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12.0 THE MOLE: FORMULAS AND CHEMICAL EQUATIONS

Chemistry is a quantitative science. To know how much of a reactant to use or how much product to expect, we need a way to count atoms and molecules. But atoms are far too small to count individually. The mole is the chemist's counting unit – a way to link the mass of a substance to the number of particles it contains. This chapter introduces the mole, relative atomic and molecular masses, and lays the foundation for all stoichiometric calculations.

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12.1 RELATIVE ATOMIC MASS (R.A.M.)

Atoms are incredibly tiny, so their actual masses in grams are very small numbers (e.g., a hydrogen atom weighs about 1.67 × 10⁻²⁴ g). It's not practical to work with such numbers. Instead, we use relative masses.

Relative Atomic Mass (R.A.M. or Aᵣ) is defined as:

The average mass of an atom of an element, compared to 1/12th of the mass of an atom of carbon-12.

• It is a ratio and therefore has no units.
• The carbon-12 atom is chosen as the standard reference and is assigned a mass of exactly 12 atomic mass units.
• For most elements, the R.A.M. takes into account the different isotopes and their relative abundances. It is the weighted average mass of all the atoms of that element.

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