SAMPLE EXERCISE 17.2 Calculating Ion Concentrations When a Common lon s Involved
ID: 1022029 • Letter: S
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SAMPLE EXERCISE 17.2 Calculating Ion Concentrations When a Common lon s Involved Calculate the fluoride ion concentration and pH of a solution that is 0.20 M in HF and 0.10 M in HCL SOLuRON Anaryzs We are asked to determine the concentration of FT and the pH in a solution containing the weak acid HF and the strong acid HCI In this case the common ion is H". Ptan We can again use the four steps outlined in Sample Exercise 17.1. Seive Because HF is a weak acid and HCl is a strong acid, the major species in solution are HF, H, and Cl. The CI, which is the conju gate base of a strong acid, is merely a spectator ion in any acid-base chemistry. The problem asks for [F], which is formed by ionization of HF. Thus, the important equilibrium is HF(aq) H+(aq) + F-(aq) -= The common ion in this problem is the hydro gen (or hydronium) ion. Now we can tabulate the initial and equilibrium concentrations of each species involved in this equilibrium: 0.10 +x (0.10 + x) 0.20 Initial (M) Change (M) Equilibrium (M) -X +x . (0.20-x) The equilibrium constant for the ionization of HF, from Appendix D, is 6.8 × 10-1. Substitut. ing the equilibrium-constant concentrations into the equilibrium expression gives (tri HF] (0.10 +x)(x) K,=6.8 × 10-4 = HF) 0.20 x (0.10)(x)-6.8 × 10 If we assume that x is small relative to 0.10 or 0.20 M, this expression simplifies to (0.10) (x) 0.20 0.20 (6.8 × 10-4)-1.4 × 10-3M = [F] 0.10 x This F concentration is substantially smaller than it would be in a 0.20 M solution of HF with no added HCI. The common ion, H suppresses the ionization of HF. The concen- tration of H (aq) is [H+] = (0.10 + x) M pH 1.00 0.10 M Thus, Cumment Notice that for all practical purposes, the hydrogen ion concentration is due entirely to the HCl; the HF makes a negligible contribution by comparison.Explanation / Answer
Calculate the fluoride ion concentration and ph of a solution that is 0.20M in HF and 0.10 M in Hcl
HCl is a strong acid and HF is a weak acid,
the Common-Ion Effect
As species in solution are H+, Cl–, HF, and the solvent, H2O
it is asked for [F–] , ----------which is formed by ionization of HF.
At Initial equilibrium concentration
So the initial concentration of [F–] , is 0
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