Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Caustikola on April 29, 2012, 05:58:56 PM
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I've read and found that the solubility of chlorine in water is 3.26g/L
I tried to use this value to calculate the pH of a saturated solution of chlorine water.
Here's what i did:
Cl2+H2O<--->HClO+HCl
chlorine dissolves according to the above reaction until the solution reaches maximum solubility at 3.26g/L
then calculating the molarity, i did 3.26/71, ie mass concentration/Molar mass of Cl2 which gave~0.0459M
since the saturated solution is almost an equimolar mixture of HCl &HClO, from the reaction 0.0459M of Cl2 will give 0.0459M of HCl, giving 0.0459M of H+
then considering the HClO,
HClO<---->H+ + ClO-
k=[H+][ClO-]/[HClO]
k~3*10^-8
solving for [H+] gives [H+]~ 3.71*10^-5
solving for the pH, i'm getting about 1.34. I feel this value is too low. Is there something i'm missing? Pls i need clarification
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You have failed to take into account that your first rxn is an equilibrium:
Cl2 + H2O ⇋ HOCl + H^+ + Cl^-
Ka = [HOCl][H^+][Cl^-]/[Cl2] = 4.2×10^-4 [1]
As you have shown the contribution of the weak acid HClO to the pH can be ignored.
[1] N. N. Greenwood, A. Earnshaw, Chemistry of the Elements 2nd ed. (1997) p 856
(So yes - your pH is a bit low. ;))
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@cheese.
Thanks a lot:)
although i worked with that equilibrium and i got 1.66 which still appears low to me :(
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When I looked at this the first time round I was thinking weak acid, but this equilibrium involves x^3 where x is [H^+]. Solving the equation (successive iterations) does indeed give [H^+] = 0.021 and pH 1.66. This appears reasonable to me given the concs and that the strong acid HCl is formed. You are not confusing with “Cl2” in tapwater ~1.0 ppm max conc? (Incidentally, there is essentially no Cl2 present at the pH of drinking water 6.5-8.5.)
At RT, a saturated solution of chlorine in water contains about two-thirds hydrated dichlorine molecules and one third of the acid mixture. [1] So pH ~1.82 using this different approach.
However: the solubility of Cl2 you’re using appears to be that from Wikipedia, but several other more reliable sources have the solubility at ~7 g L^-1 (~0.1 M) [2] so your pH value will be even lower using this solubility!
[1] G. Raynor-Canham, Descriptive Inorganic Chemistry 2nd ed (2000) p 386
[2] CAS; 2012 CRC Handbook 5-151 (need to do a conversion);
http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html