Question

Isotopes are the atoms of the same element; they have the same atomic number but different mass numbers. Isotopes have a different number of neutrons in their nucleus. If an element exists in two isotopes having atomic masses 'a' and 'b' in the ratio m: n, then average atomic mass will $\frac{m\times a+n\times b}{m+n}$.

Different isotopes of the same element have the same position in the periodic table. The elements which have single isotope are called monoisotopic elements. Greater is the percentage composition of an isotope, more will be its abundance in nature.

Answer the following questions:

The ratio of the mass of ${}^{12}C$ atom to that of an atom of element= X (whose atomicity is four) is 1:9. The molecular mass element X is:

• A. $480gmo{l}^{-1}$
• B. $432gmo{l}^{-1}$
• C. $36gmo{l}^{-1}$
• D. $84gmo{l}^{-1}$

Answer 1

The correct option is B $432gmo{l}^{-1}$

Given :

Atomic mass of carbon -12=12u

Molar mass of carbon -12=12g

The mass of 1 mole of carbon 12=12g

Ratio of carbon 12 atomic mass to that of X is =1:9

$\frac{12}{M_X}=\frac{1}{9}$

$M_X=108g$

The atomicity=4

Thus , one molecule will contain $4X$ atoms

Hence, the mass of 1 mole of the compound will be $4\times 108=432g$

The molecular weight of X will be $432g$.