Question

If one cubic dm of a gas weighs ${{2}}{{.5}}\,{{g}}$ at STP, its molecular weight (in grams) is:
A. ${{56}}\,{{g}}$
B. $11.2\,{{g}}$
C. $22.4\,{{g}}$
D. $224\,{{g}}$

Answer 1

Molecular weight measures the mass or weight of a given species or a molecule. It is represented in the unit of ${{Da}}\,$ or ${{\mu }}$.

Complete step by step answer:
Given that,
${{1}}\,{{d}}{{{m}}^{{3}}}$ weighs ${{2}}{{.5}}\,{{g}}$,
then we can say that ${{22}}{{.4}}\,{{d}}{{{m}}^{{3}}}$ weighs,
${{22}}{{.4}}\, \times 2.5 = \,56\,{{g}}$
Now,
We know that 1 mole of gas at STP occupies ${{22}}{{.4}}\,{{d}}{{{m}}^{{3}}}$
As we know that, the molecular weight is defined as the weight of 1 mole of the gas, so we can say that the required weight is ${{56}}\,{{g}}$ if one cubic dm of a gas weighs ${{2}}{{.5}}\,{{g}}$ at STP.
Therefore, we can conclude that out of the given four options, A is the correct option. B, C and D are incorrect options.

Additional information:
Full form of STP is standard temperature and pressure. STP is a condition that helps in comparing measurements between various sets of given data. Earlier until 1982, the STP was measured as the temperature of ${{273}}{{.15}}\,{{k}}\left( {{{0^\circ C,}}\,{{32^\circ F}}} \right)$ and an pressure of${{1}}\,{{atm}}\left( {{{101}}{{.325}}\,\,{{kPa}}} \right)$ exactly.. But IUPAC has defined that STP is a temperature of ${{273}}{{.15}}\,{{k}}\left( {{{0^\circ C,}}\,{{32^\circ F}}} \right)$ and an pressure of ${{1}}{{{0}}^{{5}}}{{Pa}}\left( {{{100}}\,{{kPa,}}\,{{1}}\,{{bar}}} \right)$ exactly since 1982.

Note: Molecular weight is also known as molecular mass. It is the mass of a substance which is based on the atomic weight of 12- carbon which is 12. Molecular weight is the total weight after summing up the atomic weights of all the atoms present in the molecular formula of a compound. For example we can calculate the molecular mass of glucose in the following way.