The direct determination of extremely low concentrations of ions can be quite di
ID: 500799 • Letter: T
Question
The direct determination of extremely low concentrations of ions can be quite difficult. A far simpler and common procedure based on a relatively simple physical technique uses cell potentials, which are easy to measure, and evaluates ion concentrations with help of the Nernst equation. Although the Nernst equation relates cell potentials to ion activities rather than to ion concentrations, the difference between them is usually fairly small and becomes negligible in solutions where the total ionic concentration is less than about 10^-3 M. A general silver-based voltaic cell is constructed as follows: Ag(s)| Ag^+ -Solution A|| Ag^+ -Solution B|Ag(s) E_cell 1) In a first cell set-up, solution A is prepared by dissolving 125mg of silver nitrate (MW: 169.9) in 125 mL of distilled water, while solution B is prepared by dissolving 133mg of silver acetate (MW: 166.9) in 135 mL of distilled water. Calculate the expected cell potential E_cell. 2) In a second cell set-up, solution A is prepared by adding 125mg of silver nitrate (MW: 169.9) to 250mL of 2.50M hydrochloric acid, while solution B comprises 100 mL of AgClO_4 (0.100M). The cell potential is measured as E_cell = 0.541 V. Calculate [Ag^+ (aq)] of solution A. 3) Use data of the second cell set-up, and calculate a value for the solubility product of silver chloride.Explanation / Answer
Using Nernst equation we get E=E0-(0.059/n)*log[Ag+(A)]/[log Ag+(B)]
Now putting all the values we get E=0.80-(0.059/1) log(0.125*166.9*135)/(125*166.9*0.133)=0.8000-0.0038=0.7962 V