Formula mass. The formula mass of a substance is the sum of the atomic masses of the atoms in its formula.

The mole concept. The mole is the chemist's counting unit. One mole of any substance - element or compound - consists of 6.02 ´ 1023 formula units of the substance. Avogadro's number is the name given to the numerical value 6.02 ´ 1023.

Molar mass. The molar mass of a substance is the mass in grams that is numerically equal to the substance's formula mass. Molar mass is not a set number; it varies and is different for each chemical substance.

The mole and chemical formulas. The numerical subscripts in a chemical formula give the number of moles of atoms of the various elements present in 1 mole of the substance.

Chemical equation. A chemical equation is a written statement that uses symbols and formulas instead of words to represent how reactants undergo transformation into products in a chemical reaction.

Balanced chemical equation. A balanced chemical equation has the same number of atoms of each element involved in the reaction on each side of the equation. An unbalanced equation is brought into balance through the use of coefficients. A coefficient is a number that is placed to the left of the formula of a substance and that changes the amount, but not the identity, of the substance.

The mole and chemical equations. The coefficients in a balanced chemical equation give the molar ratios between substances consumed or produced in the chemical reaction described by the equation.