Question
Ca(OH)2 dissolves according to the followingreaction: Ca(OH)2<=> Ca2+ +2OH-,pKsp= 5.3 Questions: 1) write equilibrium espression. 2) Write equilibrium expression for dissociation ofwater. 3) In addition to 1 and 2, you also need to satisfy anequation called the charge balance. What are the positively chargedspecies in the system? what are the negative ones? write a chargebalance for this system. 4) using Eq. from 1, 2, and 3, what is the pH in the systemwhen solid Ca(OH)2 dissolves until it can't dissolve anyfurther (i.e. you are at equilibrium and reached the solubility ofCa(OH)2) 5) if you decrease the pH could you dissolve moreCa(OH)2? How would you decrease the pH? so for (1) i got 10^-5.3=[Ca2+][OH-]2 for (2) i got 10^-5.3= [H+] [OH-] /[H2O]2 for (3), isn't the equation alreadycharge balanced? I'm not sure where to go fromhere. Ca(OH)2 dissolves according to the followingreaction: Ca(OH)2<=> Ca2+ +2OH-,pKsp= 5.3 Questions: 1) write equilibrium espression. 2) Write equilibrium expression for dissociation ofwater. 3) In addition to 1 and 2, you also need to satisfy anequation called the charge balance. What are the positively chargedspecies in the system? what are the negative ones? write a chargebalance for this system. 4) using Eq. from 1, 2, and 3, what is the pH in the systemwhen solid Ca(OH)2 dissolves until it can't dissolve anyfurther (i.e. you are at equilibrium and reached the solubility ofCa(OH)2) 5) if you decrease the pH could you dissolve moreCa(OH)2? How would you decrease the pH? so for (1) i got 10^-5.3=[Ca2+][OH-]2 for (2) i got 10^-5.3= [H+] [OH-] /[H2O]2 for (3), isn't the equation alreadycharge balanced? I'm not sure where to go fromhere. for (3), isn't the equation alreadycharge balanced? I'm not sure where to go fromhere.
Explanation / Answer
1.) Keq = 10^-5.3=[Ca2+][OH-]2 2.) Kw = [H+][OH-] =10-14 , the equation of dissociation of water is :H2O = 2H+ + OH- 3.) See you have 2 sets of equations now , that is : - a)Ca(OH)2<=> Ca2+ +2OH- b) H2O = 2H+ + OH- so just add the 2 equations you will get the charge balancedequation . And of course you can see which speciesare positively charged in the sytem and which are negativelycharged. 4) Note : when solid Ca(OH)2 dissolves untilit can't dissolve any further , implies that the rxn is atequilibrium i.e saturation is attained. so use ,Ksp = s* (2s)2 = 4s3 ( s=solubiltity). find the value of s from this equaition. then[OH-] = 2 * s . So find the pH. 5) yes , you can because if pH decreases , [OH ] alsodecreases, driving the rxn forward . you can decrease pH by adding small amount ofacid. so just add the 2 equations you will get the charge balancedequation . And of course you can see which speciesare positively charged in the sytem and which are negativelycharged. 4) Note : when solid Ca(OH)2 dissolves untilit can't dissolve any further , implies that the rxn is atequilibrium i.e saturation is attained. so use ,Ksp = s* (2s)2 = 4s3 ( s=solubiltity). find the value of s from this equaition. then[OH-] = 2 * s . So find the pH. 5) yes , you can because if pH decreases , [OH ] alsodecreases, driving the rxn forward . you can decrease pH by adding small amount ofacid.