Question

Styrofoam is made by blowing a gas into a polymer called polystyrene, which in turn is made from styrene. Styrene is 92.26% carbon and the rest hydrogen. 7.05x10^18 molecules of styrene weigh 1.22mg. What is the molecular formula for it?

Please give step by step.

Answer 1

7.05x10^18 molecules = 7.05x10^18 / (6.022*10^23) mol

= 1.171*10^-5 mol

mass = 1.22 mg = 1.22*10^-3 g

so,

molar mass = mass / number of mol

= 1.22*10^-3 g / 1.171*10^-5 mol

= 104.2 g/mol

Now calculate the empirical and molecular formula:

we have mass of each elements as:

C: 92.26 g

H: 7.74 g

Divide by molar mass to get number of moles of each:

C: 92.26/12.01 = 7.681932

H: 7.74/1.008 = 7.678571

Divide by smallest to get simplest whole number ratio:

C: 7.681932/7.678571 = 1

H: 7.678571/7.678571 = 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 = 104.2 g/mol

Empirical formula mass = 13.018 g/mol

Multiplying factor = molar mass / empirical formula mass

= 104.2/13.018

= 8

Hence the molecular formula is : C8H8