pH of KNO3 in water?
Hi guys,
I'm trying to find out what would be the pH of potassium nitrate dissolved in water? I know that the equation is:
KNO3 + H2O -> HNO3 + KOH
Lets say im going to take some KNO3 (which has a pH of around 6-7) and add it to water (ph of 7), so what would be the pH of the final solution?
This is not a calculation question. Im just asking about pH based on pure chemistry knowledge, if that makes sense...
Thanks a lot!
I'm trying to find out what would be the pH of potassium nitrate dissolved in water? I know that the equation is:
KNO3 + H2O -> HNO3 + KOH
Lets say im going to take some KNO3 (which has a pH of around 6-7) and add it to water (ph of 7), so what would be the pH of the final solution?
This is not a calculation question. Im just asking about pH based on pure chemistry knowledge, if that makes sense...
Thanks a lot!
Several answers to your question.
First, the most basic one: reaction you wrote makes no sense, pH of the solution containing a salt of a strong acid and strong base is neutral.
Second, slightly more complicated: neither KOH nor HNO3 is as strong as it is often assumed. Some slight differences due to hydrolysis can be observed, but they are typically negligible.
Third: real situation is even more complicated, as pH is a function of the ionic strength of the solution, and every ion introduced, even if it is just a spectator, changes pH slightly.
First, the most basic one: reaction you wrote makes no sense, pH of the solution containing a salt of a strong acid and strong base is neutral.
Second, slightly more complicated: neither KOH nor HNO3 is as strong as it is often assumed. Some slight differences due to hydrolysis can be observed, but they are typically negligible.
Third: real situation is even more complicated, as pH is a function of the ionic strength of the solution, and every ion introduced, even if it is just a spectator, changes pH slightly.
Original post by NekoMeowMeow
Lets say im going to take some KNO3 (which has a pH of around 6-7) and add it to water (ph of 7), so what would be the pH of the final solution?
What? If you already (pertain to) know the pH of KNO3 then why is your thread entitled "pH of KNO3 in water"? The answer to your question is just "closer to 7 than before". But I'm guessing you want to know pH of KNO3 rather than the effect of dilution.
Of course it will just be neutral (I am trying to target this at the Sixth Form level, but Borek's post is the real deal). What is Ka for the reaction K+ + 2H2O = KOH + H3O+? The answer (if you can't see why yet then look up Kw=Ka*Kb) is Kw/Kb(KOH). Since Kb(KOH) is taken as around infinity if you call KOH a strong base, then Ka is (around) 0 for the reaction I showed. So - no acidic action. What is Kb for the reaction NO3- + H2O = HNO3 + OH-? Again the answer is Kw/Ka(HNO3), and since Ka(HNO3) is taken as around infinity if HNO3 is a strong acid, then this Kb is (around) 0. So, no basic action either. Therefore the solution will be neutral.
Now let's try some numbers. pKa(HNO3)=-1.4 so Ka(HNO3)=25.119. Not exactly infinity, huh. But now when you divide Kw by this ... Kb(NO3-)=10-14-1.4=3.981*10-16. I found pKa for K+ given directly, rather than Kb for KOH, on Wikipedia, so I used that - pKa(K+)=13.5, so Ka(K+)=3.162*10-14. So finally, the acidic action does outweigh the basic action, and so the solution will indeed be acidic! You were right all along, pH will be between 6 and 7! (But I would expect it to be somewhat closer to - read: almost indistinguishable from - 7.)
With such small constants, I am sure as Borek says there will be some more noticeable activity effects. But as I am not versed in activity corrections I cannot go into that - I am sure he will if you ask him nicely.
What I've tried to do here in this long post is not so much to analyse pH of KNO3 - maybe activity effects will swing it much more heavily than my hair-splitting calculations of Ka and Kb, I wouldn't know - but supply a concrete way for you to estimate pH of a salt solution in general. Obviously we haven't done anything exact here, but it's a good thing to know.
(edited 10 years ago)
Thanks a lot guys, I really appreciate your help. 
I know it was a bit confusing question, but I had to do this experiment, where I had to measure the pH of that solution and then talk about it (describe the trend). So before writing it up, I was just wondering if it's correct by any chance as I got a pH of around 6-7. Now I can finally do it, so thanks again.
I know it was a bit confusing question, but I had to do this experiment, where I had to measure the pH of that solution and then talk about it (describe the trend). So before writing it up, I was just wondering if it's correct by any chance as I got a pH of around 6-7. Now I can finally do it, so thanks again.
Beware: if the solution is in contact with atmosphere, it will pretty fast get acidic because of the carbon dioxide dissolution, and you will end with pH somewhere around 5.5, regardless of whether you add inert salt, or not.
Actually if you got pH between 6 and 7 that is rather surprising, it should be lower. Keeping water pure and carbon dioxide free is not that easy.
Actually if you got pH between 6 and 7 that is rather surprising, it should be lower. Keeping water pure and carbon dioxide free is not that easy.
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