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Answer 1 · 2017-09-22T14:24:31

Molecular mass of water 18 amu.
Molecular mass of carbon dioxide 44 amu.

Molecular mass is the sum of the atomic masses of the elements present in that molecule .

And atomic mass of a element is the total number of protons and neutrons present in that atom .

Consider water , the molecular formula for water is given by $H_2O$

(i.e., each molecule of water contains two hydrogen atoms and one oxygen atom)

Hydrogen contains one proton so it's atomic mass is 1 amu..

Oxygen contains 8 protons and 8 neutrons so it's atomic mass is 16 amu.

atomic mass of water $M_(H_2O)$ =2 x atomic mass of hydrogen(hence there are two atoms present in a molecule ) + atomic mass of oxygen.

$M_(H_2O)$ =2 x 1+16 amu.
$M_(H_2O)$ =18 amu.
( 1 amu is atomic mass unit which is equal to $1.6xx10^-27kg$ )

From the above idea of calculating the molecular mass of water we calculate it for carbon dioxide.

Molecular formula for carbon dioxide is $CO_2$ .

So every molecule of carbon dioxide contains two oxygen atoms and one carbon atom.

atomic mass of oxygen=16 amu.

carbon contains 6 protons and 6 neutrons so it's atomic mass is 12 amu.

$M_(CO_2)$ =2 x atomic mass of oxygen + atomic mass of carbon.

$M_(CO_2)$ =2 x 16 +12.
$M_(CO_2)$ =44.