Question

Molecular mass of water_______
(A) $18u$
(B) $16u$
(C) $15u$
(D) $10u$

Answer 1

the molecular mass we come to know that it is basically the given mass of the molecule which is further measured in terms of Dalton D or u. Then in order to get the molecular mass of water we will further add the molecular weight of two hydrogen atoms and the molecular weight of oxygen.
$2u + 16u = 18u$

Complete step by step solution:
When we talk about the molecular mass we come to know that it is basically the given mass of the molecule which is further measured in terms of Dalton D or u. Basically the molecular masses differ according to the isotope as different molecules of the same compound have different molecular masses because they contain different isotopes of the element to be considered. We will calculate the molecular mass of the water as stated: the water is written as the hydrogen containing two molecules of water and one molecule of oxygen. Further as we know that the weight of water given is: $1u$ and for two hydrogen atoms we will multiply it by two which will further give: $2u$ . Further we should know that the weight of one oxygen atom is : $16u$ . Then in order to get the molecular mass of water we will further add the molecular weight of two hydrogen atoms and the molecular weight of oxygen.
Then the molecular mass of water is approximately:
$2u + 16u = 18u$
Further we should note here that the molecular weight and the molecular mass are authoritatively synonyms.
Hence, the option A is correct.

Note:
We should note here that in technical terms the molecular mass is the mass of a single or specific well defined molecule. Then the molecular weight is referred to the weighted sample or the species.