Question

Calculate the relative molecular mass of water​ ${H_2}O$.

Answer 1

Hint: In order to deal with this question first we will define molecular mass then we will proceed further by using the concept as in water compound water contains two hydrogen atoms and one oxygen atom according to it we will calculate relative molecular mass.
Complete step-by-step answer:
Molecular mass: The mass of a given molecule is the molecular mass (m): it is expressed in daltons (Da or u). Different molecules of the same compound may have different molecular masses, because they contain different element isotopes. The related sum of relative molecular mass, as described by IUPAC, is the ratio of a molecule 's mass to the unified unit of atomic mass (also known as the dalton) and is unitless.
For ${H_2}O$ molecule
Relative mass of hydrogen $\left( H \right) = 1u$
Relative mass of oxygen $\left( O \right) = 16u$
According to the formula of water $\left( {{H_2}O} \right)$ contains 2 hydrogen atoms,
So, relative mass of water = (2 × relative mass of hydrogen) + (1× relative mass of oxygen)
$
   = \left( {2 \times 1u} \right) + \left( {1 \times 16u} \right) \\
   = 18u \\
 $
Hence, Relative mass of water $ = 18u$

Additional Information-
The concept of molecular weight is more authoritatively synonymous with molecular mass; however, it is still widely subjective in general usage, such as the units used in connection with it are. Most traditional preparatory sources use g / mol and describe it effectively as a synonym for molar mass, whereas more reliable sources use Da or u and more precisely equate its meaning with molecular mass.

Note- The molecular mass and relative molecular mass are distinct from but related to the molar mass. The molar mass is defined as the mass of a given substance divided by a substance's quantity and expressed in g / mol. The molar mass is generally the most relevant measure when concerned with a substance's macroscopic (weighable) amounts.