How do you calculate the number of molecules in a substance?

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Hint:. The answer here is based on the basic concept of general chemistry which involves the fact about the Avogadro law and the Avogadro constant which is given by, one mole of a substance contains $6.023\times {{10}^{23}}$ atoms.

Complete step by step answer:
- The concept of general chemistry which deals with the chapter called as some basic concepts of chemistry, tells us about the mole, molecule and atoms and their masses along with it.
- We shall now see what one molecule of substance is and how can it be determined by taking one instance of a chemical reaction.
- According to the Avogadro law, mole concept that is one mole of a substance is defined as ‘the amount of a substance which contains as many elementary particles as that the number of carbon atoms present in 12 g of C – 12 isotope’.
- According to his law, one mole of substance contains $6.023\times {{10}^{23}}$ number of atoms which he obtained by the actual law that he proposed which states that ‘equal volumes of different gases measured under same conditions of temperature and pressure contain same number of molecules’.
- To calculate the required number of molecules, firstly calculate the molecular weight of the substance for one mole followed by dividing the molar mass value to the molecular mass and then multiplying it by the Avogadro constant.
- Thus, accordingly, if we consider the question as an example as,
How many molecules are contained in 180 g of water?
The answer would be as follows:
Molecular weight of water $\left( {{H}_{2}}O \right)$ is: $\left( 2\times 1 \right)+\left( 16\times 1 \right) = 18g/mol$
Now, the Total number of moles of water is = $\dfrac{180}{18} = 10 moles$
Number of atoms of ${{H}_{2}}O$ = $6.023\times {{10}^{23}}\times 10 = 6.023\times {{10}^{24}}$
Thus, the total number of molecules of water present in 180 g of water is $6.023\times {{10}^{24}}$
Therefore, this above statement and an example gives the required answer.

Note: Note that mole and molecule are different quantities and these terms are not to be confused. One mole is nothing but the total amount of the substance given and one molecule is the total number of molecular entities that is Avogadro number of atoms present in one mole of a substance.