The shorter way: add up the atoms
Now the good news. The real mass of a molecule does not have to be known — knowing its molecular formula is enough.
If the molecular formula of an element or a compound is known, its relative molecular mass can be calculated. That is because the relative molecular mass is the algebraic sum of the relative atomic masses of the atoms it contains.
Water, step by step
In a molecule of water (H2O), two hydrogen atoms are bonded to one oxygen atom. So the relative molecular mass of water is the sum of the relative atomic masses of two H atoms and one O atom.
Take the values from the table and add
H2O = (2 × 1) + 16
= 18
Relative atomic masses: H = 1, O = 16
Dividing two real masses on the previous page gave this same 18. The two methods agree — but this one is far quicker.
Do not lose the subscript
Writing H2O as 1 + 16 = 17 is the commonest error here. The subscript 2 is the number of times that atom's mass must be counted before adding.
No more dividing after this
For the rest of the chapter, relative molecular masses are found this way: read the formula, multiply each atom's relative atomic mass by its subscript, and add.