q's on titrations - PLEASE help :D (1 Viewer)

[Pace_T]

For the neutralisation of NaOH (0.1M) with known chemical - potassium hydrogen phthalate

1. why can't we weigh out a calculated amount of solid NaOH and then dissolve it to make a standard solution, instead of neutralising it with potassium hydrogen phthalate to find the concentration of NaOH? Why is this unsuitable?

2. Why don't we have to use a pipette for measuring 30 mL of NaOH into a conical flask? we just use a measuring cylinder.

3. Why is the burette rinsed twice with NaOH?

4. Why was phenolphthalein used instead of other indicators?

5. How would i go about preparing a standard 0.1M Hydrochloric acid solution, starting with concentrated 10M HCl solution??

Yes i know, im very dumb lol. im not very good with titrations so that sorta justifies the reasons for the easy questions
thank you

 

[illin]

These are some good questions

enter: Will Hunting or Chemcoach to answer these questions

Anyway i know the answer to a few of these but not in the right wording

 

[xiao1985]

1) NaOH is not a primary standard, as it is a strong base and it tends to obsorb water from air, hence stuffing up the weighing process

2) u certainly can for accuracy purposes... but practically it might not be viable time wise

3) u rinse it with NaOH so that any impurities /water is the burette is rinsed out, so not nothing will stuff up the concentration of NaOH in the burette... 2nd time for reason as ablove

4) u r doing titration using a strong base against a weak acid.... the salt will be basic... hence u use phenolphthalein which changes colour at around pH of 10-11 ... making the end pt most suitable for this expmt...

5) dilute it 1:100 ?!

 

[Emma-Jayde]

For 5) you'd have to start with a known volume of the 10M solution of HCl, (say 50mL), and work out the number of moles in that volume using

n_HCl = concentration X volume (= 0.5 mol)


Then to work out the volume for an 0.1M solution, use the same formula to find the new volume.

n_HCl / concentration = Volume (= 5L)

To find out the amount of water to add, take the original volume away from the new volume (5 - 0.05 = 4.95L), so you need to add that much water.

With those concentrations it's easier to say yeah, the ratio is 1:100, but with other values you'll need to use this method.

 

[shafqat]

for 1), other reasons why NaOH is not a good primary standard:
it has a low molecular mass, so weighing errors will be amplified
it reacts with carbon dioxide in the air:
NaOH + CO2 ---> Na2CO3 + H2O

 

[mitochondria]

Some additions and amendments to the answers posted above:

1. I am assuming that the question you ask was: "why do we have to find out the concentration of the 0.1M NaOH instead of making a fresh new solution?".. in addition to the points xiao has mentioend, which add extra weight to the NaOH_(s), NaOH also reacts with CO₂ to give Na₂CO₃ - you may have noticed the white solid around the neck of an NaOH bottle or at the bottom of the bottle. So one way or the other (whether you make it up before you use it) you are going to have to standardise (to find out the concentration) the NaOH before you use it for an accurate titration Gernerally, you can only use pure (or close to pure..) and unreactive solids to prepare a stardard that can be used straight away..

2. You MUST ALWAYS use a pipette unless you are just doing an estimation of some sort .. If you use a pipette, learn to use it well the amount of eople who can't actually use a pipette properly at uni surprises me..

4. As xiao said, the salt will be basic, let's take ammonia as an expmaple for week base reacting with strong acid:

NH_3(aq) + HCl_(aq) ------> NH₄Cl_(aq) + H₂O_(l)

if you examine the reaction of ammonium ions with water:

NH_4(aq)⁺ + H₂O_(l) ------> NH_3(aq) + OH^-

which gives you a basic solution.. since HCl is a strong acid and it's Ka (dissociation constant) is essentially infinite, for the reaction:

HCl_(aq) + H₂O_(l) -------> Cl_(aq)^- + H₃O_(aq)⁺

you have Ka = [Cl^-][H⁺] / [HCl]

if Ka is very very large, the demoninator will clearly be very very small and the nominator be very very big, meaning that you won't have much of HCl (practically none) and there for chloride ions doesn't react with water to give a basic solution. For the same reason a strong base (has a very very large Kb) will not give ions that produce an acid.

the general guide is:

strong acid + strong base = neutral salt
weak acid + strong base = basic salt
strong acid + weak base = acidic salt

weak acid + weak base = well.. that will depend on whether the weak acid is stronger or the weak base is stronger, I think if the weak acid is stronger you will have an acidic salt and if the weak base is stronger you will have a basic salt. Don't worry too much about this one.. it's not required for the HSC i think

5. 1mL 10M HCl into a 100mL volumetric flask and fill up to the graduation mark.. there you have your 0.1M HCl O_____o... if you don't feel comfortable doing 1mL, do a 10mL in 100 dilution and then do it again with the diluted HCl..

Whatever he said

that's rather offensive..



Edit: *frown* people always post when I'm typing.. ended up saying the same thing again x_____X I should learn to type quicker

 

[illin]

What i never said that!!

 

[Templar]

weak acid + weak base = well.. that will depend on whether the weak acid is stronger or the weak base is stronger, I think if the weak acid is stronger you will have an acidic salt and if the weak base is stronger you will have a basic salt. Don't worry too much about this one.. it's not required for the HSC i think

You can't accurately titrate a weak acid against a weak base. The end point is not very distinct and the margin of error is quite large as colour change can occur across a wider range of volumes.

Phenolphthalein is also used in strong acid strong base titrations.

1mL 10M HCl into a 100mL volumetric flask and fill up to the graduation mark.. there you have your 0.1M HCl O_____o... if you don't feel comfortable doing 1mL, do a 10mL in 100 dilution and then do it again with the diluted HCl..

Don't think most people have access to 1mL pipettes (if they even exist). Pipette 25mL into 250mL volumetric flask, fill rest with water. Repeat.

 

[mitochondria]

You can't accurately titrate a weak acid against a weak base. The end point is not very distinct and the margin of error is quite large as colour change can occur across a wider range of volumes.

Phenolphthalein is also used in strong acid strong base titrations.

hmm.. nobody ever said about doing a weak acid - weak base titration.. I just was just commenting the acidity/basicity of the salt as a result of reacting a weak acid with a weak base..

and just out of curiosity.. I have never come across or read about using phenolphthalein as an indicator for a strong acid - strong base titration.. it has an indication range of 8.3-10.0 I don't suppose a salt that comes from a strong acid - strong base can be "off neutral" for that much.. at least not mathematically but then again I might well be wrong.. sooo.. do we stop when it changes from pink to colourless or do we stop when it just reaches pink?

Don't think most people have access to 1mL pipettes (if they even exist). Pipette 25mL into 250mL volumetric flask, fill rest with water. Repeat.

on the contrary.. I think most labs should have access to 1mL pipettes and they do exist.. not to mention some of them come in 0.01 mL (I just used one 2 weeks ago ) increments if you didn't think it's possible (or if it's too small to be possible for you)..

 

[shafqat]

remember in strong acid base the change in pH is very steep, so rapidly changes from high to low pH (is that the right way round? ). so phenolpthalein can be used as an indicator.

 

[Templar]

and just out of curiosity.. I have never come across or read about using phenolphthalein as an indicator for a strong acid - strong base titration.. it has an indication range of 8.3-10.0 I don't suppose a salt that comes from a strong acid - strong base can be "off neutral" for that much.. at least not mathematically but then again I might well be wrong.. sooo.. do we stop when it changes from pink to colourless or do we stop when it just reaches pink?

In strong acid to strong base titration the phenolphthalein should turn from clear to crimson as the result of addition of one drop.

on the contrary.. I think most labs should have access to 1mL pipettes and they do exist.. not to mention some of them come in 0.01 mL (I just used one 2 weeks ago ) increments if you didn't think it's possible (or if it's too small to be possible for you)..

I assumed most people are in high school (this is a HSC thread). I doubt high school labs has 1mL or 0.01mL pipettes.

 

[mitochondria]

oh thankie to both of you xD hmm.. I forgot about that *sticks with bromothymo blue and phenol red* and.. *deleted*

now you mention it, I seem to remember doing that in the titration competition during the standardising step.. hmm.. enough of talking to myself.. that was embarrassing

Edited: as for the pipette.. you missed my point.. my point was there was no need to yell

 

[tam89]

i'm also doing a titration prac and the method stated:

1) Accurately weigh out about 1g of potassium hydrogen phthlalate (the primary standard) into a clean and dry 250ml beaker. Record the exact mass.

2) Dissolbe the potassium hydrogen phthalate in 100ml of distilled water in a beaker, warm on a steam bath until dissolved (~15 mins) and then transfer the solution to a clean 250ml volumetric flask. Remember to rinse the beaker several times and transfer all washings to the flask.

Just wondering.... why is it necessary that the 100ml beaker used for weighing the primary standard potassium hydrogen phthalate be dry and clean?

 

[Pace_T]

umm im not sure but... ill try anyway
you don't want the potassium hydrogen phthalate to get mixed with other substances because it affect its pH and thus the end point of the titration.
What you want to do is put the hydrogen phthalate in the beaker without anything else but distilled water (as this doesn't affect the pH of the phthalate solution) and obviously, the end point will be accurate as possible
-someone correct me if im wrong.
hope it helps

 

[Templar]

Just wondering.... why is it necessary that the 100ml beaker used for weighing the primary standard potassium hydrogen phthalate be dry and clean?

You don't want any contamination from either tap water or any residue of chemicals in the beaker.

 

[britt-eck]

2) u certainly can for accuracy purposes... but practically it might not be viable time wise


actually no measuring cylinders are not as accurate as pipettes hence why they are used.