Peter J. Mikulecky

Chemists aren’t satisfied with measuring length, mass, temperature, and time alone. On the contrary, chemistry often deals in calculated quantities. These kinds of quantities are expressed with derived units, which are built from combinations of base units.Here are some examples: Area (for example, catalytic surface).

A conversion factor uses your knowledge of the relationships between units to convert from one unit to another. For example, if you know that there are 2.54 centimeters in every inch (or 2.2 pounds in every kilogram or 101.3 kilopascals in every atmosphere), then converting between those units becomes simple algebra.

In chemistry, you can add and subtract extreme numbers by using exponential notation, and expressing your numbers as coefficients of identical powers of 10. To wrestle your numbers into this form, you may need to use coefficients less than 1 or greater than 10. Adding with exponential notation To add two numbers by using exponential notation, you begin by expressing each number as a coefficient and a power of 10.

In the world of chemistry, not all acids and bases are created equally. Some have an innate ability to neutralize more effectively than others.Consider hydrochloric acid (HCl) and sulfuric acid (H2SO4), for example. If you mixed 1 M sodium hydroxide (NaOH) together with 1 M hydrochloric acid, you'd need to add equal amounts of each to create a neutral solution of sodium chloride (NaCl) and water (H2O).

When gases mix, each individual gas within the mixture contributes a partial pressure, and adding the partial pressures yields the total pressure. This relationship is summarized by Dalton's law of partial pressures for a mixture of individual gases:This relationship makes sense if you think about pressure in terms of kinetic molecular theory.

For the chemist, Hess's law is a valuable tool for dissecting heat flow in complicated, multistep reactions. For the confused or disgruntled chemistry student, Hess's law is a breath of fresh air. In essence, the law confirms that heat behaves the way we'd like it to behave: predictably.Imagine that the product of one reaction serves as the reactant for another reaction.

Increasing temperature magnifies the effects of entropy on a system. Because the entropy of a solute is usually increased when it dissolves, increasing temperature usually increases solubility — for solid and liquid solutes, anyway. Another way to understand the effect of temperature on solubility is to think about heat as a reactant in the dissolution reaction:Heat is usually absorbed into solute particles when a solute dissolves.

Like the difference in their names, the practical difference between molarity and molality is subtle. Take a close look at their definitions, expressed next to one another in the following equations:The numerators in molarity and molality calculations are identical, but their denominators differ greatly. Molarity deals with liters of solution, while molality deals with kilograms of solvent.