PLEASE HELP:Some questions : Titration problems (1 Viewer)

[xx06]

Hi ive got my half yearlies coming up reli reli soonI really dont understand these titration problems involving stoichiometry. Refer to Conquering Chemistry edition3- chapter 5 q16-23 pg 155
I would type them up so you ppl dont have to drag yourselves to look at the questions but i am reli short of time.

-Im really desperatly in need of help so if anyone kind please help....plz plz plz plz!!!

 

[Dreamerish*~]

I have edition 4. Please type up the question.

 

[lourai*87]

I just did a titration prac the other day, it should still be fresh in my mind. Im just working out stoichiometry too, so i might be able to share some of my discoveries

(on second thought, Dreamerish's sig implies higher intelligence where chem is concerned)

 

[sikeveo]

i might be able to do the lot tomorrow then post up worked solutions. I dont think i have the newest edition though...

 

[xx06]

okiez,

well this is one
16.
Phthalic acid is a weak diprotic acdi for which we can writethe formula h2(c8h4o6). Poattassium hydrogen phthlate, KH(C8H4O4), is a good primary standard for standardising alkali solutions. It contains one acidic hydrogen per formula unit (may i ask WHAT IS THAT????). 0.917g pottassium hydrogen phthalate was dissolved in water and titrated with an approximately 0.2 mol/L sodium hydroxide solution; 27.2 ml hydroxide solution was neededd to reach the end point. Calculate teh accurate molarity of the hydrogen solution.

 

[lourai*87]

Im out :|

 

[xx06]

its from the Third edition. the one prior the latest (orange cover) not to sound rude..

 

[Dreamerish*~]

okiez,

well this is one
16.
Phthalic acid is a weak diprotic acdi for which we can writethe formula h2(c8h4o6). Poattassium hydrogen phthlate, KH(C8H4O4), is a good primary standard for standardising alkali solutions. It contains one acidic hydrogen per formula unit (may i ask WHAT IS THAT????). 0.917g pottassium hydrogen phthalate was dissolved in water and titrated with an approximately 0.2 mol/L sodium hydroxide solution; 27.2 ml hydroxide solution was neededd to reach the end point. Calculate teh accurate molarity of the hydrogen solution.

Per formula unit is like saying per molecule, except ionic compounds such as potassium hydrogen phthalate don't exist as molecules - hence "formula unit".

 

[sando]

i've got the 3rd edition too. Our school's too scabby to upgrade lol.

But sorry i hav no idea how to do it

 

[xx06]

OoOohhhhhh......GaWwwSssHHHH

i Really don't understand how do to them, i attempted them n all yet i still got the wrong answer. n theres galston h.s for ya...simply useless

 

[.ben]

one quick question, what's the point of using phenolpthalein for strong acid-strong base titrations? wouldn't it be better to use bromothymol blue? But my chem teacher insists on phenolpthalein???

 

[Riviet]

one quick question, what's the point of using phenolpthalein for strong acid-strong base titrations? wouldn't it be better to use bromothymol blue? But my chem teacher insists on phenolpthalein???

If you use bromothymol blue in a strong acid-base titration, it is hard to determine the colour change since the pH range of bb is 6.0-7.6. We expect the endpoint to be approximately at 7, and the change between blue and yellow means it's hard to determine the equivalence point because the pH lies right in the middle of that range for the indicator.
Using phenophthalein is better because the range of the equivalence point is much larger for a strong acid-base titration. Also, it is easier to see the colour change from colourless to pink or pink to colourless. I hope that helps.

 

[.ben]

but wouldn't you want teh end point and teh equilvalenct point to be at the exact same spot so you stop immediately when it changes colour? what happens in the middle of the pH range btw?

 

[Riviet]

but wouldn't you want teh end point and teh equilvalenct point to be at the exact same spot so you stop immediately when it changes colour? what happens in the middle of the pH range btw?

If you plot a graph of pH vs. volume of substance added, you get a graph shaped like this:
/
| <-- the vertical bit here is the equivalence endpoint, and the midpoint of this vertical
/ distance is exactly when the acid neutralises the base completely, for a strong acid-base titration, this equivalence point would be a pH of 7. Now at the top end of the equivalence point (vertical bit), the pH is approximately 10, and this is where phenolphthalein just changes from colourless to pink. Therefore, in your titration, you would be able to determine the endpoint by stopping the burette as soon as you see the first sign of colour change.