$A$ gas jar contains $1.7 \, g$ of ammonia gas. Calculate the following:
$(i)$ Molar mass of ammonia.
$(ii)$ How many moles of ammonia are present in the gas jar?
$(iii)$ How many molecules of ammonia are present in the sample?

$(i)$ Molar mass of $NH_3 = 14 + (3 \times 1) = 17 \, g/mol$.
$(ii)$ Number of moles $= \frac{\text{Given mass}}{\text{Molar mass}} = \frac{1.7 \, g}{17 \, g/mol} = 0.1 \, mol$.
$(iii)$ Number of molecules $= \text{Number of moles} \times \text{Avogadro's number} = 0.1 \times 6.022 \times 10^{23} = 6.022 \times 10^{22}$ molecules.