Question

The number of water molecules is maximum in-
(A) 18 grams of water
(B) 18moles of water
(C) 18 molecules of water
(D) 1.8 grams of water

Answer 1

Hint: Water is a small bent molecule with the molecular formula $H_{2}O$. It’s found that it’s structure consists of two light hydrogen atoms attached to an oxygen atom. To calculate the number of molecules we first have to calculate, the number of moles. And one mole consists of about $6.02\times {10}^{23}$ molecules.

Complete step by step answer:
In above question four options are given, let’s find out water molecules present in each option-
- Let’s see the first option -18 grams of water –
Let’s first calculate the number of moles present in this first. Number of moles has formula-
     $$Numberofmoles={\displaystyle \frac{weight}{molecularweight}}$$
So, as weight is given, we have to first calculate the molecular weight of water-
     $$\begin{array}{rl}& H_{2}O\\ & =(2\times 1)+16\\ & =2+16\\ & =18\end{array}$$

Now putting the values of weight and molecular weight in formula we will calculate number of moles, so we get-
     $$\begin{array}{rl}& Numberofmoles={\displaystyle \frac{18}{18}}\\ & =1\end{array}$$

- We can say that 18 grams of water means same as 1 mole of water , and in this $N_A$ Avogadro number of water molecules are present , which means $6.02\times {10}^{23}$ molecules.

- Now let see next option- 18 moles of water
Above, we have seen that 1 mole of water has $6.02\times {10}^{23}$ molecules. So, 18 mole of water will have
     $$\begin{array}{rl}& 18\times N_{A}molecules\\ & =18\times 6.023\times {10}^{23}molecules\end{array}$$

- The third option is 18 molecules of water-
Here, already 18 molecules are present in water.

- Last option is 1.8 gram of water-
Here, we will get the number of moles =
     $$\begin{array}{rl}& {\displaystyle \frac{1.8}{18}}\\ & =0.1mole\end{array}$$

As we know that 1 mole of water has $6.02\times {10}^{23}$ molecules. So, 0.1 mole of water will have
     $$\begin{array}{rl}& 0.1\times N_A\\ & =0.1\times 6.023\times {10}^{23}molecule\end{array}$$
Hence, we can conclude that option (B) 18moles of water is correct, as it has the maximum number of water molecules. So, the correct answer is “Option B”.

Additional Information:
- Water has a dipolar nature, as we know that oxygen has a higher electronegativity than hydrogen, so the side of an oxygen atom has a partial negative charge. A molecule with such a charge difference is called a dipole.
- The charge difference in water molecules causes water molecules to be attracted to each other and to other polar molecules. This attraction is called hydrogen bonding.

Note: - We should not get confused in the terms moles and molecules, number of moles = $6.02\times {10}^{23}$ molecule of water.
- To solve any question of finding molecules we need to first find the number of moles present .
-We must not forget to write the unit after solving any question.