Basis

Have you noted in previous examples that the word basis has appeared at the top of the computations? This concept of basis is vitally important both to your understanding of how to solve a problem and also to your solving it in the most expedit ious manner. The basis is the reference chosen by you for the calculations you plan to make in any particular problem, and a proper choice of basis frequently makes the problem much easier to solve. The basis may be a period of time-for example, hours, or a given mass of material-such as 5 kg of CO2 or some other convenient quantity. In selecting a sound basis (which in many problems is predetermined for you but in some problems is not so clear), you should ask yourself the following
questions:

(a) What do I have to start with?
(b) What do I want to find out?
(c) What is the most convenient basis to use?

These questions and their answers will suggest suitable bases. Sometimes, when a number of bases seem appropriate, you may find it is best to use a unit basis of I or 100 of something , as, for example, kilograms, hours, moles, cubic feet. For
liquids and solids when a weight analysis is used , a convenient basis is often I or 100 Ib or kg; similarly, I or 100 moles is often a good choice for a gas. The reason for these choices is that the fraction or percent automatically equals the number of pounds, kilograms, or moles, respectively, and one step in the calculations is saved.