Titration query... (1 Viewer)

[Pimpcess.Snaz]

Alright well, i'm doing a titration assessment on wednesday where i'm trying to find my the concentration of HCl (which i've got in my conical flask) and i'm using Na2CO3 as my weak base (got in the burette). Now don't ask why my unknown concentration solution is in my conical flask while my known is in my burette cause that's how my teacher wants it.

Anyways, amidst my method, I rinse my conical flask three times with distilled water. Now obviously, there will be a volume of water remaining after it is rinsed. My question is, if i measure 25 mL of HCl and put it into my 'wet' conical flask, surely my volume and concentration, that i will be reacting with Na2CO3, will change. Hence it stuffs up my calculations? Cause then I'll be calculating with 25 mL for the supposed volume of HCl but in fact, i've got maybe 26 or 27 mL of HCl reacting.

Or am i completely and utterly wrong, and have absolutely nothing to worry about? (Please say yes )

anyways, thanks guys

 

[Pwnage101]

you may leave the conical flask wet - and ill explain to u y

by pipetting 25mL of HCL - that is a known amount - now let us assume there is water left in the concial flask - THIS WILL NOT CHANGE THE NUMBER OF MOLES of HCL IN THE CONICAL FLASK AND WILL NOT STUFF UP UR CALCULATIONS SO LONG AS U TAKE 'VOLUME OF HCL' to be 25mL, as u are asked to find teh conc of the original HCL solution, not the one in the conical flask (which is slightly diluted, as u mentioned, but that doesnt matter)

it's a little hard to explain on the net, but i assure u it will not change the calculations:

when u standardise the HCL, u use a known volume of a known concentraation of NA2CO3, so u know the number of moles used (n=cv) - double the moles to find the number of moles of HCL in that conical flask (1:2 molar ratio), now is where the added water DOES NOT make a difference - u ADDED 25mL of the original solution of HCL into the conical flask - so all the moles of HCL in the flask now are from that 25mL volume of HCL - even if there is now a greater volume due to extra water - so since u now have number of moles and u have volume (= 0.025L), n=cv therefore c=n/v, using v=o.o25, and the 'n' u worked out, ull find the concentration of THE ORIGINAL SOLUTION OF HCL THE 25mL aliquot was pipetted from, which is what u need, u get me?

i hope that helps, ive tried to explain it comprehensively, but put simply, u just gotta understand the concept that u know all the moels of HCL are from that 25mL aliquot so no matter wat water is added/there in the conical flask after this 25mL has been mesuerd out, THERE WONT BE ANY DIFFERENCE

Please ask if u have a specific question from that, but i hope i helped u (albeit in some little way)

 

[Pimpcess.Snaz]

yeah.. it did.. i understand now.. thank you so very much.. i was getting worried that my experiment wouldn't be valid cause of that

 

[Hollieee]

Titration confuses the absolute fuck out of me, and I have a prac test on it on Friday =)

I just don't completely understand how I can find concentrations and stuff.
And does it actually neutralise it or what?

-shoots self in foot-

 

[brenton1987]

I just don't completely understand how I can find concentrations and stuff.

What part of calculating the concentration dont you understand?

There is a solution of known volume and known concentration.
In the equation n = C * V both C and V are known. Therefore n can be calculated.

There is a second solution of known volume.

By knowing the identity of solution 1 and solution 2 a reaction formula can be deduced. Using the stoichiometry from this reaction, n₂ can be determined.

Now, in the equation n₂ = C₂ * V₂, both n₂ and V₂ are known so C₂ can be calculated.

---

And does it actually neutralise it or what?

In the flask there is an indicator. An indicator is a large molecule that can be either protonated or neutral. The indicator is a different colour when it is protonated compared with when it is neutral.

When the indicator is in acid it will be coloured. As base is added from the burette the protons within the solution are neutralised first. After all of the solution protons have reacted the protons on the indicator are removed which results in the change in colour.

Or acid is added from the burette to a solution of base which neutralises the base first. After all of the base has reacted the indicator will be protonated which results in the change in colour.

The colour change indicates when the reaction is complete and is used to find the V variable in the above equation.

The only reason a titration is performed is to find the variable V.

 

[Hollieee]

Ahh, thanks.
That helps. I got a bit of a better grasp on it, discussing it in class today and whatnot.
It's weird, I have flashes with it, where I'll have a brainwave, and understand it well, and then I'll just completely forget what I worked out lol.
Hopeless, I tell you.
But I understand the calculations now, but still manage to confuse myself a little.
Does the solution you don't know the concentration of go in the conical flask?
=I

Blasted chemistry.

 

[advanced sam]

p.s water has no effect on the cocentration, that is the chage of colour

 

[Pimpcess.Snaz]

how did you go?

screwed up if you're question is aimed at me.. well like i was aiming for 100% in this assessment so anything else lower is a screw up lol..
basically cause i took too long to do the experiment.. all that rinsing is useless and hence i wasn't exactly patient for my accurate readings...

a TIP for people doing a titration experiment.. even though in your method you might say that you've rinsed 3 times, just rinse one and get the hell on with it cause its the biggest waste of time!

anyways, with the questions part of the assessment i had 10 minutes to race through it but all over, i did my best and the only way i could've done better was if i practised the actual experiment more times but i was unable to due to a few bitches in my class.. but anyways as i said, i did my best so i don't care anymore

 

[Hollieee]

Heey, well as long as you did your best!
And I have mine tomorrow, and I haven't actually had any practice.
Done it in class once, so I'm going to go and mentally rehearse and practice those blasted equations, lol.
=D

 

[brenton1987]

Does the solution you don't know the concentration of go in the conical flask?

The calculations will work no matter which way you run the experiment. The unknown can be in the flask or the burette.

Personally I always have acid in the burette because bases can affect the tap mechanism. The only exception is a solution containing the fluoride ion. Anything with fluoride should be in a teflon coated plastic flask.

a TIP for people doing a titration experiment.. even though in your method you might say that you've rinsed 3 times, just rinse one and get the hell on with it cause its the biggest waste of time!

How long did you have to do the experiment?

 

[Pimpcess.Snaz]

How long did you have to do the experiment?

had an hour and half to write up method for both preparing my standard solution and the titration, draw and label all my equipment, prepare my standard solution, do the titration (3 times excl rough run) and then answer questions for my discussion and conclusion

 

[advanced sam]

for our assesment on the titrations our teacher said it would take about 20 minutes. we ended up taking over an hour and a half. so dont worry you took so long
at least we know that the teachers know what they are talking about
go hsc!

 

[loki911]

omg you guys make it sound so easy my prac test is well today x.X and we have to do it on indicators and determining the ph with several indicators (easy), a tritation and solution of heat or something where you get a data logger and you measure the temp as you add salts and im sure there is something else. The timeframe no idea i'll find all that today =[.

 

[ronash]

we did a titration prac assessment around a month ago ................ it was deemed invalid as the science department had bought new conical flasks which some classes used and others didnt. when they came to marking it they couldnt tell who was actually wrong so they just gave everyone 12/12...as for the theory part, the calculations and questions were easy...i did lose a mark however for not knowing that HCl standard solutions are found by secondary titration not diluting a primary standard...i was so angry

 

[brenton1987]

I dont see how new conical flasks would invalidate a prac assessment.

 

[minijumbuk]

They probably didn't rinse it? I don't know =\

 

[jordankuai22]

we did a titration prac assessment around a month ago ................ it was deemed invalid as the science department had bought new conical flasks which some classes used and others didnt. when they came to marking it they couldnt tell who was actually wrong so they just gave everyone 12/12...as for the theory part, the calculations and questions were easy...i did lose a mark however for not knowing that HCl standard solutions are found by secondary titration not diluting a primary standard...i was so angry

lol, which shit school was that?

 

[ronash]

yeah they didnt rinse the new conical flasks...so u can understand the variance that would hav caused

 

[ronash]

lol, which shit school was that?

a very good school actually. it was a rare hiccup haha
your a sick man for having goatse.cz as ur homepage btw. ive seen that before and it aint good at all haha

 

[jordankuai22]

private school?