Question

How many atoms of oxygen are there in 18 g of water?

Answer 1

In order to find the atoms, we will calculate the number of moles by the formula of dividing the value of mass of sample by molar mass. We can say that in order to solve this question we should have the idea of mole concept.

Complete step by step answer:
- As we know that the chemical formula of water is ${{H}_{2}}O$ . this means that one molecule of water is made up of one oxygen atom and two hydrogen atoms.
- Then we will calculate the moles of water that are there in 18 g. We will divide 18 g by the number of grams in one mole of water, also called as molar mass of water:
- Number of moles= mass of sample divided by molar mass.
- Now, by substituting the values we will get:
Number of moles =
$\begin{aligned} & \dfrac{18g}{molar\text{ }mass\text{ }of\text{ }H\times 2+molar\text{ }mass\text{ }of\text{ }O} \\\ & =\dfrac{18g}{1.008g/mol\times 2+16g/mol} \\\ & =\dfrac{18g}{18.02g/mol} \\\ & = 1mole \\\ \end{aligned}$
- As in one mole of water there are$6.022\times {{10}^{23}}$ molecules, that’s how the mole was defined.
- And as the chemical formula for water is ${{H}_{2}}O$, and one mole of water corresponds to one mole of oxygen atoms. Hence, there will be $6.022\times {{10}^{23}}$ oxygen atoms in one mole, or we can say 18 g of water.

Note: - We should not get confused in the terms mole and molecule. As mole is used to measure the quantity of a substance, and is having the unit mol. Whereas, molecules are used to name a combination of some atoms and can be measured as the number of ‘mol’ present.