Question

Calculate the empirical and molecular formula of the compound containing
80% Carbon, 20% Hydrogen. If the molecular mass of the compound is 30 then determine the molecular formula
please answer my question ​

Answer 1

Answer:We have  ethane .

Explanation:  As with all these problems, it is usually assumed that we have a  100 ⋅ g  mass of unknown compound, and we work out the molar quantities:

And thus  moles of carbon ≡ 80 ⋅ g  / 12.011 ⋅ g ⋅ mo l ^− 1 = 6.66 ⋅ m o l .  

And thus  moles of hydrogen ≡ 20 ⋅ g /  1.008 ⋅ g ⋅ m ol^ − 1 = 19.8 ⋅ m o l .

We divide the molar quantities thru by the SMALLER molar quantity:

C : 6.66 ⋅ m o l /  6.66 ⋅ m o l = 1

H : 19.8⋅ m o l  / 6.66 ⋅ m o l = 2.97

And thus the  empirical formula   =   C H 3  

And we know that the molecular formula is always a multiple of the empirical formula, and thus is terms of mass:

i.e.  molecular formula   =   n × empirical formula  

But we have a molecular mass of  30 ⋅ amu  

So,30 ⋅ amu   =   n × ( 12.01 + 3 × 1.01 ) ⋅ amu  

Clearly ,  n = 2 , and the MOLECULAR FORMULA is C 2 H 6 .

This is not a realistic problem, as few analysts would perform combustion on a liquid, and no analyst could perform combustion on a gas.