We titrated Sprite with 0.1051 M standard NaOH. I've ploted three different titrations and am supposed to use the titration curve to identify the acid. I assume the acid is Citric since it's Sprite, but the lab manual also says "to recall that two K values must be far enough apart ( at least 10^ -3) to see two equivalence points. What am I supposed to use to figure out the K values. I have pH's vs. mL of NaOH and started with 25mL of Sprite. Help PLEASE!
Why not plot the graph and see what you get? The pK value is the pH at the half-way point to the equivalence point. If you can identify the equivalence point, then take half that volume and read the pH for the pKa. If you have two equivalence points just do that twice. Could the acid be carbonic and not citric acid? Carbonic acid has two ks that are separated by at least 10^3.
Okay, thanks. The choices offered are phosphoric acid, citric acid, a oxalic.
Let me make sure I'm understaning you, for example: I have an equilvenc point at 4.80mL and the pH at that point read 6.93. So I should go to 3.5 and the pH at that point is 5.69. So K= 5.69? or am I still missing something? I did plot the graphs, but honestly, they look a LOT alike, so I'm thinking that's not a big help. Besides I really would like to know the math behind this. Thank you for all you help.
Close but not quite there. If the equivalence point is at 4.80 mL, then go to 2.40 mL (the half-way point) and the pH at that point equals the pKa (not Ka). The theory is simple enough.
A weak acid HA ==> H^+ + A^- and
Ka = (H^+)(A^-)/(HA). Solve for
(H^+)=Ka*(HA)/(A^-). At the half way point, the acid, HA, has been exactly half neutralized forming an equal amount of A^-; therefore, (HA) = (A^-) so they cancel and (H^+) = Ka. If we take the negative log of both sides, we get pH = pKa. I can't help much more, and this may not be much help, since I can't see the data or graph.
That does help greatly. Thank you so much!
To identify the acid in Sprite, you can use the information from the titration curve and the concept of acid dissociation constants (Ka values). The presence of two equivalence points indicates the presence of two acidic components in the solution.
To determine the Ka values, you need to find the half-equivalence points of the titration curve. The half-equivalence point is the point at which exactly half of the acid has reacted with the base, resulting in a buffer solution containing equal concentrations of the acidic and conjugate base species.
Since the titration curve plots pH against mL of NaOH added, locate the point on the curve where the pH is half-way between the initial pH of the solution (before adding any NaOH) and the pH at the equivalence point. This point represents the half-equivalence point.
Once you've identified the half-equivalence point, note down the corresponding volume of NaOH added (in mL). Let's denote this volume as V1.
Now, consider the buffered region around the half-equivalence point. In this region, the concentration of the acidic component is approximately equal to the concentration of its conjugate base, resulting in a buffer solution. The Henderson-Hasselbalch equation can be used to relate the pH of a buffer solution to its acid dissociation constant (Ka):
pH = pKa + log([A-]/[HA])
In the case of Sprite, assume the two components to be HA and H2A, reflecting the presence of two acidic hydrogens. By plugging in the pH value at the half-equivalence point, the concentration ratio of the conjugate base ([A-]) to the acid ([HA]) can be calculated.
Next, consider the second half-equivalence point. Similarly, locate the point on the curve that is half-way between the pH at the first equivalence point and the pH at the second equivalence point. Note down the corresponding volume of NaOH added (in mL). Let's denote this volume as V2.
Now, you can calculate the concentration ratio of the conjugate base ([A-]) to the acid ([HA]) at the second half-equivalence point, using the Henderson-Hasselbalch equation with the pH value at the second half-equivalence point.
Finally, you need to compare the calculated concentration ratios of [A-]/[HA] at the two half-equivalence points. Recall that the lab manual states that the Ka values must be far enough apart (at least 10^-3) to observe two equivalence points. If the concentration ratios differ by at least a factor of 10^-3, you can conclude that there are two acidic components, and the acid in Sprite is likely citric acid (H3Cit).
To summarize the steps:
1. Identify the half-equivalence points on the titration curve by locating the points where pH is halfway between the initial pH and the pH at the equivalence points. Note down the corresponding volumes of NaOH added (V1 and V2).
2. Use the Henderson-Hasselbalch equation to calculate the [A-]/[HA] concentration ratios at the two half-equivalence points, utilizing the pH values at each point.
3. Compare the concentration ratios obtained at the two half-equivalence points. If the ratios are significantly different (at least 10^-3 apart), there are likely two acidic components present, supporting the conclusion that citric acid is the acid in Sprite.
Remember that these steps are based on general principles, and you may need to adjust them based on the specific details of your experiment or any instructions provided by your instructor or lab manual.