Question

What is the molecular weight of water?
A. 21
B. 18
C. 20
D. 16

Answer 1

Hint: To calculate the molecular weight of any compound, first of all we must know its molecular formula and then atomic weight of each type of atom that combine together to form that molecule. After that we can add the atomic weight of each atom present in the molecule to find out its molecular weight.

Complete step by step solution:
When a group of atoms bind together in a specific proportion, they form a compound and the smallest fundamental unit of that compound is known as a molecule.
Molecular formula of a compound is a chemical formula that gives the information about the number of atoms of each element present in the molecule of that compound.
Example: $N{{H}_{3}}$ is the molecular formula of an ammonia compound’s molecule that clearly tells us that to form one molecule of ammonia, 1 nitrogen atom and 3 hydrogen atoms combine together.
${{H}_{2}}O$is the molecular formula of a water compound’s molecule that tells us that to form one molecule of water 2 hydrogen atoms and 1 oxygen atom combine together.

Molecular weight: molecular weight of a molecule is actually the sum of the weight of all the atoms of that compound.
When we measure the weight of one molecule we call is molecular weight and we generally represent it by the unit amu or u. 1 amu is equal to \[1.66\times {{10}^{-24}}\]g. Full form of amu is an atomic mass unit.

When we measure the weight of 1 gram mole molecules of a compound we call it gram molecular weight. It is represented in gram.
Both the molecular weights are equal in numerical digits but different in units.

Molecular formula of water: ${{H}_{2}}O$
Atomic weight of hydrogen = 1 amu
Atomic weight of oxygen = 16 amu
Molecular weight of water= 2 (weight of hydrogen atom) + weight of oxygen atom
${{M}_{{{H}_{2}}O}} = 2{{M}_{H}}+{{M}_{O}}$
\[{{M}_{{{H}_{2}}O}} = 2\times 1+16\]
\[{{M}_{{{H}_{2}}O}} = 2 + 16\]
${{M}_{{{H}_{2}}O}} = 18amu$
So the molecular weight of water will be 18amu.
So, the correct answer is “Option B”.

Additional Information:
1 amu is equal to $1.66\times {{10}^{-24}}$g that is actually equal to the reciprocal of avogadro number and that’s why when we calculate the gram molecular weight of a molecule in gram, numerically it comes equal to molecular weight. Let’s calculate the gram molecular weight of water to understand it completely.
Weight of one molecule of water = 18 amu
Weight of 1 gram mole of water = $18{{N}_{A}}$ amu
Here ${{N}_{A}}$ is the Avogadro number
Weight of 1 gram mole of water = $18\times 6.022\times {{10}^{23}}$amu
Now let’s convert this amu into gram
Weight of one gram mole of water = $18\times 6.022\times {{10}^{23}}\times 1.66\times {{10}^{-24}}$ g
Weight of one gram mole of water = $18\times 10\times {{10}^{-1}}$g
Weight of one gram mole of water = $18$g

Note: Water is a very good solvent, it dissolves ionic compounds in it very readily. To dissolve any ionic compound in it, water releases energy called hydration energy to break the molecule into its ion and after the ionization water molecule surrounds the ions and dissolves them. As the dielectric constant is very high, that is the reason why it is a very good polar protic solvent for many chemical reactions.