California State University Northridge Acid Base Titration Experiment Lab

Experiment 6 online with BeyondLabz
Part I
We are not able to do part I.
Part II: Base/Acid Ra?os of HCl and NaOH
In BeyondLabz, clear the lab bench by clicking on the red Cleanup Bench trash can in the righthand corner. Then, go to the stockroom and grab the NaOH and HCl boBles from the Base and Acid
Shelves, respecGvely. Make sure they do not have the same molariGes; if they do, close the program and
start again (it is OK if they are similar, but they should not be exactly the same). Make sure you record
these molariGes in your lab book!
Grab an empty beaker and place it on the sGr plate. Fill the buret with HCl but make sure the
volume is at or below the 0.00 marking; if it is above the 0.00 marking, you will need to empty some acid
into a waste beaker unGl the reading is at or below the 0.00 mark. Record your iniGal volume of HCl and
then deliver approximately 20 mL of HCl to an empty reacGon flask. Record your final volume of HCl.
Add phenolphthalein to the beaker.
Empty your buret by dragging it over to the red Cleanup Bench trash can and refill it, this Gme
with NaOH. Record your iniGal volume of NaOH and then Gtrate your acid with your base. These are
similar molariGes, so you should need roughly the same volume of base as you did with your acid; as you
approach where the equivalence point should be, slow down, even to the point of adding base drop by
drop. When you have reached the equivalence point, your soluGon should be a light pink. If it is red,
you have probably overshot! When the soluGon has turned either pink (ideal!) or red (less ideal, but OK
in Part II), record the final volume of NaOH.
For this part of the experiment, if you overshoot your GtraGon, you do not need to start over! If
you overshoot your GtraGon, empty the buret of base and refill it with acid. Record your new iniGal acid
volume and then slowly add acid, a drop at a Gme, unGl the soluGon just barely turns clear again. Record
your new final acid volume and calculate the total acid you just added. Now, empty the buret of acid,
replace it with base, record your new iniGal base volume, and do the exact same thing with your base,
being careful to add base dropwise unGl the soluGon just barely turns pink. This is called “back-GtraGng”,
and is something we can do when we have soluGons of both our acid and our base—we can cross back
and forth across the equivalence point to fine tune our GtraGon!
ConGnue to Gtrate unGl you are happy with the shade of pink/red in your reacGon flask; the goal
is to get to the point where a single drop causes a color shi_. Ideally, it causes the color to shi_ from
clear to pink, but if the molariGes of your two soluGons are very close, you may not see the pink color for
Part II, only. When you are finished with your GtraGon, calculate the total volume of acid you used and
the total volume of base you used. Empty your reacGon flask and buret and start your next trial!
As before, do enough trials that you have trials in which you are confident. Then, calculate your
Base/Acid raGos and ensure that your three trials are within 6 ppt of one another; if they are not, do
addiGonal trials unGl you have 3 trials that you are happy with.
Calcula)ons
1. For each trial: calculate your base / acid raGo by dividing the total volume of base you used
in that trial by the total volume of acid you used in that trial.
2. Average all the trials you intend to use; you should have at least three that you are confident
in. If you don’t, you’ll want to do addiGonal GtraGons unGl you do!
3. For each trial: calculate the deviaGon from the average
d e vi at ionTrial X = a ver age ba se : a ci d r at io − ba se : a ci d r at ioTrial X
4. For each trial: calculate the relaGve deviaGon from the average, in parts per thousand (ppt)
r el at ive d e vi at ionTrial X =
d e vi at ionTrial X
× 1000
a ver age ba se : a ci d r at io
a. If your trials have relaGve deviaGons greater than 6 ppt, you will want to do
addiGonal trials and will need to recalculate your average based on the three trials
that are closest in value.
5. Once you have three trials that “agree” to within 6 ppt, calculate the average relaGve
deviaGon of those three trials.
Part III: Standardizing a Sodium Hydroxide Solu?on using KHP
Go to the BeyondLabz program and select the TitraGons opGon. When the lab room loads, click
on the red Cleanup Bench trash can in the right-hand corner to make sure no materials are out on your
bench. Then, go to the stockroom and find the clipboard on the right-hand wall; the clipboard is labeled
“Open preset list.” From the list of opGons, choose “NaOH StandardizaGon.” You should now have the
following chemicals and equipment on your bench: NaOH Unknown #, full buret, KHP solid, an empty
beaker by the scale, and a weighed amount of KHP already on the scale. Make sure you record your
unknown number in your lab book!
General notes: if you make a mistake at any Gme, you can always empty glassware by dragging it
over to the red Cleanup Bench trashcan to dispose of it. You can also completely reset the simulaGon by
clicking on the red Cleanup Bench trash can to clear your bench, but you will need to select the preset
experiment and start over. Your unknown number should not change when you do this, but do verify
this before conGnuing. This program also has some tools that we won’t use, like the pH meter,
conducGvity meter, and Plot. You can close these windows.
Click on the yellow scale to access the scale view. Remove the weigh paper containing KHP from
the scale and set it aside or dispose of it in the red trashcan; you will not be using it. Place the empty
beaker on the scale and tare the scale. Weigh out 1 to 1.5 grams of KHP into your beaker and record
your mass in your Lab Book; note that the video says to round it to 3 decimal places, but you should
actually keep all of the significant figures provided by the scale. Remove the beaker from the scale.
Add water to the beaker either by placing the beaker under the sink faucet or by placing a 25 mL
beaker under the sink faucet and transferring those 25 mL of water to the beaker. Place the beaker with
water on the sGr plate and make sure the sGr plate is turned on (you should see an animaGon of a
spinning bar). Select the appropriate indicator (phenolphthalein in this case) and use the dropper to add
it to the beaker; if you have done this correctly, that indicator name should be bolded and the rest of the
indicator names should be greyed out like this.
Ensure that your buret contains your unknown NaOH. If it does not, add your unknown NaOH to
fill the buret to the top. Do not assume that the simulaGon will fill the buret to precisely 0.00 mL! If you
have too much base in your buret, you can replace your reacGon beaker with an empty beaker for waste
and open the stopcock slightly to remove some NaOH from your buret and then dispose of this waste
beaker.
Once you are ready to begin, record the full buret reading as your Base Vi (iniGal volume). Make
sure you use the correct number of sig figs! Now you can begin adding base at differing rates depending
on where and how much you click the stopcock. You’ll want to add base unGl the soluGon has just barely
passed the equivalence point, when the soluGon is just barely neutral. When you have reached this
point, your soluGon will change from clear to light pink; if your soluGon turns red, you have gone too far
(this is called “overGtraGng” or “overshooGng”) and will need to start over.
For an iniGal GtraGon, it is common to do an approximate or rough GtraGon, where you Gtrate
quickly to determine the approximate equivalence point based on your current amount of KHP. If you
choose to do your first GtraGon as an approximate GtraGon, you can add your NaOH quickly, being
careful to stop as soon as the soluGon turns pink or red. Record your new volume as your Base Vf (final
volume). Your Total Base Volume is equal to your Base Final Volume minus your Base IniGal Volume. You
now have a way to get a very rough idea of the total volume required to Gtrate the amount of KHP you
measured out. You can set a raGo to apply this relaGonship to future GtraGons:
volu m e NaOH t r i a l 1
Ex pected volu m e NaOH t r i a l 2
. Once you’ve measured out your
=
g K HP t r i a l 1
g K HP t r i a l 2
KHP for Trial 2, you can solve for Expected Volume NaOH Trial 2, which will tell you the approximate
volume you can expect to use for trial 2. When you start GtraGng, you’ll want to slow your addiGon of
NaOH as you approach this expected volume.
When you do a precise GtraGon, you’ll again want to record your Base IniGal Volume. Then, you
can begin adding base; you can sGll usually add your first 5-10 mL of NaOH fairly quickly, but you’ll want
to slow way down as you approach where you think the equivalence point might be (or where you
expect it to be, if you ran a rough iniGal GtraGon). As you get closer and closer, you’ll want to add NaOH
“dropwise,” in other words drop by drop, unGl the soluGon turns the faintest pink. It is very easy to
overshoot a GtraGon, and if you do this, you’ll have to start over! This is another benefit to doing an
approximate first GtraGon.
Once you have finished your GtraGon, record your Base Final Volume and calculate the Base
Total Volume Added. Do at least two more GtraGons unGl you are confident that you have 3 good
GtraGons. If you did an approximate GtraGon for your first GtraGon, this means you’ll end up doing at
least 4 GtraGons: 1 for your approximate GtraGon and 3 “for real”, with the possibility of addiGonal
GtraGons if any of your “for real” GtraGons were overGtrated.
Calcula)ons and results
Note that the formula for the reacGon between KHP and NaOH is given below, with the acidic hydrogen
in bold.
K HC8 H4O4 (a q) + NaOH(a q) → K NaC8 H4O4 (a q) + H2O(l )
1. For each trial: use your mass of KHP and the volume of NaOH you used to determine the
molarity of your NaOH for that trial.
2. Average the results of your best three trials.
3. For each trial: calculcate the deviaGon from the average
d e vi at ionTrial X = a ver age m ol ar it y − m ol ar it yTrial X
4. For each trial: calculate the relaGve deviaGon from the average, in parts per thousand (ppt)
r el at ive d e vi at ionTrial X =
d e vi at ionTrial X
× 1000
a ver age m ol ar it y
a. If your trials have relaGve deviaGons greater than 6 ppt, you will want to do
addiGonal trials and will need to recalculate your average based on the three trials
that are closest in value.
5. Once you have three trials that “agree” to within 6 ppt, calculate the average relaGve
deviaGon of those three trials.
Part IV: Concentra?on of an Unknown HCl Sample
In BeyondLabz, clear the lab bench by clicking on the red Cleanup Bench trash can in the righthand corner. Then, go to the stockroom and find the clipboard on the right-hand wall; the clipboard is
labeled “Open preset list.” From the list of opGons, choose “Strong Acid Strong Base Unknown.” You
should now have the following chemicals and equipment on your bench: HCl Unknown #, NaOH with
provided molarity, and a full buret. Make sure you record your unknown number and your base
concentraGon in your lab book!
Grab two empty beakers and a 25 mL pipet from the lab bench drawers. Place the 25 mL pipet
in the le_-hand side of the ring stand and place one of the beakers beneath the pipet. Pour a generous
amount of your HCl into the beaker beneath the pipet. Click the top of the blue bulb on the pipet to
draw up precisely 25 mL of HCl into the pipet. Dispose of your HCl beaker and replace it with the empty
beaker. Again click the blue bulb, which will now dispense precisely 25 mL of HCl into what is now your
reacGon beaker. Transfer your reacGon beaker to the sGr plate and add phenolphthalein indicator.
Titrate your HCl with NaOH exactly as you did in in Part III. The only difference is that you are
using an unknown acid that was already in liquid form and you now know your base concentraGon. You
should sGll have 3 good trials within 6 ppt of one another.
Calcula)ons and results
6. Write a balanced chemical equaGon for the reacGon between HCl and NaOH.
7. For each trial: calculate the HCl molarity.
8. Calculate the average HCl molarity from your three closest trials
9. For each trial: calculate your deviaGon and relaGve deviaGons
a. Do addiGonal trials if you do not have three trials that agree within 6 ppt
10. Calculate your average relaGve deviaGon using your three closest values.
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