Question

What is the molecular mass of ${C_2}{H_4}$ ?
A. $26$
B. $24$
C. $28$
D. $22$

Answer 1

Hint: To calculate the molecular mass of any substance we should know the atomic mass of each of the elements present in the molecule . In case of ethene the mass of carbon and hydrogen should be known .

Complete step by step answer: The molecular mass (m) is the mass of a given molecule it is measured in Daltons (Da or $\mu$). We may observe that different molecules of the same elements may have different molecular mass , the reason behind this is because there exist different isotopes of the same element which have different masses .
Molecular mass of ${C_2}{H_4}$ = (2 $\times$ atomic mass of carbon) + (4 $\times$ atomic mass of Hydrogen)
$\Rightarrow$ $ 2 \times 12 + 4 \times 1$ $ = 28$
The way of calculating molecular mass is from the atomic masses of each nuclide present in the molecule, while molar masses can be calculated from atomic weights of each element
So, the correct answer is “Option C”.

Additional Information:
Molar Mass: Molar mass of an element is simply the atomic mass in $(g/mol)$ .The other way of calculating molar mass is by multiplying the atomic mass in $amu$ by the molar mass constant $\left( {1g/mol} \right)$ . The way of calculating the molar mass of a compound with multiple atoms is by taking the sum of atomic mass of all the constituent atoms .
Determining the molar mass of a compound:
In a compound of $NaOH$, the molar mass of $Na$ alone is $23g/mol$ , the molar mass of $'o'$ is $16g/mol$ and $'H'$ is $1g/mol$ .
Molar mass of $NaOH$ is.
$mass\,of\,Na$ + $mass\,of\,O$ + $mass\,of\,H$ = $mass\,of\,NaOH$
$23g/mol + 16g/mol + 1g/mol = 40g/mol$

Note: When any chemical change takes place there is rearrangement of billions of atoms . It is impossible to count or visualize all these atoms , but we need some way so that we can refer to the entire quantity . They also need a way to compare these numbers and relate them to the weights of the substance, which can be measured and observed.