Question

A hydrocarbon contains 17.2% of hydrogen. if the vapour density of the compounds is 29 then calculate its molecular formula.

Answer 1

The molecular formula of a compound represents the actual number of atoms of various elements present in a molecule of a compound.

Whereas, Empirical formula represents the simplest whole number ratio in which the atoms are present in a molecule of that compound.

In order to derive molecular formula of any unknown compound, we need to know the following relations:

Molecular formula = n * Empirical formula, where n = molecular formula mass/empirical formula massMolecular mass = 2 * Vapor density

Now we know the percentage of carbon in the hydrocarbon which is 82.76% so we can assume the rest of the element is hydrogen only and then its percentage will be 17.24%.

Considering 100 g of the sample

mass of carbon = 82.76 g

mass of hydrogen = 17.24 g

We can convert these masses in terms of moles by dividing with respective molar masses

Number of moles of carbon = 82.76/12 = 6.8 moles

Number of moles of hydrogen = 17.24/1 = 17.24 moles

We need to convert these number of moles in terms of simple whole number ratio, we can divide both by smaller one in order to find it. Hence,

6.8/6.8 = 1

17.24/6.8 = 2.5

We multiply both by 2 to convert them in simple whole numbers

Carbon=2 and hydrogen = 5

Thus Empirical formula of the compound is given by, C2H5

Empirical formula mass of C2H5 = 2*12 + 5*1 = 29 g

Now we can determine molecular formula mass from vapor density = 2 * 28 = 58 g

Therefore n = 58/29 = 2

Thus Molecular formula of the compound = 2 * C2H5 = C4H10

Now,

Molecular mass = Vapour Density x 2 = 29 x 2 = 58

One mole of given hydrocarbon has 58 grams of mass. In that 82.76% is carbon. So mass of of carbon is 58 X 82.76 /100 = 48 g

Now find the number of moles present is 48g of carbon. Number of moles = Mass/GramMolecularMass = 48/12 = 4

Therefore, there are 4 moles of Carbon atoms in 1 mole of given Hydrocarbon molecules.

Same way , calculate number of moles of Hydrogen. { Hydrocarbon mass is 58 and carbon mass is 48. So hydrogen mass is 10. Since Gram Molecular Mass of Hydrogen is one, number of moles is 10/1 = 10 }

So there are 4 moles of Carbon atoms and 10 moles of Hydrogen atoms in one mole of hydrocarbon molecule.

Answer 2

Answer:

CH3

Explanation:

Name %comp moles. Simple Ratio

C. 82.8% 6.9. 1

H. 17.2%. 17.2. 3

Empirical Formula CH3