Question

What is the mass formula for water?

Answer 1

Hint : We know that the molecular mass is the total mass of the given molecule. We should know the atomic mass of every element that is present in the molecule and by adding the atomic mass of all the elements we can get the molecular mass of the molecule.

Complete Step By Step Answer:
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 unit less.
The molecule of water consists of two molecules of hydrogen and one molecule of oxygen. -From the periodic table, we know that the atomic mass of the hydrogen is $1g/mol$ and oxygen has the atomic mass $16\text{ }g/mol.$ We know that a total of two molecules of hydrogen makes the bond with the oxygen, so the molecular mass of water will be:
$(No.\text{ }of\text{ }molecules\text{ }of\text{ }hydrogen\text{ }\times \text{ }mass\text{ }of\text{ }hydrogen)\text{ }+\text{ }atomic\text{ }mass\text{ }of\text{ }oxygen$
$\Rightarrow (2\times 1)+16$
$\Rightarrow 2+16=18$
Therefore, the molecular mass of water is $18\text{ }g/mol$

Note :
Note that to calculate the correct molecular mass it is necessary to have a correct molecular formula. In water, hydrogen bonding is present. Oxygen consists of a negative charge whereas hydrogen consists of a positive so bond formation takes place between the neighboring oxygen and hydrogen. SI unit of molecular mass is $g/mol.$