Calculations in chemistry ebook

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Learn more. Kortext home Store Log in Sign up. Don't have an account? Sign up. Already have an account? Log in. You have no items in your shopping cart. Home Books Calculations in Chemistry. Calculations in Chemistry. Add to cart Buy now. Confirm your account. Not you? The relative masses shown in the bottom line establish the stoichiometry of the reaction, that is, the relations between the masses of the various components.

Since these masses vary in direct proportion to one another, we can define what amounts to a conversion factor sometimes referred to as a chemical factor that relates the mass of any one component to that of any other component. But the same relation applies to any common set of mass or weight units, so we can simply say that the mass ratio of CO 2 -to-CO expressed by the above equation is 1. This, in turn, allows us to easily handle such problems as the following:.

Don't expect to pass Chemistry unless you can handle problems such as the ones below; they come up frequently in all kinds of contexts. If you feel the need for more guidance, see one of the video tutorials listed near the bottom of this page. Most chemical reactions that take place in the real world begin with more or less arbitrary amounts of the various reactants; we usually have to make a special effort if we want to ensure that stoichiometric amounts of the reactants are combined.

This means that one or more reactant will usually be present in excess; there will be more present than can react, and some will remain after the reaction is over. At the same time, one reactant will be completely used up; we call this the limiting reactant because the amount of this substance present will control, or limit, the quantities of the other reactants that are consumed as well as the amounts of products produced.

Limiting reactant problems are handled in the same way as ordinary stoichiometry problems with one additional preliminary step: you must first determine which of the reactants is limiting— that is, which one will be completely used up. To start you off, consider the following very simple example. The concept of limiting reactants touches us all in our everyday lives — and as we will show in the second example below, even in the maintenance of life itself!

Combustion is an exothermic process in which a fuel is combined with oxygen; complete combustion of a hydrocarbon fuel such as methane or gasoline yields carbon dioxide and water:.

Complete combustion of each kg of methane consumes 4 kg of dioxygen, which is supplied by the air. In the classic Bunsen burner, this air is admitted through an adjustable opening near the base.

When it is fully open, the flame is blue in color and achieves its maximum temperature, indicating that combustion is approximately stoichiometric. If the opening is gradually closed, the appearance of the flame changes as illustrated. Under these conditions, oxygen becomes the limiting reactant and combustion is incomplete.

Incomplete combustion is generally undesirable because it wastes fuel, produces less heat, and releases pollutants such as carbon soot. Energy-producing combustion processes should always operate in fuel-limited mode. In ordinary combustion processes, the source of oxygen is air. Because only about 20 percent of the molecules in dry air consist of O 2 , the volume of air that must be supplied is five times greater than what would be required for pure O 2.

For the detailed calculation, see here. In order to ensure complete combustion, it is common practice to maintain a slightly lean mixture. However, practical considerations necessitate differing ratios at various stages of operation. Typical values vary from a rich ratio for starting or acceleration to slightly lean ratios for ordinary driving.

These ratios are set by the carburetor , with additional control by the engine computer and exhaust-line oxygen sensor in modern vehicles, or by a manual choke in earlier ones. Our bodies require a continual supply of energy in order to maintain neural activity, synthesize proteins and other essential biochemical components, replace cells, and to power muscular action.

The "fuel" — the carrier of chemical energy — glucose, a simple sugar which is released as needed from the starch-like polymer glycogen , the form in which the energy we derive from food is stored. Arterial blood carries dissolved glucose along with hemoglobin-bound dioxygen to individual cells which are the sites of glucose "combustion":. The net reaction and the quantity of energy released are the same as if the glucose were burned in the open air, but within the cells the reaction proceeds in a series of tiny steps which capture most of this energy for the body's use, liberating only a small fraction of it as thermal energy heat.