If it takes $1$ second to count $4$ grains of wheat,then the time required to count one mole of wheat grains is ...... years.

Consider two sealed jars of equal volume. One contains $2 \ g$ of hydrogen at $200 \ K$ and the other contains $28 \ g$ of nitrogen at $400 \ K$. The gases in the two jars will have

Sucrose has molecular formula $C_{12}H_{22}O_{11}$. How many moles of carbon atoms are present in its $34.2 \ g$ sample?

How many $CaF_2$ formula units are present in $146.4 \ g$ of $CaF_2$? (Molar mass of $CaF_2 = 78.08 \ g/mol$)

Assertion $(A)$: Equal moles of different substances contain the same number of constituent particles.
Reason $(R)$: Equal weights of different substances contain the same number of constituent particles.
The correct answer is

How many moles of $Mg_3(PO_4)_2$ will contain $0.50 \ mol$ of oxygen atoms?