Question

How many grams are in $4.500$ moles of $L{i_2}O$ ?

Answer 1

We must know the molecular mass of the given compound. Molecular mass is calculated as the sum of the atomic weights of each individual element constituting a molecule. For example, the molecular weight of water which has two atoms of hydrogen and one atom of oxygen has a molecular weight of $18$ since the atomic weight of each hydrogen is $1$ and that of oxygen is $16$. Multiplying this molecular mass by the given number of moles will give the weight in grams of the substance.

Complete step by step answer:
The given molecule is $L{i_2}O$ . We can get the atomic mass of the constituent elements of $L{i_2}O$ from the periodic table. The elements are Lithium, $Li$ and oxygen $O$. The atomic mass of Lithium is $6.49g/mol$ and that of oxygen is $15.99g/mol$ . Therefore, the molecular mass of $L{i_2}O$ is as follows:
Since there are two molecules of lithium, therefore the mass is $2 \times$ $2 \times 6.49 = 13.882g/mol$
There is one molecule of oxygen whose mass is $15.99g/mol$ . Hence the total molecular mass is $13.882 + 15.99 = 29.881g/mol$

Therefore $4.500$ moles $L{i_2}O = 29.881g/mol \times 4.500moles = 134.46grams$.

Note: It must be noted that the rules given above for the molecular mass of a compound is applicable only for molecules non-ionic compounds only which carry no charge. Also, there are different names for molecular mass of a compound. The name may be molar mass, molecular weight, gram formula mass, etc but they all account for the mass of one mole of a substance in grams per mole. It must also be noted that sometimes the atomic masses of the elements are not given but we can obtain them from the periodic table.