Question

If we assume that one-sixth the mass of an atom of ${}^{12}C$ isotope is taken as the reference, the mass of one molecule of oxygen will:

• A. be double its original value
• B. be half its original value
• C. be the same
• D. increase by four fold

Answer 1

The correct option is C be the same

We know that $12gofCarbon=6.023\times {10}^{23}atoms$

So $1g=\frac{6.023X{10}^{23}}{12}$

$6gofcarbon=6\times \frac{6.023X{10}^{23}}{12}$

or

$=\frac{1}{2}\times 6.023\times {10}^{23}$

This is our new Avogadro's number according to the given conditions.

Or

$1atomicmassunit=\frac{1}{N_A}=\frac{2}{6.023X{10}^{23}}$

Mass of $1$ mole of $O_2=32\times Avograd{o}^{\prime }snumber\times \frac{2}{6.023\times {10}^{23}}$

$=32\times \frac{6.023\times {10}^{23}}{2}\times \frac{2}{6.023\times {10}^{23}}$

$=32g$

Hence, there is no change in the mass number of $O_2.$

Option $\text{C}$ is correct.