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

Natural abundances of $^{12}C$ and $^{13}C$ isotopes of carbon are $99\,\%$ and $1\,\%$,respectively. Assuming they only contribute to the molecular weight of $C_2F_4$,the percentage of $C_2F_4$ having a molecular mass of $101$ is $.....$

The number of atoms in $0.004 \ g$ of magnesium is:

Number of atoms present in $224 \, dm^3$ of oxygen gas at $STP$ is

The mass of $2 \times 10^{22}$ nitrogen atoms is ..... grams.

The mass and volume of $1/4$ mole of oxygen gas at $STP$ are respectively .....