Calculate the molecular mass of the following:
$(i)$ $H_{2}O$
$(ii)$ $CO_{2}$
$(iii)$ $CH_{4}$

$(i)$ $H_{2}O$
The molecular mass of water,$H_{2}O$ is calculated as:
$= (2 \times \text{Atomic mass of H}) + (1 \times \text{Atomic mass of O})$
$= [2(1.008 \, u) + 1(16.00 \, u)]$
$= 2.016 \, u + 16.00 \, u = 18.016 \, u \approx 18.02 \, u$
$(ii)$ $CO_{2}$
The molecular mass of carbon dioxide,$CO_{2}$ is calculated as:
$= (1 \times \text{Atomic mass of C}) + (2 \times \text{Atomic mass of O})$
$= [1(12.011 \, u) + 2(16.00 \, u)]$
$= 12.011 \, u + 32.00 \, u = 44.011 \, u \approx 44.01 \, u$
$(iii)$ $CH_{4}$
The molecular mass of methane,$CH_{4}$ is calculated as:
$= (1 \times \text{Atomic mass of C}) + (4 \times \text{Atomic mass of H})$
$= [1(12.011 \, u) + 4(1.008 \, u)]$
$= 12.011 \, u + 4.032 \, u = 16.043 \, u$