Question

An organic compound having empirical formula CH3 has a molar mass 30g/mol.What will be it's molecular formula?

C2H4

C3H8

C2H6

CH4

​

Answer 1

Answer:

Let the mass of unknown compound be 100 g.

Given that the compound contains 80% of carbon and 20% of hydrogen.

Therefore, weight of carbon in the compound =80g

weight of hydrogen in the compound =20g

Molecular weight of carbon =12g

Molecular weight of hydrogen =1g

∴ moles of carbon in the compound =

12

80

=6.67

Moles of hydrogen in the compound =

1

20

=20

Ratio of no. of moles of carbon to no. of moles of hydrogen-

No. of moles of carbon : no. of moles of hydrogen =6.67:20≈1:3

∴ 3 moles of hydrogen are present in the compound for 1 mole of carbon.

Hence, the empirical formula of the compound is CH

3

.

empirical mass=15 g/mol

Molecular formula =(CH

3

)

n

whereas,

n=

empirical mass

molecular mass

=

15

30

=2

Thus, the molecular formula of the compoud is C 2H

6

Answer 2

The correct option is C2H6.

• The empirical formula of a compound gives the simplest whole-number ratio of the atoms present in it.
• In this case, the empirical formula is CH3, which means that the compound contains one carbon atom and three hydrogen atoms.
• The molar mass of the compound is given as 30 g/mol. This is the mass of one mole of the compound.
• We can use this information to calculate the molecular formula of the compound.

To do this, we need to find the actual molecular mass of the compound. We can do this by dividing the molar mass by the empirical formula mass:

Molecular mass = (Molar mass) / (Empirical formula mass)

The empirical formula mass of CH3 is:(1 x atomic mass of C) + (3 x atomic mass of H) = (1 x 12.01) + (3 x 1.01) = 15.04 g/mol

Substituting the values given in the question, we get:

Molecular mass = 30 g/mol / 15.04 g/mol = 1.995 g/mol

This tells us that the molecular formula of the compound is twice the empirical formula. Therefore, the molecular formula is C2H6.

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