Question

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

Answer 1

We could express the concept of mole as the amount of substance. For a molecule, molecular weight in grams is equal to the Avogadro number and for transforming between moles and grams, we could use the molar mass of the substance.

Complete step by step answer:
- Let's begin with the notion of Avogadro number and concept of moles. Avogadro’s number can be expressed as the number of elementary particles such as molecules, atoms, compounds, etc. per mole of a substance and Avogadro number is often denoted as ${{N}_{A}}$.
- Or in other words, the number of units in one mole of any substance is called Avogadro’s number or Avogadro’s constant and its value is given as $6.023\times {{10}^{23}}$.
- A further property of Avogadro’s number is that the mass of one mole of a substance is equal to that substance’s molecular weight. Also, the mass of a single mole of the compound refers to the gram molecular mass of a compound and the gram atomic mass of an element is the mass of one mole of that element.
- Let's find out the number of moles in 18 grams of water. It can be obtained by dividing the given mass (18 g) by molecular mass of water (18 g).
$Number\text{ }of\text{ }moles=\dfrac{Given\text{ }Mass}{Molar\text{ mass}}=\dfrac{18g}{18{g}/{mol}\;}=1mol$
- Thus 18 g of water will contain one mole of water, that is $6.023\times {{10}^{23}}$ molecules.
- In the second option it’s given that the number of moles is 18. Therefore 18 mole of water will contain $18\times 6.023\times {{10}^{23}}$ number of molecules.
- The third option is given as 18 molecules of water and hence the number of water molecules is 18.
- The number of moles in 1.8 grams of water can be given as follows
$Number\text{ }of\text{ }moles =\dfrac{1.8g}{18{g}/{mol}\;}=0.1mol$

1.8 gram of water will contain $0.1\times 6.023\times {{10}^{23}}$ number of molecules.
From the above discussion it’s clear that the number of water molecules is maximum in 18 moles of water. The number of molecules is $18\times 6.023\times {{10}^{23}}$.
So, the correct answer is “Option B”.

Note: It should be noted that the concept of moles can be defined in terms of volume also. That is in an STP (standard pressure and temperature) environment, one mole of any gas ($6.023\times {{10}^{23}}$ particles) will occupy a volume of 22.4 litres.