$3.42 \, g$ of sucrose $(C_{12}H_{22}O_{11})$ is dissolved in $18 \, g$ of water $(H_2O)$. The number of oxygen atoms in the solution is:

Give one example each of:
$(a)$ Triatomic element
$(b)$ Monoatomic molecule
$(c)$ Polyatomic element
$(d)$ Polyatomic ion

Find the ratio by mass of the combining elements in the following compounds. (You may use Appendix $-III$)
$(a)$ $CaCO_{3}$ $(b)$ $MgCl_{2}$
$(c)$ $H_{2}SO_{4}$ $(d)$ $C_{2}H_{5}OH$
$(e)$ $NH_{3}$ $(f)$ $Ca(OH)_{2}$

$(a)$ Calculate the mass of $0.5$ mole of oxygen atoms.
$(b)$ Calculate the number of molecules of glucose present in its $90 \ g$ (molecular mass of glucose is $180 \ u$). (Avogadro constant $= 6.022 \times 10^{23} \ mol^{-1}$)
$(c)$ Calculate the number of moles of water in $2 \ g$ of water.
(Atomic mass of $H = 1 \ u, O = 16 \ u$)

Define Avogadro's constant. How is it related to a mole?

The chemical symbol for nitrogen gas is