What is the mole fraction of oxygen in a mixture of $7 \ g$ of nitrogen and $8 \ g$ of oxygen?

If $500 \ mL$ of $NH_3$ gas at $STP$ contains $6.0 \times 10^{21}$ molecules,how many molecules will $100 \ mL$ of $CO_2$ gas contain at $STP$?

Calculate the mass of $0.25 \ mol$ of iron (molar mass $= 56 \ g \ mol^{-1}$).

The number of mole atoms and mole molecules in $56 \ L$ of oxygen gas at $STP$ are respectively:

$A$ mixture has $18 \ g$ of water and $414 \ g$ of ethanol. The mole fraction of water in the mixture is (assume ideal behavior of the mixture):

The number of $H$ atoms in $27 \ g$ of water is .........