Question

How many molecules are there in 39 grams of water?

Answer 1

We are already provided with the compound which is water. We need to find out the number of molecules. For that we need the basic formula of mole concept. It is a part of physical chemistry. We shall calculate the number of moles present and multiply it with Avogadro’s number.

Formula used: ${\text{no of moles = }}\dfrac{{{\text{given mass}}}}{{{\text{molar mass}}}}{\text{ Avogadro number}}$
Here Avogadro number is $6.023 {10^{23}}$

Complete step by step answer:
We already know that:
Molar mass of water is 18 g/mol.
Also, there are $6.023 {10^{23}}$molecules of water.
On multiplying the given mass of water times, the conversion factors that cancel grams and moles of water, leaving molecules of water, as shown below:
So, ${\text{no of moles = }}\dfrac{{{\text{given mass}}}}{{{\text{molar mass}}}}{\text{ Avagadro number}}$
${{ = 39g }}\dfrac{{{\text{1mol}}}}{{{\text{18}}{\text{.015g}}}}{\text{ }}\dfrac{{{\text{6}}{\text{.023 1}}{{\text{0}}^{{\text{23}}}}}}{{{\text{1mol}}}}$
${\text{ = 1}}{{.3 1}}{{\text{0}}^{{\text{24}}}}{\text{ molecules of }}{{\text{H}}_{\text{2}}}{\text{O}}$
So, there are $1.3 {10^{24}}$ molecules of water.
Additional Note: We should know that in order to calculate the number of moles of any substance present in the sample, we simply divide the given weight of the substance by its molar mass. Where 'n' is the number of moles, 'm' is the given mass and 'M' is the molar mass. A mole is defined as the mass of the substance which consists of the equal quantity of basic units. Example atoms in 12 grams are the same as 12C. The basic units can be molecules, atoms or formula units based on the substance. A mole of a substance is equal to as many molecules of that substance as there are atoms of carbon-12 in exactly 12 g of carbon-12. Most important of all, by this definition, 1 mole of any substance contains the same number of molecules.
One mole of any substance is equal to the value of $6.023 {10^{23}}$ (Avagadro number). It can be used to measure the products obtained from the chemical reaction. The unit is denoted by mol.

Note:
We already know that to know how many moles of a material you have, divide the mass of the material by its molar mass. The molar mass of a substance is the mass in grams of one mole of that substance. A group of two or more atoms held together by chemical bonds. So, minimum 2 atoms are required to form a molecule.