A certain compound contains carbon and oxygen. Analysis show it has 36% carbon and 64% oxygen.If its molecular weight is 400 then what is the molecular formula of the compound?

We are given that the percentage of carbon in the compound is 36 % while that of oxygen is 64 %. Hence 100g of compound would contain 36 g of carbon and 64 g of oxygen. 

So number of moles in 36 g of Carbon = mass / molar mass of carbon = 36 / 12 = 3

similarly number of moles in 64 g of oxygen = mass / molar mass of oxygen = 64 / 16 = 4

We will now calculate the simplest ratio. This will be done by dividing the number of moles of each atom by smallest number which is three. Hence for carbon we have  

 = 3 / 3 = 1

for oxygen

 = 4 / 3 = 1.33

Therefore the empirical formula of the compound is C₁O_1.33. In terms of whole number, this will be = (C₁O_1.33)₃ = C₃O_3.99

= C₃O₄.

Now the empirical mass is 100. We are given that molecular weight is 400. Let the molecular formula of the compound = [C₃O₄]_n.

 n = molecular weight / empirical mass

 = 400 / 100 = 4

Thus the molecular formula is [C₃O₄]₄ = C₁₂O₁₆.