Question

How many molecules of aspirin (formula mass aspirin= $180amu$) are in a $0.250gm$ sample?

Answer 1

We know that in chemistry we study about various elements, their physical and chemical properties, their nature and reactivity with other elements. Also, we know that substance or a matter has some mass. And each element has a unique atomic mass or so-called molar mass. Molar mass is the mass of a chemical compound or element divided by the amount of sample in that substance (in moles).

Complete step by step answer:
Like elements, molecules of compounds too have a molar mass. As a compound is formed by different elements. Thus, the molar mass of elements in the compound are all sum-up to obtain the molar mass of the compound.
We know that aspirin is also called as acetylsalicyclic acid and it has a molecular formula ${C_9}{H_8}{O_4}$.
Thus, the molar mass of aspirin is calculated as $180.16g/mol$.
We will use mole concept to calculate the moles of aspirin.
We have the mass of the sample as $0.250g$.
Moles is calculated as the mass of the sample divided by the molar mass of the aspirin.
Thus,
$n = \dfrac{{0.250}}{{180.16}}$
On simplification we get,
$n = 0.001388mol$
To find the number of molecules we will multiply the number of moles with Avocado’s number.
Thus,
Number of molecules = $0.001388 \times 6.02214 \times {10^{23}}$
Number of molecules = $8.36 \times {10^{20}}$
Thus, there are $8.36 \times {10^{20}}$ molecules of aspirin present in $0.250gm$ sample.

Note:
We need to remember that the mole concept is the most efficient measure to calculate the moles of a substance or even to calculate the concentration or mass of a substance. In mole concept there are various terminologies like molarity, molality, normality, concentration, moles, molar mass, molecular mass, etc. All these terms are used in measure of substance in any composition or mixture. Mole concept gives the accurate measure of the elements involved in a reaction.