Question

What is the mass of:
i.0.5 mole of water molecules
ii.10 moles of sodium sulphite($N{{a}_{2}}S{{O}_{3}}$)?

Answer 1

We need to know about the molecular weight of the molecules to solve this type of questions and then simply apply the concept of mole and molecular formula. For the molar mass of a compound, the elements respective molar masses are added.

Complete answer:
I. First we need to know how to calculate the molecular weight of water molecules. So first let's see how to find out the molecular weight of water.
A molecule of water contains 2 hydrogen atoms and one oxygen atom.
Molecular weight of hydrogen is 1 and that of oxygen is 16 which we get to know from the periodic table.
Now molecular weight of water= $(2\times 1)+(1\times 16)$
=18
Now 1 mole of water weighs 18g
Now we need to apply simple unitary method to get to the answer
One mole of water weighs 18 gram
Their 0.5 mole of water weighs $18\times 0.5$ =9gram
So the answer is 0.5 mole of water weighs 9 gram.

II. A molecule of sodium sulphite contains 2 sodium atoms 1 sulphur atom and 3 oxygen atoms.
Molecular weight of sodium is 23 g
Molecular weight of sulphur is 32 g
Molecular weight of oxygen is 16 g
Now molecular weight of sodium sulphite is $(2\times 23)+(1\times 32)+(3\times 16)$= 126g
Now 1 mole of sodium sulphite contains 126 g
So 10 moles of sodium sulphite contains 1260g.
So the answer is 1260 g of sodium sulphite is present in 10 moles.

Note:
The mole is an amount unit kind of like familiar units like pair, dozen, gross, etc. It provides a selected measure of the amount of atoms or molecules during a bulk sample of matter. A mole is defined as a substance containing the identical number of discrete entities (atoms, molecules, ions, etc.)