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Hint: 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 $$2 \times 6.49 = 13.882g/mol$
There is one molecule of oxygen whose mass is $15.99g/mol$ . Hence the total molecular mass is $13.882 + 15.99 = 29.881g/mol$
Therefore $4.500$ moles $L{i_2}O = 29.881g/mol \times 4.500moles = 134.46grams$.
Note: It must be noted that the rules given above for the molecular mass of a compound is applicable only for molecules non-ionic compounds only which carry no charge. Also, there are different names for molecular mass of a compound. The name may be molar mass, molecular weight, gram formula mass, etc but they all account for the mass of one mole of a substance in grams per mole. It must also be noted that sometimes the atomic masses of the elements are not given but we can obtain them from the periodic table.
Complete step by step answer:
The given molecule is $L{i_2}O$ . We can get the atomic mass of the constituent elements of $L{i_2}O$ from the periodic table. The elements are Lithium, $Li$ and oxygen $O$. The atomic mass of Lithium is $6.49g/mol$ and that of oxygen is $15.99g/mol$ . Therefore, the molecular mass of $L{i_2}O$ is as follows:
Since there are two molecules of lithium, therefore the mass is $2 \times $$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.
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