂ = xCO₂ + (y/2)H₂O

Moles of CO₂ = 41.98/44 = 0.95 milli moles

Moles of H₂O = 6.45/18 = 0.36 milli moles

Moles of C_xH_yO_z = 19.81/(12x+y+16z) milli moles

0.95/x = 19.81/(12x+y+16z) equation (1)

0.36/(y/2) = 19.81/(12x+y+16z)

=>0.72/y = 19.81/(12x+y+16z) equation (2)

0.95/x=0.72/y

=>1.32y=x

0.72(12x+y+16z) =19.81y

=>(15.84y+y+16z) =27.5y

=>16z= 10.66y

=>z=0.667y

=>1.5z = y

x ,y and z should be integers

Hence, z=2, y=1.5z=3, x=1.32y =2

Empirical formula= C2H3O₂

Mass of C2H3O₂ =12x2+3+16X2=59

hence Terephthalic acid will be twice of this with weight =59X2 =118g

Terephthalic acid =C4H6O4