The weight of one carbon dioxide molecule is

One carbon dioxide molecule has one carbon atom and two oxygen atoms. One carbon atom has 12 a.m.u and each oxygen atom has 16 a.m.u. Therefore, the total weight of one carbon dioxide molecule, ${\mathrm{CO}}_2$ is

$1\times \left(12 \mathrm{a}.\mathrm{m}.\mathrm{u}\right)+2\times 16\left(\mathrm{a}.\mathrm{m}.\mathrm{u}\right) =44\mathrm{a}.\mathrm{m}.\mathrm{u}$