In the Laboratory 1644 Journal of Chemical Education • Vol. 81 No. 11 November 2004 • www.JCE.DivCHED.org The relationship between solution ionic strength and solubility of an electrolyte is important in biochemistry, in- dustrial chemistry, and environmental chemistry. It affects diverse topics such as wastewater treatment, trace metal trans- port, and aquatic toxicology. The theory of activity versus concentration is usually taught to undergraduate students in physical chemistry, a course most biology and environmen- tal science students do not take. The increase in solubility of an electrolyte in a solution of a second electrolyte compared with pure water is not an easy concept to grasp because it seems to be counterintuitive: more salt can dissolve in a salty solution (assuming no common ion, and concentrations be- low approximately 1 M). The simple experiment described here illustrates this principle visually and dramatically. Experimental Overview The experiment can easily be completed in one three- hour lab period, and it uses safe and inexpensive reagents. Students attempt to dissolve 0.40 g of CaSO 4 2H 2 O (gyp- sum) into 100 mL of deionized water, and into 100 mL of a 0.25 M solution of NaCl. This quantity of gypsum dissolves almost completely in the sodium chloride solution and makes a clear solution with only a few visible solid grains remain- ing, however the pure water remains turbid with obvious undissolved solid. Students then analyze filtered samples of each solution (by EDTA titration or flame atomic absorp- tion analysis) to determine the total calcium concentration. The students see that gypsum really is more soluble in the NaCl solution than in pure water. Solubility calculations (Table 1) performed after completion of the experiment al- low students to explain what they have seen, and students also learn a method by which mean activity coefficients can be measured experimentally. Equilibrium Constants and Activity Coefficients This experiment can be used to emphasize that the real power of equilibrium constants is that they can be used in any aqueous solution as long as activity coefficients are con- sidered. Other experiments have taken a different approach, of measuring the effect of ionic strength on the value of an equilibrium constant when activity coefficients are neglected (K c ) (1–3), including extrapolation to obtain the equilibrium constant at zero ionic strength (1, 4–5). This approach can leave the student thinking that one must use a different equi- librium constant value for every solution, which greatly di- minishes the usefulness of the equilibrium constant. The Effect of Ionic Strength on the Solubility of an Electrolyte W Joan D. Willey Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington, NC 28403-5932; willeyj@uncw.edu O S a C f o n o i t a l u c l a C e h t r o f s n o i t a u q E d n a s t n a t s n o C . 1 e l b a T 4  H 2 2 n o i t u l o S s u o e u q A n i y t i l i b u l o S ) s ( O 5 2 t a ° s n o I n o m m o C o N h t i w C a e r A n o i t a l u c l a C s n o i t a u q E r e b m u N n o i t a u q E n o i t a m r o f n I d n u o r g k c a B (i = ) γ i [i] 01 I 5 . 0 = ∑ [i]z i 2 02 g o l γ i =  ( 9 0 5 . 0 z i ) 2 [ { I 5 . 0 + 1 ( / I 5 . 0 3 . 0 – ] ) I} 03 a C [ + 2 O S [ = ] ) q a ( 4 2 d n a ] ) q a ( 04 γ– = γ + 2 a C = γ 2 4 O S  t c u d o r P y t i l i b u l o S K 0 s 0 1 x 3 6 . 2 = 5 K 0 s a C ( = + 2 O S ( ) ) q a ( 4 2 a C ( = ) ) q a ( + 2 ) ) q a ( 2 05 a C ( + 2 = ) ) q a ( K 0 s 5 . 0 06 a C [ + 2 = ] ) q a ( K 0 s 5 . 0 /γ a C + 2 a 07 t n a t s n o C n o i t a m r o F r i a P - n o I K 4 0 2 = K O S a C ( = 4 a C ( ( / ) ) q a ( + 2 O S ( ) ) q a ( 4 2 ) ) ) q a ( 08 O S a C ( 4 O S a C [ = ) ) q a ( 4 ] ) q a ( 09 O S a C [ 4 = ] ) q a ( K a C ( + 2 ) ) q a ( 2 = K K 0 s 0 1 = y t i l i b u l o S S S O S a C [ = 4 a C [ + ] ) q a ( + 2 ] ) q a ( 1 1 S = K K 0 s + K 0 s 5 . 0 /γ + 2 a C 2 1 NO : e t . s e i t i v i t c a e t a c i d n i s e s e h t n e r a p d n a s n o i t a r t n e c n o c e t a c i d n i s t e k c a r B i n o i = i, γ i n o i f o t n e i c i f f e o c y t i v i t c a = i, I , h t g n e r t s c i n o i n o i t u l o s = z i = n o i f o e g r a h c i O S a C d e g r a h c n u e h t f o t n e i c i f f e o c y t i v i t c a e h T . 4 . 9 q e n i 0 . 1 e b o t d e m u s s a s i r i a p n o i ) q a ( γ– = γ Ca 2+ = γ SO 4 2