Overview:
Atoms and molecules are so tiny that scientists do not usually measure how many of them are needed in a chemical reaction atom by atom or molecule by molecule. The amount of atoms or molecules are usually measured in a standard unit called a mole. The number of atoms, molecules, or ions in a mole is always the same, but the mass of a mole differs based upon the atomic weight of the substance it contains.
What Is Atomic Mass?
The atomic mass of each element can be found on the periodic table under the symbol for the element. For example, the atomic mass of hydrogen is 1.0 amu (atomic mass units), the atomic mass of carbon is 12 amu, and the atomic mass of sodium is 23 amu, rounded to the nearest amu. It is measured in relative terms, as the weight in grams of any atom is so tiny.
What Is Formula Mass?
The formula mass of a molecule is the sum of atomic mass units of the elements in that molecule. For example, hydrogen atoms are usually in pairs, so the formula mass of a H2 molecule is 2. The formula mass of a water molecule (H2O) is the 2 amu for 2 atoms of hydrogen plus 16 amu for 1 atom of oxygen, or 18 amu. The formula mass of a salt molecule (NaCl) is 23 amu for sodium plus 35.5 amu for chlorine or 58.5 amu.
What Is A Mole?
A mole in chemistry is the same number, no matter what is being measured: 6.02 X 1023 atoms or molecules of a substance, also known as Avogadro’s number of molecules. The mass of one mole of a substance is related to the atomic mass of the substance. For example, the number of atoms in 12 grams of carbon is 6.02 X 1023. It was experimentally measured and used as the definition of a mole. The number of hydrogen atoms in 1.0 gram of hydrogen is the same, 6.02 X 1023, as are the number of oxygen atoms in 16.0 grams of oxygen.
What Is the Molar Mass?
The molar mass is the mass of a mole of a substance in grams. The type of substance must always be specified, because the masses of substances differ so much. Therefore, the molar mass of water is the mass of one mole of water, or 18 grams, and the molar mass of NaCl is the mass of one mole of NaCl or 58.5 grams.
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