Achiever G-Chem Problem Test 3 #55

[justwu86]

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What will be the concentration of OH - at equilibrium if a mixture consisting 0.20 mole of NH4Cl and 0.30 mole of NaOH is diluted to 1.0 liter? (Assume ionization constant for ammonia, Kb = 1.8 x 10-5) NH3 + H2O « NH4+ + OH -


So apparently the answer is just .10 moles? Why wouldn't you do the whole Kb = [.20-x][.30-x]/[x] gibberish? I'd assume at equillibrium that though all of the OH reacts with NH4 to generate NH3, some of NH3 would also shift back to producing OH- + NH4 which is why you'd have to use Kb to get the concentration of [OH]


This is problem seems way to easy for me to be missing it....

 

[doc toothache]

What will be the concentration of OH - at equilibrium if a mixture consisting 0.20 mole of NH4Cl and 0.30 mole of NaOH is diluted to 1.0 liter? (Assume ionization constant for ammonia, Kb = 1.8 x 10-5) NH3 + H2O « NH4+ + OH -


So apparently the answer is just .10 moles? Why wouldn't you do the whole Kb = [.20-x][.30-x]/[x] gibberish? I'd assume at equillibrium that though all of the OH reacts with NH4 to generate NH3, some of NH3 would also shift back to producing OH- + NH4 which is why you'd have to use Kb to get the concentration of [OH]

In the reaction 0.2 moles of NaOH will be consumed leaving 0.1M. With a Kb (18 x 10-^6) the contribution of OH- (0.004M) from ammonium hydroxide will be negligible.