Wednesday, November 17, 2010

How many moles of HCl do you need to add to 1 L of a certain buffer to change its pH by 1 unit?

A buffer solution is 1.10 M in NH3 and 1.20 M in NH4Cl. How many moles of HCl must be added to 1.00 L of the buffer to change its pH by 1.00 unit? Assume the volume remains constant. Kb of NH3 = 1.8 10-5.



I don't know how to deal with the change by one unit. Please help!How many moles of HCl do you need to add to 1 L of a certain buffer to change its pH by 1 unit?
I believe the way you would do this would be to use henderson-hasselbach to first find the pH of the solution and then use it again with the new pH to find the new ratio of conjugate base to conjugate acid in the buffer (so NH3/NH4Cl):



(1) pH = pka + log [Base]/[Acid]



(2) pH + 1 = pka + log[Base]/[Acid]' (where this ratio is now different)



Once you have that new ratio... you can think of it as a titration with strong acid weak base...wherein you'll react a certain amount of HCl with NH3 to get new amount of acid (that has caused the ratio to change)... thus you can say... [base - HCl]/[acid + HCl]... make sure you correct all the concentrations for the change in volume and i think you have enough equations for unknowns to solve it then...How many moles of HCl do you need to add to 1 L of a certain buffer to change its pH by 1 unit?
You will have to figure it out yourself, it's cheating having someone else figure it out for you.

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