Question

Twenty molecule of ${\text{ S}}{{\text{O}}_3}{\text{ }}$ will weigh as much as_______ molecule of oxygen.
A.100
B.50
C.15
D.8

Answer 1

Calculate weight of given molecules of ${\text{ S}}{{\text{O}}_3}$ first, then calculate the number of molecule of oxygen weight that much by using weight of 1 molecule of oxygen.

Complete step by step answer:
The mass/weight of smaller entities like atoms, molecules, and elements are expressed in atomic mass units (amu). However the mass of moles of atom, molecule or compound can be expressed in terms of gram. The atomic mass of Sulphur is 32 amu, that is mass of 1 atom of Sulphur weigh 32 amu and atomic mass of Oxygen is 16 amu that is mass of 1 atom of Oxygen weigh 16 amu. But the molecular mass of Oxygen is 32 amu, as Oxygen exists in ${{\text{O}}_2}$ form, so 1 molecule of Oxygen will weigh 32 amu. Thereby we can calculate the mass of 1 molecule of ${\text{ S}}{{\text{O}}_3}$ as $= 32 + \left( {16 \times 3} \right) = 80{\text{ amu}}$.
The mass of 20 molecules of ${\text{ S}}{{\text{O}}_3}$ is $= 80 \times 20 = 1600$.
As the mass of 1 molecule of oxygen is 32 amu. In order to find how many molecule of oxygen will weigh 1600 amu; we can do $= \dfrac{{1600}}{{32}} = 50$
That is 50 molecules of oxygen weigh as much as 20 molecules of ${\text{ S}}{{\text{O}}_3}$.
It is important to remember that the molecular mass of oxygen is 32amu and the atomic mass of oxygen is 16 amu. The molecule of oxygen is ${{\text{O}}_2}$, but the atom of oxygen is only ${\text{ O }}$.

Thus, option B is correct.

Note:
$1{\text{ amu}} = 1.67 \times {10^{ - 24}}{\text{gram }}$.In order for proper representation of mass/weight of smaller entities like atom or molecule; amu is used. But the moles of such entities can be represented in gram. $1{\text{ mole}} = 6.022 \times {10^{ - 23}}$ atoms, molecules or ions.