Name: Law of Multiple Proportions
Uploaded: 2019-07-19
Duration: 6 min 43 s
Description: The Law of multiple proportions is applicable only when two elements combine to form two or more compounds. Watch this video to know how this law is applicable.

Question

What do you understand by empirical and molecular formula of a compound? How are they related? A compound contains carbon, hydrogen, and nitrogen in the ratio $18 : 2 : 7$ . Calculate its empirical formula. If the molecular mass of the compound is $108$ , what is its molecular formula?

Accepted Answer

The empirical formula shows the simplest whole number ratio of atoms of each element present in one molecule of the compound. Whereas, the molecular formula shows the exact number of atoms of each element present in one molecule of the compound.
For example, the molecular formula of hydrogen peroxide is $H_{2} O_{2}$ , while, its empirical formula is $HO$ .

Hence, from the given example we can say that, Molecular formula $=$ (Empirical formula)_n

Where the value of n is a whole number. For $H_{2} O_{2}$ , the value of n is $2$ .

For some compounds, the molecular formula is the same as its empirical formula. Both formulae for hydrogen chloride is $HCl$ and have the value of n as $1$ .

As the given compound contains carbon, hydrogen, and nitrogen in the ratio $18 : 2 : 7$
Therefore, the percentage composition of the compound will be,
$C = \frac{18}{27} \times 100 = 66.66 %$
$H = \frac{2}{27} \times 100 = 7.40 %$
$N = \frac{7}{27} \times 100 = 25.92 %$

Element Percentage
Composition Atomic mass Relative number of atoms Simplest ratio

$C$ $66.66$ $12$ $\frac{66.66}{12} = 5.55$ $\frac{5.55}{1.85} = 3$

$H$ $7.40$ $1.008$ $\frac{7.40}{1.008} = 7.34$ $\frac{7.34}{1.85} \approx 4$

$N$ $25.92$ $14$ $\frac{25.92}{14} = 1.85$ $\frac{1.85}{1.85} = 1$

Hence, the empirical formula of the compound is $C_{3} H_{4} N$ .
And, the empirical formula mass $= (3 \times 12) + (4 \times 1) + 14 = 54$

We Know, $n = \frac{Molecular mass}{Empirical formula mass} = \frac{108}{54} = 2$

Thus, the molecular formula of the compound is ${(C_{3} H_{4} N)}_{2}$ i.e, $C_{6} H_{8} N_{2}$ .

What do you understand by empirical and molecular formula of a compound? How are they related? A compound contains carbon, hydrogen, and nitrogen in the ratio $18 : 2 : 7$ . Calculate its empirical formula. If the molecular mass of the compound is $108$ , what is its molecular formula?

Important Questions on Basic Concepts Atoms & Molecules

Learn and Prepare for any exam you want !

What do you understand by empirical and molecular formula of a compound? How are they related? A compound contains carbon, hydrogen, and nitrogen in the ratio 18:2:7. Calculate its empirical formula. If the molecular mass of the compound is 108, what is its molecular formula?

Important Questions on Basic Concepts Atoms & Molecules

Learn and Prepare for any exam you want !

What do you understand by empirical and molecular formula of a compound? How are they related? A compound contains carbon, hydrogen, and nitrogen in the ratio $18 : 2 : 7$ . Calculate its empirical formula. If the molecular mass of the compound is $108$ , what is its molecular formula?