Hello,
I have data here:
tube # Contents Initial color (after 2 drops of universal indicator) Color after adding HCl Color after adding NaOH (after the HCl had been poured)
1 CO32-/HCO3- Dark purple Purple Purple
2 CO32-/HCO3- Dark purple -- --
3 HCH3CO2/C H3CO2- red Light red Purple
4 HCH3CO2/C H3CO2- Red -- --
5 NAHCO3 Blue-green Light green Light blue
6 HCH3CO2 Red brighter red Orange
7 Distilled H2O Light green Bright red purple
A better version is at...
tinypic. com/ view. php?pic=20tqyv8&s=5. (Tubes 1 and 2 each had actual pHs of 9.5, while tubes 3 and 4 had 3.5, which is something not mentioned on the table.) How do I find the expected initial pH of the buffered solutions (tubes 1 and 3) with this data and the molarities (M) of NaCH3CO2, NaHCO3, and Ns2CO3, which are .1002, .09999, and .09992 respectively?
I was thinking about ICE tables or the Henderson-Hassalbalch equation, but I don't have any Ka values. Should I look up pH values online that correspond to colors like dark purple, blue, red, etc. and just use those as the expected initial values? That would give me a very large percent error because of possible large ranges...
First, I tried the tiny pics and didn't get anything.
Second, from the chart which is difficult to understand due to spacing problems, I think you can use the HH equation, substitute the concns you have listed in your narrative as well as the pH you have and that will allow you to calculate the pKa value. You don't have a question (as to what you are trying to do) but with a pKa value I'm sure you can work something out.
Hi Dr. Bob! I actually cleared it all up, but thank you!
To find the expected initial pH of the buffered solutions in tubes 1 and 3, we can make some assumptions and use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation relates the pH of a buffered solution to the pKa of the weak acid and the ratio of its conjugate base to acid forms.
First, let's make an assumption that in tube 1, the carbonate/bicarbonate (CO32-/HCO3-) is the buffering system, and in tube 3, the acetate/acetic acid (HCH3CO2/C H3CO2-) is the buffering system.
Now, we can use the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
Where:
- pH is the expected initial pH of the buffered solution.
- pKa is the dissociation constant of the weak acid - in this case, it is the pKa of the buffered system.
- [A-] is the molar concentration of the conjugate base.
- [HA] is the molar concentration of the weak acid.
Since we don't have the exact pKa values for the buffered systems, we'll use approximate values and calculate the expected initial pH based on those assumptions:
For tube 1 (CO32-/HCO3-):
- Assume the pKa value for the carbonate/bicarbonate system to be around 10.3 (a commonly accepted value)
- The bicarbonate ion (HCO3-) is the conjugate base, and the carbonate ion (CO32-) is the weak acid.
Using the Henderson-Hasselbalch equation, we have:
pH = 10.3 + log([HCO3-]/[CO32-])
Now, let's calculate the concentrations of HCO3- and CO32- using the given molarities:
- The molarity of NaHCO3 is 0.09999 M (approximately 0.1000 M).
- The molarity of Na2CO3 is 0.09992 M (approximately 0.1000 M).
Assuming complete dissociation of the salts:
- [HCO3-] = Molarity of NaHCO3 = 0.1000 M
- [CO32-] = Molarity of Na2CO3 = 0.1000 M
Plugging in these values, we have:
pH = 10.3 + log(0.1000/0.1000)
pH = 10.3 + log(1)
pH = 10.3
Therefore, the expected initial pH of tube 1 is approximately 10.3.
Similarly, for tube 3 (HCH3CO2/C H3CO2-):
- Assume the pKa value for the acetate/acetic acid system to be around 4.76 (known approximate value for acetic acid)
- The acetate ion (C H3CO2-) is the conjugate base, and acetic acid (HCH3CO2) is the weak acid.
Using the Henderson-Hasselbalch equation again, we have:
pH = 4.76 + log([C H3CO2-]/[HCH3CO2])
Now, let's calculate the concentrations of C H3CO2- and HCH3CO2 using the given molarity of NaCH3CO2 (0.1002 M, approximately 0.1000 M).
Assuming complete dissociation of sodium salt:
- [C H3CO2-] = Molarity of NaCH3CO2 = 0.1000 M
- [HCH3CO2] = 0.1000 M (since weak acid concentration is assumed to be the same as the sodium salt concentration)
Plugging in these values, we have:
pH = 4.76 + log(0.1000/0.1000)
pH = 4.76 + log(1)
pH = 4.76
Therefore, the expected initial pH of tube 3 is approximately 4.76.
Using the Henderson-Hasselbalch equation with assumptions about the buffering systems and using approximate pKa values allows us to estimate the expected initial pH values of the buffered solutions. However, keep in mind that the actual values may vary depending on the specific pKa values and other factors that may affect the system.