Question

Butene, used as the monomer for polybutene, is composed of 85.7% and 14.3% by mass. If the density of butene is 2.50 g/L at STP, what is the molecular formula of butene?

Answer 1

C₄H₈

Step-by-step explanation:

In the first, you should know the ideal gas law. One of the formula for this concept is:

MW = (δ*R*T) / P where:

MW is molecular weight, δ is density 2,50 g/L, R is the gas constant (0,082 (atm*L) / (mol*K) ), T is temperature (273,15 K at STP) and P is pressure (1 atm).

So:

MW = [2,50 g/L * 0,082 (atm*L) / (mol*K) * 273,15 K ] / 1 atm

MW = 56 g/mol

Then the problem says than butene is composed by 85,7% (of carbon) and 14,3 % (of hydrogen). So:

Carbon mass: 56 g/mol * 85,7% = 48 g/mol / 12 g/mol ( atomic mass of carbon = 4 carbon atoms.

And, hydrogen mass: 56 g/mol * 14,3% = 8 g/mol / 1 g/mol ( atomic mass of hydrogen = 8 hydrogen atoms.

Thus, the molecular mass of butene is: C₄H₈

I hope it helps!