What is a mole?

One mole of a gas represents......

How many molecules of ammonia gas are present in $67.2 \ dm^3$,measured at $S.T.P.$?

How many electrons are present in $2 \times 10^{-3} \ mol$ of ${}_{8}^{18}O^{2-}$?

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

There are $6.02 \times 10^{22}$ molecules each of $N_2$,$O_2$,and $H_2$ which are mixed together at $760 \ mm$ and $273 \ K$. The mass of the mixture in grams is: