Chemistry 3/4 2013 Thread

[saba.ay]

A student conducts two titration experiments. In experiment 1, a 20.00 mL sample of 0.100 M hydrochloric acid was placed in a conical flask, and a 0.100 M sodium hydroxide solution was slowly added until a total of 50.0 mL had been added. In experiment 2, a 20.00 mL sample of 0.100 M ethanoic acid was used instead of hydrochloric acid.

Would the volume of NaOH needed for complete reaction in experiment 2 be equal to, greater than or less than the value in experiment 1? Explain your choice.


I know that ethanoic acid is a weak acid, so you would have to add in extra. But there seems to be an excess amount of NaOH solution added to the hydrochloric acid in experiment 1.

Would it be unchanged? I mean the volume of NaOH used for both experiments should be the same. Though the second experiment uses a weak acid, the concentration of the ethanoic acid is the same as that of HCl. Therefore, the mole of H+ ions present is the same in both experiments and the volume of NaOH (or OH- ions) required to neutralise the two experiments too should be the same. :?

I don't know if my reasoning is right, so can someone please explain the answer.

[Bad Student]

Would it be unchanged? I mean the volume of NaOH used for both experiments should be the same. Though the second experiment uses a weak acid, the concentration of the ethanoic acid is the same as that of HCl. Therefore, the mole of H+ ions present is the same in both experiments and the volume of NaOH (or OH- ions) required to neutralise the two experiments too should be the same. :?

I don't know if my reasoning is right, so can someone please explain the answer.

Since ethanoic acid is a weak acid, not all of the hydrogen ions it has will be donated.

[thushan]

Guys, it's the same.

Remember the equivalence point definition - the point at which the reactants have been added in their stoichiometric ratios. This means that if we had 20.00 mL of a 0.1000 M solution of NaOH titrated with 0.1000 M of HCl, you'd get a 20.00 mL titre and if you had titrated it with 0.1000 M of CH3COOH, you'd also get a 20.00 mL titre. In each case, you've added the reactants (NaOH + HCl) and (NaOH + CH3COOH) in their stoichiometric ratios.

Where the weak-strong stuff comes in is the pH at equivalence point:

strong acid + strong base --> EXTREMELY weak conjugate base + EXTREMELY weak conjugate acid. Hence, pH at equivalence point is 7.
strong acid + weak base --> EXTREMELY weak conjugate base + weak conjugate acid. Hence, pH at equivalence point would be < 7.
weak acid + strong base --> weak conjugate base + EXTREMELY weak conjugate conjugate acid . Hence, pH at equivalence point would be > 7.

On each occasion, we are adding the reactants in their stoichiometric ratios. That's our aim. We don't care about what happens afterwards. In fact, we acknowledge that the pH of the solution would be different depending on the strength of the acid and the base.

[Stick]

So does that mean that what Bad Student said is not actually the case when the weak substance is not reacting with water?

[Bad Student]

So does that mean that what Bad Student said is not actually the case when the weak substance is not reacting with water?

I think what I said before is wrong.

[Stick]

I thought you were right in regards to water. :|

[Bad Student]

Lets summon Thushan to clear this up for us.

THUSHAAAAAAAAAAAAN!!!!!!!!!!!!!!!!

[Nobby]

Equivalence point means the amount of each species satisfies the stoichiometric ratio, it doesn't matter how strong or weak your acids/bases are. Volume for each titre is equal.

[Dedicated]

The weak and strong monoprotic acid problem of equal concentrations being added to a strong base yet giving the same titre bugged me a lot last year as well. "Since ethanoic acid is a weak acid, not all of the hydrogen ions it has will be donated.'' I think this is only true in water. It should under normal conditions donate all its H ions to the strong base leaving the CH3COO- to be the weak conjugate base. I hope I was clear still a bit (possibly a lot)  rusty on chem as haven't revised in a while.

[Dedicated]

CH3COOH + H20 <arrows both ways> CH3C00- + H30+
Initially when weak acid is in the solution there is a low concentration of H30+ ions. However when you are adding the strong base these are removed to form h20. However the equation above will shift to the right to produce more h30+. This occurs when close to no more CH3C00H remains. The only difference for a strong acid is nearly all that H30+ ions are in the solution intially while for a weak acid they are not. This results in the same titre number. (Sorry for the horrible notation)

[hardworker]

Can someone please quickly go over how to calculate PH please regarding Acids and Base

[Daenerys Targaryen]

- used when given conc. of H+ or OH-
can be used interchangeably with :

[McFleurry]

Could someone please explain to me how H2PO4 works as a catalyst in esterification?

Also, why are catalysts not required for reactions with alkenes and halogens? (e.g. C2H4 + Br2 --> 1, 2, dibromoethane)

Thanks

[hardworker]

Thanks Haters gonna hate

Can someone please help me with this question-A concrete leaner contains concentrated HCl. A  25.00ml of volume of the cleaner was diluted to 250.00ml in a volumetric flask. A 20.00 ml of aliquot of 0.4480 M Na2CO3 solution was placed in a conical flask. Methyl Orange was used as an indicator the colour changed from yellow to pink when 19.84 ml of cleaner was added. 


Determine the ammount in mol of HCl present in the average titre? (Where do you get the average titre from).

Thanks in advance

[teletubbies_95]

hardworker: Great questionn! 19.84 mL? But i dont think you need it to answer the question( since you are asking for the moles , not the concentration)   , because you will find the moles by doing mole ratios with the Na2CO3


mcfluerry: great question! Shall try to answer it later !