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General : Acid-Base Equilibrium Questions
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 Message 1 of 2 in Discussion 
From: MSN NicknameN_2006  (Original Message)Sent: 4/14/2008 3:50 AM
1:

What is the hydronium ion concentration in a solution prepared by mixing 50.00 mL of 0.10 M HCN with 50.00 mL of 0.050 M NaCN? Assume that the volumes of the solutions are additive and that Ka = 4.9 × 10-10 for HCN.

2.  What is the [CH3CO2-]/[CH3CO2H] ratio necessary to make a buffer solution with a pH of 4.44? Ka = 1.8 ×10-5 for CH3CO2H.
For this one do I just use the Hasselbaclch equation?

Thanks again..



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 Message 2 of 2 in Discussion 
From: MSN Nickname·Steve·Sent: 4/14/2008 5:04 PM
1.  Use the Henderson-Hasselbalch to calculate the pH:
 
pH  =  pKa  +  log ( moles of CN�?/SUP> / moles of HCN)
 
Use Volume X Molarity to get the moles of CN�?/SUP> and HCN.  You can divide them by the total volume to get the molarities, but in the ratio in the HH equation, this volume cancels (since the mole values in the numerator and denominator are each divided by this same volume), leaving just the ratio of the moles, which is why I wrote it that way.
 
Once you have the pH, you can calculate [H+] (which is the same thing as [H3O+] in aqueous solution) using [H+] = 10–pH.
 
2.  That's right, just plug into the HH equation and solve for the ratio [CH3CO2�?/SUP>] / [CH3CO2H]. 
 
 
Steve