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Perhaps no scientific term is as commonly used and commonly misunderstood as pH. This term, which is used to sell consumer products (e.g., "pH adjusted"), is simply an abbreviation for a mathematical expression of the concentration (or power, p) of hydrogen ions (H+ ) in a solution:
pH = - log(H+ )
where (H+ ) is the molar concentrations of hydrogen cations, and
molarity (M) is the number of moles of solute per liter of solution
Molarity is used in chemical calculations, because it reduces ratios of reactants to their simplest common denominator (i.e., number of reactive units). For example, 1 hydrogen atom will combine with 2 oxygen atoms to for 1 water molecule. This is independent of the masses of the hydrogen or oxygen atoms, which are quite different (i.e., the atomic mass of hydrogen is 1.0079 and the atomic mass of oxygen is 15.9994). Moreover, there are different isotopes of hydrogen and oxygen, which markedly different masses (e.g., 1H, 2H, 3H), that may be involved in the formation of a water molecule.
In a sense, this is no different that the job of the host of "The Love Connection". Their job is to match up singles to form couples (and sell a lot of soap in doing so). It doesn't matter whether the individuals weigh 60 pounds or 300 pounds, as long as they are mutually attractive. In contrast, trying to pair them up by matching the weights of the contestants could result in some unnecessary surgeries or some unstable combinations.
The reason for using the power (p) term (which simply means taking the negative of the logarithm of the concentration) is also simplicity. Scientists avoid writing out very little or very big numbers whenever possible, and this is an easy way of doing it *. Hydrogen ion concentrations commonly range from 0.1 M to 0.00000000000001 M, which is equivalent to -1 (10 -1) to -14 (10 -14) when converted to logarithms. Similarly, taking the negative of those logarithmic numbers [i.e., - (-1) = 1, and 1(-14) =14] makes the numbers even simpler. So, as much as it may seem that pH is designed to confuse and annoy students and consumers, it really is designed to make things simpler for everyone.
* Another is to change units. For example, the units used to express the amount may change from ounces to pints to quarts to gallons as the volume increases. Again scientists simplify such unwieldy conversions by using the metric system, which makes all unit conversions occur in increments of ten (e.g., 1000 mm = 100 cm = 10 dm = 1 m).
atomic mass unit (amu) a unit used to express the mass of individual atoms and molecules. It is based on the amount (1 mole) of carbon-12 atoms that weigh 12 grams. This is Avogadro's number 602,000,000,000,000,000,000,000 or 6.02 x 10 23 atoms. If these were marshmallows, they would cover the United States to a depth of 650 miles. If these were pennies that were equally distributed among 5 billion people inhabiting the Earth, every one of them could spend one million dollars every hour of the day, and half of the pennies would not be spent in their lifetime. In comparison, the equivalent number of carbon atoms amounts to a teaspoon of soot (from Schwartz et al., 1994).
mole Avogadro's number (6.02 x 1023 ) of anything: atoms, molecules, apples, oranges,…
Schwartz, A.T. et al.1994. Chemistry in Context: Applying Chemistry to Society. American Chemical Society, Wm. C. Brown Communications, Inc., Dubuque, IA.
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