The DensstySpecific Gravity Connection
Recently a student asked,'"Why do we use both density and specific gravity? They have about the same value". The answer can be found in our inability to measure volumes a s effectively as we can measure mass. Amilliliter sample of liquid can be weighed to four decimals on a n ordinary analytical balance but to only one decimal for the volume using a small graduated cylinder. This disparity between mass and volume can be eliminated by a pycnometer, which allows very small samples to be analyzed and requires only weighings. In essence a pycnometer provides density comparisons between a standard liquid, usually distilled water, and an unknown sample. Specific gravity =
Mass of X Volume Mass of Water Volume
Since the volume is the same for both density measurements, it will cancel out leaving only the more valuable mass data. To mnvert specific gravity to density, simply multiply the specific gravity of the sample by the density of the standard a t the temperature of use. A compromise is often encountered by the use specific gravity with exponential notation, which indicates the temperatures a t which the two weighings were made, for example, 0.901 2a5.
Reference MeCullough,T, J. C k m . Edue ls?S,bO, 546.
Thomas McCullough, CSC St. Edward's University Austin, TX 78704
46
Journal of Chemical Education
