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Formula weight is a measure of a chemical compound’s weight based on its formula. It is calculated from the molecular formula and is also known as the empirical formula. The mole is a constant used to determine the number of molecules in a given amount of substance. Formula weight is used to calculate weight percent and to determine the ratio of each atom within a molecule.
Formula weight is a measure used to identify how much a specific chemical compound weighs based on its formula. For example, the formula weight of the oxygen molecule is about 32 grams per mole; this is because the oxygen molecule consists of two oxygen atoms and each oxygen atom weighs about 16 grams per mole. Formula weight is usually calculated from the molecular formula, which is the lowest number of atoms required to comprise a molecule. It is also known as the empirical formula.
Formula weight describes the molecular weight of a compound. Molecular weight is determined by dividing the number of grams of the specific chemical by the number of moles of the specific chemical. The mole is a constant that establishes how many molecules are present in a given amount of substance. It is called Avogadro’s number and is approximately 6.02×10^23 molecules or atoms.
As mentioned, one mole of the oxygen atom weighs about 16 grams. This means that 16 grams of oxygen will contain 6.02×10^23 oxygen atoms. Similarly, one mole of the sodium atom weighs about 23 grams. This means that 23 grams of sodium will contain 6.02 × 10^23 sodium atoms.
In terms of formula weight, the number of moles of an unknown sample can be determined by weighing the sample and using the known formula weight. For example, water has a molecular weight of 18 grams per mole. If the amount of water in a sample weighs 30 grams, the number of moles in the compound is obtained by dividing 30 grams by 18 grams per mole. The answer is about 1.67 moles of water.
The calculation can be verified by determining whether moles appear to be related to the number of grams. For example, if one mole of water has 18 grams, then surely a sample with 30 grams of water, which is about 60% heavier, will have more water molecules. Also, formula weight can be used to determine the ratio of each atom within a molecule.
Generally, formula weight is used to calculate weight percent, which is the amount of weight each atom contributes to the overall molecule. It is usually determined by dividing the single atom weight by the total weight of the molecules and then multiplying by 100. For example, water has a total molecular weight of 18. The percentage of that weight that is attributed to oxygen is approximately 89 percent.
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