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What Is Formula Weight?

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  • Written By: Vasanth S.
  • Edited By: R. Halprin
  • Last Modified Date: 05 September 2014
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Formula weight is a measurement 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 approximately 16 grams per mole. The 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.

The 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 which establishes how many molecules are present in a specific amount of substance. It is called Avogadro's number and is about 6.02×10^23 molecules or atoms.

As mentioned, one mole of the oxygen atom weighs about 16 grams. That means 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.

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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 has a weight of 30 grams, then the number of moles in the compound is found by dividing 30 grams by 18 grams per mole. The answer is about 1.67 moles of water.

The calculation can be checked by determining whether the moles appear to correlate with 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 percent heavier, will have more molecules of water. Also, the formula weight can be used to determine the ratio of each atom within a molecule.

Generally, the formula weight is used to calculated the percentage by weight, which is the amount of weight each atom contributes to the overall molecule. It is usually determined by dividing the weight of the individual atom 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 which is attributed to oxygen is about 89 percent.

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