\^ K^ ±^ J. \^ X\/X-J 1.WX JUt X-^X iX. 



while a mole of sucrose is 342 g. A liter of solution containing one mole 

 of a compound is said to be one molar (il/). Equimolar solutions contain 

 the same number of molecules. In problems in physiology, such as 

 osmotic pressure, which have to do with the numbers of molecules it is 

 necessary to use this way of expressing concentration. If it is desired to 

 compare the effect of the osmotic pressure due to glucose and sucrose, the 

 concentration must be expressed in terms of molar strengths, for the 

 osmotic pressure is a function of the number of molecules of solute in a 

 solution. If it is the purpose to compare the effect of glucose and sucrose 

 on the amount of growth of a fungus, this method of expressing concen- 

 trations should not be used. Media of equal molarity with respect to 

 sucrose and glucose do not contain the same amount of carbon. The 

 first contains twice as much carbon as the second. Just as a milligram is 

 one-thousandth of a gram, a millimole is one-thousandth of a mole. The 

 meaning of micromole and millimicromole should be obvious. 



If the weight of a compound is given in grams, this datum may be con- 

 verted into moles. If a medium contains 50 g. of glucose per liter, the 

 glucose concentration may be expressed as 50/180 or 5/18ilf. Con- 

 versely, if the concentration of sucrose in a medium is stated to be 0.15ilf, 

 the weight of sucrose is 0.15 X 342 or 51.3 g. per liter. These conversions 

 imply that the molecular weight is known or can be calculated. In pre- 

 paratory work compounds are weighed on a balance as grams, not as 

 moles, and unless the interpretation of the results demands conversion to 

 moles, it is better to record the weights than to convert these data to 

 derived units. The mole and molar solutions are particularly useful in 

 dealing with non-ionizing compounds. 



Another derived unit, the equivalent, is frequently used to express the 

 concentration of ionized compounds. An equivalent is the atomic weight 

 of an ion expressed in grams divided by the valence of the ion. If an ion 

 is composed of more than one atom, the ion weight is computed by adding 

 together the atomic weights. It is important to remember that, if an 

 element has more than one valence, the equivalent weight depends upon 

 the valence. An equivalent of ferrous (Fe++) ion is 55.8/2 or 27.9 g., 

 while an equivalent of ferric (Fe+++) ion is 55.8/3 or 18.6 g. A normal 

 solution (A^) is one which contains one equivalent in a liter of solution. 

 In dealing with small amounts it is convenient to use milliequivalents or 

 microequivalents. 



In preparing a series of media for the purpose of comparing the growth 

 of a fungus on different nitrogen sources, the nitrogen content of the media 

 should be equal. If urea, CO(NH2)2, and aspartic acid, HOOC — CH2 — 

 CH(NH2) — COOH, are used, it is obvious that different weights of these 

 nitrogen sources must be used if the media are to contain equal amounts of 

 nitrogen. Whenever media are modified by replacing one compound by 



1 



