When Is a Strong - Electrolyte Strong? -
An electrolyte can he defined as a substance that results in an electrically conducting solution when dissolved in water or some other nolar solvent. When several solutes are dissolved so that the resultine solutions have eoual concentrations, the higher the conductivity, the stronger the electn>lp.TheArrhenius theory c&cludcs that fm st& electrolytes thedegreeofdissociation, a,isequal to m e . Thequestimarises: ifarolute is 100"; ionized. 1s it a strong electrolyte? This question may arise when considering sparingly soluble ssltr such as AgCI. M r teach the student that 100% of the AgCl in solution is ionized. Is AgCl n stnmg electrolyte? To answer chis question. we should consider the electrical conductivity of silver chloride aolutionn as well as the complete ionization. 'Thesolubility of AgCl inwater, calculated from theK,,, is I..( X 10-'M,The limiting molareonductnnceof AgCIcan hecalculated from the limitine molar imiccmductancesuf the Ax' and CI-. which wlll be 138ohm-'cm'mol-:. Asanapproximation, the eonductivit;of a saturated AgCl solution is given by1 nAgawt. = ho X M = 1.8 X ohm-'cm-' Consider a 1.3 X 10W M acetic acid solution. At this concentration we consider the activities of the ions equal to the M. concentrations and calculate or = 0.65 with the respective concentrations of hydrogen and acetate ions as 8.5 X Using the limiting molar ionic conductances of Hf and CH3CO;, A. = 391 ~hm-~cm~mol-'. Applying the same approximation as with AgCI, the conductivity of the acetic acid solution is 5.1 X 10-6 ohm-'em-' as compared to 1.8 X 10-6 for AgCI. Is acetic acid a stronger electrolyte than AgCl? These calculations show the importance of stressing concentration when discussing strength of electrolytes. An appropriate definition for strong and weak electrolytes should include a minimum conductivity andlor concentration and/or solubility as well as degree of ionization. We might define a strong electrolyte as a substance that dissolves in water such that u = 1,and has same minimum conductivity value. Or we might define our,strong electrolyte as a substance that can form 0.001 M aqueous solutions with a conductivity greater than s reference solution. ~~
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' Brornherg, J. P. Physical Chemistry, 2nd. ed.; Allyn & Bacon: Boston, 1981; pp 315-316. Albert Kowalak University 01 Lowell Lowell. MA 01854
Volume 65
Number 7
July 1988
607
