Calculate the molecular masses of $H_2$,$O_2$ and $Cl_2$.

Calculate the number of aluminium ions present in $0.051 \, g$ of aluminium oxide.
[Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of $Al = 27 \, u$.]

Convert the following into moles:
$(a)$ $12\, g$ of oxygen gas
$(b)$ $20\, g$ of water
$(c)$ $22\, g$ of carbon dioxide.

Hydrogen and oxygen combine in the ratio of $1:8$ by mass to form water. What mass of oxygen gas would be required to react completely with $3 \,g$ of hydrogen gas (in $,g$)?

If one mole of carbon atoms weighs $12 \, g$,what is the mass (in grams) of $1$ atom of carbon?

What are polyatomic ions? Give examples.