A compound is made up of 80.0 % C and 20.0 % H. Its molar mass is 30.0 g. What is its molecular formula? a. C2H6

b. CH

c. C4H12

d. CH3

Molecular formula = C₂H₆

Explanation:

Given data:

Percentage of hydrogen = 20.0 %

Percentage of carbon = 80.0 %

Molar mass = 30.0 g  

Empirical formula = ?

Solution:

Number of gram atoms of H = 20 / 1.01 = 19.8  

Number of gram atoms of C = 80 /12 = 6.7

Atomic ratio:

            C              :           H            

         6.7/6.7        :        19.8/6.7  

            1               :           3        

C : H = 1 : 3

Empirical formula is CH₃.

Molecular formula:

Molecular formula = n (empirical formula)

n = molar mass of compound / empirical formula mass

Empirical formula = CH₃ = 12×1 +3×1.01 = 15.03 g/mol

n = 30 g/mol / 15.03 g/mol

n = 2

Molecular formula = n (empirical formula)

Molecular formula = 2 (CH₃)

Molecular formula = C₂H₆

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