Question

The molar mass of ${{O}_{2}}$ is 32 gram/ mole. How many moles are there in 10 grams of oxygen?
a.) 32
b.) 320
c.) 0.3125
d.) 0.42

Answer 1

Hint: By using the molecular formula we can find the molar mass and to obtain the number of moles, divide the mass of the compound by the molar mass of the compound expressed in grams.

Complete step by step solution:

A mole is defined as the mass of the substance which consists of the equal quantity of basic units, these basic units can be atoms, molecules or formula units based on the substance.
One mole of any substance is equal to the value of $6.023\times {{10}^{23}}$ (Avogadro number). It can be used to measure the products obtained from the chemical reaction. The unit is denoted by mol.
It can be given by:
$Number\,of\,moles=\dfrac{Given\,mass}{Molar\,mass}$
Let us take an example to get a clear idea about it, suppose there is 25g of water, the molar mass of water is 18, by adding the standard atomic masses (in g/mol) of the constituent atoms.
Number of moles of water = $\dfrac{25}{18g/mol}\,$= 1.39
Similarly, in the given question the molar mass of ${{O}_{2}}$ is 32g/mole and the given mass is 10g
So, by applying the formula
Number of moles of Oxygen = $\dfrac{10}{32g/mol}\,$= 0.3125

Therefore, from the above solution we can conclude that the correct option is (c).

Note: Always keep in mind to change all the units in the SI unit while solving the problems as a general practice. Keep in mind that molar mass and molecular mass is two different concepts, don’t get confused.