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How would you make 100 mL of a carbonic acid buffer at 0.5 M and pH = 6.0 using 1.0 M NaHCO3 and either 1.0 M NaOH or 1.0 M HCl and water?
so far, I have 50 mL NaHCO3. I plugged that into the Henderson-Hasselbalch equation of pH=pKa + log [A-]/HA] and knowing that [Buffer]= [A-] + [HA] , and get [HA] = 0.5 M. Meaning that [A-] is zero... And that doesn't sound right. I'm thinking I used the wrong pKa value? I have the equation for the dissociation of H2CO3 with the pKa being 6.35, and the equation of HCO3 dissociating with a pKa of 10.33. I would think I would use 10.33 since I am starting with NaCO3, but clearly I'm messing up somewhere. Anything would help me at this point!! Thanks in advance for your time.
so far, I have 50 mL NaHCO3. I plugged that into the Henderson-Hasselbalch equation of pH=pKa + log [A-]/HA] and knowing that [Buffer]= [A-] + [HA] , and get [HA] = 0.5 M. Meaning that [A-] is zero... And that doesn't sound right. I'm thinking I used the wrong pKa value? I have the equation for the dissociation of H2CO3 with the pKa being 6.35, and the equation of HCO3 dissociating with a pKa of 10.33. I would think I would use 10.33 since I am starting with NaCO3, but clearly I'm messing up somewhere. Anything would help me at this point!! Thanks in advance for your time.
Answers
Answered by
DrBob222
You should use k1. You are starting with NaHCO3, not Na2CO3. And (base) + (acid) = 0.5M
For buffers you want to use a pKa within +/- 1 of the desired pH.
For buffers you want to use a pKa within +/- 1 of the desired pH.