What is the mass of one mole of oxygen atoms (in $g$)?

Mass of one atom of oxygen is

Which of the following species have $18$ electrons? $Ca^{2+}, K^{+}, Na, Cl^{-}, Ar$. Also,state the reason why the chemical properties of all isotopes of an element are similar.

Who proposed the laws of chemical combination?

One mole of $H_{2}O$ means:
$(i)$ $18 \text{ g}$ of $H_{2}O$
$(ii)$ $6.022 \times 10^{23}$ molecules of $H_{2}O$
$(iii)$ $180 \text{ g}$ of $H_{2}O$
$(iv)$ $1.8 \text{ g}$ of $H_{2}O$

The chemical formula of the chloride of a metal $M$ is $MCl$. Write the chemical formula of its oxide,bicarbonate,and sulphide.