# 4.S: The Mole and Measurements in Chemistry (Summary)

• The mole is defined as the number of atoms contained in exactly 12 grams of carbon-12 (the isotope ). There are 6.0221415 x 1023 particles in a mole. Remember, a mole is just a number (like dozen) and you can have a mole of anything.
• The concept of a mole is based on Avogadro’s Hypothesis (equal volumes of all gases at the same temperature and pressure contained the same number of molecules) and the number of particles in a mole (6.0221415 x 1023) is commonly referred to as Avogadro’s number (typically rounded to 6.02 x 1023 for most calculations).
• Because atomic masses, and the number of particles in a mole, are both based on the isotopic atomic mass of the isotope carbon-12, the mass of any substance expressed in atomic mass units is numerically equal to the molar mass of the substance in grams per mole. Thus, exactly 12 grams of carbon-12 contains exactly a mole of carbon atoms; likewise, 31.9988 grams of O2 contains 6.02214 x 1023 oxygen molecules (note, six significant figures), etc.
• To convert the number of moles of a substance into the mass of a substance, you simply need to multiply (moles x molar mass).
• To convert the mass of a substance into the number of moles, you simply need to divide the mass by the molar mass.
• To convert the number of moles of a substance into the number of particles of that substance, you simply need to multiply (moles x Avogadro’s number).
• The percentage composition of a compound, simply tells us what percent of the total mass arises from each element in the compound. To do the calculation, simply take the atomic mass of the element in question and divide it by the molar mass of the molecule.
• The empirical formula for a compound is the lowest whole-number ratio of the elements in that compound. For example, the molecular formula for glucose is C6H12O6, but the simplest whole-number ratio of the elements in glucose is CH2O.

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