40 MEASUREMENTS 



Energy exists in other forms: as electrical energy, as the kinetic energy 

 of motion, as radiant energy, or in one of several forms of potential 

 energy. The interrelationships of the various kinds of energy and trans- 

 formations from one form to another will not be considered here. Meas- 

 urements of energy depend very strongly upon electrical instruments of 

 one kind or another. Electrical measurements will be taken up in Chap- 

 ter 13, and measurements of radiant energy are dealt with in Chapters 

 12 and 13. Potential energy actually cannot be measured directly but 

 instead is calculated from the work required to transform the energy to 

 this storage form or from the energy released when the potential energy 

 is transformed into one of the other kinds. Examples of potential energy 

 are the energy possessed by a body held in an unstable position and the 

 bonding energy of chemical compounds. We might say that a molecule 

 of glucose contains a certain amount of energy, but almost certainly we 

 mean the energy that will be liberated if and when this molecule is 

 converted chemically into some more stable, lower energy compound. 

 Chemical potential energy is usually expressed in kilocalories per mole 

 of reacting material. 



Chemical Quantities and Concentrations: Chemical compounds exist 

 as molecules, but molecules are much too small to be treated as units. 

 A much larger unit, the mole, is Avogadro's number (6.02 X 10^^) of 

 molecules. Avogadro's number (N) was originally devised in studies 

 with gases when it was found that a given volume of any gas at a given 

 temperature and pressure contains the same number of molecules. One 

 mole of any gas at 0° C and a pressure of one atmosphere occupies 22.4 

 liters. Molecular weight is a relative figure, calculated by adding atomic 

 weights, which in turn are determined relative to oxygen, which is given 

 an atomic weight of 16. An amount of a compound with a mass equal 

 to its molecular weight expressed in grams contains N molecules. 



Most biological reactions occur in solutions; that is, the reacting mole- 

 cules are dissolved in water or occasionally in some other solvent. Con- 

 centration is a measure of the amount of the dissolved substance (solute) 

 in a unit volume of solution; several methods of expressing concentration 

 are in common use. If we dissolve one gram-molecular weight of glucose 

 (180 g) in enough water to make one liter of solution, the concentration 

 is one mole per liter (1 m/1), which is frequently contracted to one 

 molar (1m). The physical chemist uses an expression, molal, for a solu- 

 tion in which one mole of a material is dissolved in 1000 g of solvent. 



Solutions of acids which ionize to produce only one H"*" per mole- 

 cule can be labeled in molar concentrations, but if the acid should 



