Question

how many moles of oxygen in $40.0$grams of oxygen gas?

Answer 1

the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance in that sample, measured in moles.
The molar mass is a bulk, not molecular, property of a substance.
The mole is the unit of measurement for the amount of substance in the International System of Units.
A mole of a substance or a mole of particles is defined as containing exactly $6.02214076\times 10{}^\text{2}{}^\text{3}$ particles, which may be atoms, molecules, ions, or electrons

Complete step by step answer:
$n=$ given mass or molar mass/ molecular mass The molecular mass (m) is the mass of a given molecule: it is measured in daltons (Da or u).
The molar mass of a substance is the mass in grams of one mole of that substance.
This mass is given by the atomic weight of the chemical unit that makes up that substance in atomic mass units(amu).
For example,silver has an atomic weight of$107.8682\text{ amu}$, so one mole of silver has a mass of $107.8682\text{ grams}\text{.}$
To solve the question:
First, we need to calculate the molar mass of oxygen gas.
${O_2}$ has a molar molar mass of $32g/mol$
So, $32g$ of oxygen = $1mol$
Then, $40g$ of oxygen will be have $\dfrac{{40}}{{32}}moles$
$\Rightarrow 1.25mole$

There are $1.25$ moles of ${{\text{O}}_{2}}$ in $40\text{g}$ of oxygen gas.

Note: Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. If you want to know how many moles of a material you have, divide the mass of the material by its molar mass.
make sure you know the formula of calculating molar mass to get the correct ans and approach.