Question

The number of moles of water in 1.8 litres of pure water is (density of water=1g/ml)
A) 18
B) 100
C) 36
D) 55.4

Answer 1

The answer here is based on the calculation of number of moles of water based on the density value where one milliliter of water weighs one gram and then calculate for 1.8 litres.

Complete answer:
In the lower classes of chemistry, we have come across the basic concepts that deal with the calculation of number of moles, normality of solution, molality of solution, molarity of the solution and also the mole fraction of the solution.
We shall now see the calculation of the number of moles of water in the given litres of water based on the data given about the density of water.
The calculation is based on the simple cross multiplication rule by converting the given litres of solution into milliliters.
- Density of the solution is defined as the ratio of mass of the compound to the volume of the solution that is density $d=\dfrac{m}{V}$
- According to the data given, we have a density of pure water as 1 g/ml.
This means that 1 ml of water weighs one mg.
Now, since the question is asked for the number of moles of water present 1.8 litres of pure water, we can write the litres in terms of milliliters as,
$1.8l=1.8\times {{10}^{3}}ml=1800ml$
Here, since 1 ml of water weighs 1 gm then 1800 ml of water will weigh 1800 gm.
Now, the molecular mass of one mole of water is 18 g/mol.
Thus, 1800 gm of water will have $=\dfrac{1\times 1800}{18}=100moles$
Therefore, 1.8 litres of pure water contains 100 moles of water.

Thus, the correct answer is option B) 100.

Note:
Do not get confused with moles of water and molecules of water as one mole of a substance is the term used to measure quantity of a substance and one molecule is the term used to measure the number of moles of the substance present. Molecules are used to name the combination of several atoms.