Why is a balanced equation necessary to solve any stoichiometry problem?

Basically; because a balanced equation will show how many molecules of one compound will react with how many molecules of another. This can be stated as the number of moles of one compound that will react with a given number of moles of another. You need to know the number of moles; so that you can translate the number of grams of one compound into moles; and then you can convert the number of moles of another into grams. eg: hydrogen combining with oxygen to produce water H2 + O2 = H2O Balanced equation: 2H2 + O2 = 2H2O Two moles of hydrogen react with one mole of oxygen If we have 8 grams of oxygen Molecular mass O2 = 32 (ie, 2x16) 8/32 = 1/4 mole O2 therefore, if 2 moles of hydrogen combine with one mole oxygen; 1/4x2 moles H2 combine with 1/4 mole O2 1/4X2 = 1/2 mole H2 Molecular mass H2 = 2.016 1/2 mole = 1.008g Therefore 1.008 g H2 reacts with 8 grams O2