Calculate the molecular formula of compound which contains $20\% \mathrm{Ca}$ and $80\% \mathrm{Br}$ (by weight), if molecular weight of compound is $200$. (atomic weight of $\mathrm{Ca}=40, \mathrm{Br}=80$)

A mixture contains $5.4 \mathrm{g}$ of $\mathrm{Al}$, $1.2 \mathrm{g}$ of $\mathrm{Mg}$ and $4.6 \mathrm{g}$ of ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$. The ratio of their moles is

(Atomic weights of $\mathrm{Al}=27 \mathrm{u}, \mathrm{Mg}=24 \mathrm{u}, \mathrm{C}=12 \mathrm{u}, \mathrm{O}=16 \mathrm{u}, \mathrm{H}=1 \mathrm{u}$)

The number of atoms in $3.2 \mathrm{g}$ of oxygen gas is

$1$ mole of ${}_7{}^{14}\mathrm{N}^{-3}$ ions contains:

a. $7\times 6.023\times {10}^{23}$ electrons.

b. $7\times 6.023\times {10}^{23}$ protons.

c. $10\times 6.023\times {10}^{23}$ electrons.

d. $14\times 6.023\times {10}^{23}$ protons.