Question

(a) Calculate the molar mass of the following:
i.${H_2}O$
ii.$C{O_2}$
iii.$C{H_4}$
(b) Calculate the mass percent of different elements present in Sodium Sulphate $(N{a_2}S{O_4})$.

Answer 1

In chemistry, Molar mass is defined as the mass of one mole of substances. One mole of substances has $6.022\, \times \,{10^{23}}$ atoms or molecules or ions in it. Mass percentage is used to calculate the concentration of an element in a compound.
Formula used – To calculate mass percentage of an element-
$Mass\,percentage\% \, = \,\dfrac{{Mass\,of\,the\,element}}{{Molar\,mass\,of\,the\,element}} \times 100$

Complete answer:
Molar mass is the weight of one mole of the substances. It is defined as the sum of atomic masses of all the atoms present in the molecules. First, multiply the subscripts (number of atoms) with the atomic mass of an element, then add the mass of all the elements present in the molecule to obtain molecular mass.
Molar mass of water $({H_2}O)$
Atomic mass of $H=1$
Atomic mass of $O=16$
${H_2}O\, = \,2 \times \,H + 1 \times \,O$
Molar mass of water ${H_2}O\,\, = \,2 \times \,1 + 16\, = 18\,g/mol$
Molar mass of Carbon dioxide $(C{O_2})$
Atomic mass of $C=12$
Atomic mass of $O=16$
$C{O_2}\, = \,12 + \,2 \times 16$
Molar mass of Carbon dioxide $C{O_2}\, = \,12 + \,2 \times 16\, = 44\,g/mol$
Molar mass of methane $(C{H_4})$
Atomic mass of $C=12$
Atomic mass of $H=1$
$C{H_4}\, = \,12 + \,1 \times 4$
Molar mass of methane $C{H_4}\, = \,12 + \,1 \times 4\, = 16\,g/mol$
(b) Mass percentage of a substance is simply the ratio of the mass of an element with respect to the molar mass of that element. It is just a way of expressing concentration of the component in a particular mixture. We can calculate the mass percentage of each element in a compound.
Mass percentage of different elements of sodium sulphate $(N{a_2}S{O_4})$
$Mass\,percentage\% \, = \,\dfrac{{Mass\,of\,the\,element}}{{Molar\,mass\,of\,the\,element}} \times 100$
Molar mass of Sodium sulphate $(N{a_2}S{O_4})$
Atomic mass of sodium $Na=23$
Atomic mass of sulphur $S=32$
Atomic mass of oxygen $O=16$
Molar mass of Sodium sulphate $N{a_2}S{O_4}\, = \,23\, \times 2 + \,32\, + 16 \times 4\, = 142\,g/mol$
Mass percentages of Sodium $Na$ in $N{a_2}S{O_4}=\dfrac{{46}}{{142}} \times 100$
Mass percentages of Sodium $Na$ in $N{a_2}S{O_4}=32.39\%$

Similarly, Mass percentages of Sulphur $S$ in $N{a_2}S{O_4}=\dfrac{{32}}{{142}} \times 100$
Mass percentages of Sulphur $S$ in $N{a_2}S{O_4}=22.53\%$

Similarly, Mass percentages of Oxygen $O$ in $N{a_2}S{O_4}=\dfrac{{64}}{{142}} \times 100$
Mass percentages of Oxygen $O$ in $N{a_2}S{O_4}=45.07\%$

Note:
Do not forget to multiply the subscript (number of atoms) with the element’s atomic masses. Molar mass is the mass of one mole of substances. Remember one mole of substances has $6.022\, \times \,{10^{23}}$ atoms or ions or molecules present in it.