Question

A hydrogen x has relative molecular mass 56 consist of 87.8 percent by mass of carbon and 12.2 percent hydrogen determine the empirical formula of x and hence give its molecular formula and the iupac nomenclature of the compound formed.

Answer 1

$$w(C) = \frac {Ar(C)\times x}{Mr(C_xH_y)}$$

$$\therefore x = \frac {w(C) \times Mr(C_xH_y)}{Ar(C)} = \frac{0.878 \times 56}{12} = 4$$

$$w(H) = \frac {Ar(H)\times y}{Mr(C_xH_y)}$$

$$\therefore y = \frac {w(H) \times Mr(C_xH_y)}{Ar(H)} = \frac{0.122 \times 56}{1} = 6.832$$

y should be integer. There obviously should be a mistake in the task as there is no such compound C₄H_7.

Mollar mass 56 g/mol corresponds to alkene C₄H₈, where w(C) = 85.7 % and w(H) = 14.3 %.

If so, we can determine x and y:

$$x = \frac {w(C) \times Mr(C_xH_y)}{Ar(C)} = \frac{0.857 \times 56}{12} = 4$$

$$y = \frac {w(H) \times Mr(C_xH_y)}{Ar(H)} = \frac{0.143 \times 56}{1} = 8$$

The molecular formula is $C_4H_8$ butene-1 or butene-2 , empirical formula is $CH_2$