Likewise, there are 2E atoms of hydrogen present on the right side and A atoms on the left, yielding
Equations of this type should be written for each element present. The equation resulting from the consideration of oxygen is
Finally, the equation resulting from the consideration of potassium is
The equating of elements yields four equations in five unknowns. The next step is to solve for four of the variables in terms of the fifth. It is irrelevant which of the variables is the "fifth," although some choices may provide easier calculations than others. In this case, let E be the "fifth" variable. Choosing E simplifies calculations because one of the variables is already present in terms of E. Equation (8) defines A as equal to 2E. Substituting this value for A into equation (9) yields a definition of D in terms of E.
