Please Help! Im confused

Question

A compound has a molar mass of 28.1 g/mol and an empirical formula of CH2. What is the molecular formula?

Answer 1

The question is basically asking how many times will the empirical formula go into the given molar mass of 28.1 g.  When you figure out how many times the mass of the empirical formula will go into the given molar mass just multiply that throughout the empirical formula.

 

Find the mass of the empirical formula: CH₂ = 12.01 + 2(2.02) = 14.03. 

 

Divide the molar mass by the mass of the empirical formula: 28.1/14.03=2

 

Multiply the empirical formula by the factor that you just got of 2: 2(CH₂) = C₂H₄

Answer 2

So far, you know there are 2 hydrogen atoms for each carbon atom, so the formula is something like (CH₂)_n, where n is unknown.  But you also know the molecular weight is 28.1 g/mole.  Use the atomic weights of C and H, and try a few values of n until you get a molecular weight of 28.1

 

C = 12.01

H = 1.01

CH₂ = 2(1.01) + 12.01 = 14.03

 

Note n = 1 gives a molecular weight of 14.03 g, so n > 1.

 

Try n = 2:

 

(CH₂)₂ = 2*14.03 = 28.06 ≅ 28.1

 

This is close, so the formula is likely to be (CH₂)₂.  It could also be C₂H₄.