Archive – Q4 week 1 – 19-20
KHP or potassium hydrogen phthlalate was the acid we used to standardize the strong base earlier in the year.
We use KHP to standardize strong bases like NaOH or Ca(OH)2 because
Potassium hydrogen phthalate (KHP) lab requirements:
Weak Acid – Strong base Titration Lab activity- Due Next Monday, Extra credit.
25.o ml KHC8H4O4 potassium hydrogen phthalate (KHP) with 0.06 M Ca(OH)2
From the graph of the provided Titration Curve in your Google doc, complete the following:
A) Write the Net Ion Reaction of the titration
B) Graphically identify the 5 points.
C) Write the net ion reaction for the titration.
D) Determine the concentration of the KHP
D)- Verify with calculations the following points in the graph (PLEASE NUMBER THEM in your doc!)
1) initial pH
2) half equivalence point – Determine the Ka of the weak acid.
3) a pH value in between max buffering position and below the equivalence point @ 20 ml(this new!)
4) pH at the equivalence point.
5) final pH
E)- determine the appropriate indicator(s) if an ENDPOINT was to be reached from the list below.
*Remember that an Equivalence Point (moles of H+ = moles of OH–) is not the same as the Endpoint!
The Endpoint is point in the titration that you estimate the equivalence point by using chemical acid base indicators that are acid and conjugate bases themselves with Ka’s and pKa’s.
Chemical acid/base indicators which are weak acid base equilibrium reaction are written the following way:
HI <—–> H+ + I–
Conjugate acid Conjugate Base
If Litmus is the chemical acid/base indicator used the conjugate acid is RED while the Conjugate Base is Blue. These color changes are due to the changes in the chromophore that we have exhaustively covered. The point here is that we can use this color change to stop a titration (ENDPOINT) If the color change occurs near the Equivalence point.
When would litmus have a color change?
It would occur when there is 50% conjugate acid and 50% conjugate base. This would be at ITS HALF EQUIVALENCE POINT! At the half equivalence point the pH = pKa! So all you need to do is choose an appropriate chemical indicator that has a pKa at +1 or -1 of the Equivalence point.In the case of the litmus we would use this indicator if the titrations equivalence point was about 5.5 to 7.5.
*Remember if the color change is occurring near the equivalence point it is occurring ON the asymptote and thus the volume determined by stopping the titration (Endpoint) will be very similar to the volume determined on the Equivalence Point.
If you want more on this please view my notes on this topic
Please use this chart to determine best indicator to use with this titration.
Show all work on graph.
Lecture on how to complete the Lab:
Play in Youtube and look under the video for the links (time code in blue) to take you where you need go in the video to complete. Most of you need points 3 and 5.
Wait? Isn’t this the auto ionization of water that has a Kw = 1 x 10-14 ???
So what this means that as the conjugate acid strength increases its conjugate base strength decreases!! The ability of conjugate acid to act like an acid is inversely related to the ability of the conjugate base to act like a base.
So now it should be clear why A conjugate acid /base pair will never neutralize each other because their ability to donate a proton (acid) and ability to accept a proton (base) are inversely related to each other in terms of Kw (1 x 10-14)!
The conjugate acid is delivered by the salt:
potassium phosphate monobasic:
The conjugate base is delivered by the salt:
sodium phosphate Dibasic:
Na2HPO4 —–> 2Na+ + HPO4-2
This product contains a mixture of these 2 salts in a plastic capsule that is to be dissolved in 100 ml of water.
-Notice that the points labeled the “Buffered Region” is where a Buffer solution is being made as this is the place in the graphs that both the conjugate acid AND conjugate base are both present.
–Notice that the half equivalent points determine the pKa of the acid WHICH also tells us the maximum buffer pH or the median pH range of that particular buffer. We determine appropriate based on the pKa’s or Ka’s of the conjugate acid! This means if I need a buffer to maintain a pH at 9 I would use a buffer solution with approximately equal amount of NH4+ and NH3 .
If we titrate “partially” meaning we add enough base to drive the reaction forward enough to produce some conjugate base but do not add enough to reach the equivalence point we create a buffer and we are in the “Buffer Region”!
1. Equal amounts of Conjugate Pairs – This would allow for a greater range buffering ability. Think about the half- equivalence point. At that position the solution can equally buffer an acidic or basic additions.
Electrolytic – (electroplating/electrolysis)
∆G = POSITIVE
Voltaic/Galvanic Cell –
∆G = NEGATIVE