Question

What is the mass of:
I.$0.2$ mole of oxygen atoms?
II.$0.5$ mole of water molecules?

Answer 1

To solve this question, we should have knowledge about the molecular mass of some of the common elements of the periodic table. The weight of a given moles of an element or compound can be calculated by the mole concept formula (given below). Molecular masses of elements are found on the periodic table. In the case of molecules, we can calculate their molecular mass by adding the molecular weight of the constituent atoms present in it.
Formula Used:
${\text{Moles }}\left( {{\text{mol}}} \right){\text{ = }}\dfrac{{{\text{mass }}\left( {\text{g}} \right)}}{{{\text{Molecular mass }}\left( {{\text{g mo}}{{\text{l}}^{{\text{ - 1}}}}} \right)}}$ …………….(Eq. 1)

Complete step by step answer:
The mole concept states that 1 mole of an atom or molecule consists of the same amount of particles in it. The number of particles in 1 mole is known as Avogadro’s number. It is equal to $6.023 \times {10^{23}}$ atoms or molecules. The mass of 1 mole differs for each atom and molecule. It is equal to the atomic mass of an element or compound. In case of atomic mass, the unit is in amu while for molar mass, it is in ${\text{g mo}}{{\text{l}}^{{\text{ - 1}}}}$.
In option a), oxygen atom is present as ${\text{O}}$ and it has a molecular mass of ${\text{16 g mo}}{{\text{l}}^{{\text{ - 1}}}}$ . Putting this value in Eq. 1, for $0.2$ mole of oxygen atoms we get:
$\Rightarrow$ $0.2 = \dfrac{{mass}}{{16}}$
Solving this for mass of oxygen we get:
$mass = 0.2 \times 16$
$\Rightarrow mass = 3.2g$
$\therefore$ Mass of $0.2$ mole of oxygen atoms is $3.2$ g.
In option b), a water molecule is present as ${{\text{H}}_{\text{2}}}{\text{O}}$. It contains 2 atoms of hydrogen and 1 atom of oxygen. The molecular mass of water is $1 + 1 + 16 = 18{\text{ g mo}}{{\text{l}}^{{\text{ - 1}}}}$ . Putting this value in Eq. 1, for $0.5$ mole of water molecules we get:
$\Rightarrow$ ${\text{0}}{\text{.5 = }}\dfrac{{{\text{mass}}}}{{{\text{18}}}}$
Solving this for mass of water, we get:
$\Rightarrow$ $mass = 0.5 \times 18$
$\Rightarrow mass = 9g$
$\therefore$ Mass of $0.5$ mole of water molecules is 9 g.

Note:
The molecular mass of an atom and molecule of the same element can be different. For example, an atom of oxygen has a mass of 16${\text{g mo}}{{\text{l}}^{{\text{ - 1}}}}$ while a molecule of oxygen is present as ${{\text{O}}_{\text{2}}}$ and thus, has a mass of $16 + 16 = 32{\text{ g mo}}{{\text{l}}^{{\text{ - 1}}}}$. So, while solving the question, we should not get confused between the terms atom and molecule as it will lead to a different answer.