These are also given in slightly different form on page 150. Both sets of rules give the same results. All oxidation numbers must add up to the real charge on the ion or molecular ion. Molecules oxidized have had their oxidation number increase in a chemical reaction, while molecules reduced have had their oxidation number decrease. Always an increase must be accompanied by a decrease.
Oxidizing agent is a species that is reduced and thus enables something else to be oxidized.
Similarly a reducing agent is one that suffers oxidation and by so doing enables something else to be reduced. The amount of a compound present in solution is given by a unit of concentration called molarity. To find concentration, divide moles by liters. A very important volumetric laboratory technique is titration. When enough of a reagent has been added to reach the end-point, an indicator changes color.