VCE Chemistry Question Thread

[Eiffel]

can someone please give a precise definition(s) of equivalence point vs end point

[lzxnl]

The answer to this question is A, however I don't really understand why since I thought metal oxide + acid ---> salt + water

Thanks in advance

Reactions can't be rote learned. Your reaction with a salt forming is when your acid is written like HCl, with the base there present. If you react MgO with HCl, you would get MgCl2 and H2O. However here you just have MgO reacting with H+. The oxide ion reacts with the protons to make water and the magnesium ions are released. There's no anion for it to make a salt with

can someone please give a precise definition(s) of equivalence point vs end point

Equivalence point = when you've added reactants in stoichiometrically the right amounts
End point = when the indicator changes colour as a visual signal to stop the titration

[thushan]

You're basically 'tacking on' protons to the O^2- ion to form H₂O.

[Eiffel]

Reactions can't be rote learned. Your reaction with a salt forming is when your acid is written like HCl, with the base there present. If you react MgO with HCl, you would get MgCl2 and H2O. However here you just have MgO reacting with H+. The oxide ion reacts with the protons to make water and the magnesium ions are released. There's no anion for it to make a salt with
Equivalence point = when you've added reactants in stoichiometrically the right amounts
End point = when the indicator changes colour as a visual signal to stop the titration

would it be correct to say equivalence point is theoretical and end point is practical? both same but one is theoretical etc etc

[Eiffel]

with polyprotic acids in general, e.g. di protic H2SO4, what is the reaction between that and NaOH. i know its salt and water, but is their and intimidate step? is so do we need to show it and could you show it.

Being diprotic, what significance does it have to donate exactly two protons, how can this be shown?

[lzxnl]

would it be correct to say equivalence point is theoretical and end point is practical? both same but one is theoretical etc etc

It's not quite correct. You CAN theoretically work out where the end point should be, given enough information about the indicator's absorption of light, the acid/base used in the titration and the amount of coloured that needs to be present before you say it's coloured. It's quite impractical, but it's possible to do such a calculation theoretically.

However, if that helps you remember, you can use it as a guide.

with polyprotic acids in general, e.g. di protic H2SO4, what is the reaction between that and NaOH. i know its salt and water, but is their and intimidate step? is so do we need to show it and could you show it.

Being diprotic, what significance does it have to donate exactly two protons, how can this be shown?

Sulfuric acid's first dissociation is strong and will go to completion. Its second dissociation is weak-mildly strong, but when reacted with a strong base like NaOH it will go to completion. It depends on how much sulfuric acid you have; if the amount of sulfuric acid used is twice the amount of NaOH, you'll get Na2 SO4 as a product. If you only use a 1:1 ratio, you'll get NaHSO4

As for how to show it can donate exactly two protons, if you make a titration curve, it'll have two steep points corresponding to two neutralisation reactions.

[Eiffel]

sorry for the questions 

last one!!


Question 10 & 11 refer to the following information
A student wishes to determine the concentration of ethanoic acid in vinegar. The student titrates a 20.00 mL sample of a standard sodium hydroxide solution with a diluted vinegar solution from a burette. Four experiments were carried out and the following titration results were obtained, 22.16 mL, 22.30 mL, 22.35 mL, 22.35 mL.

10.   To make the solution of diluted vinegar, the student should

A  use a pipette to deliver a known volume of commercial vinegar into a volumetric flask and add water until it reaches the mark.

B  use a measuring cylinder to deliver a known volume of commercial vinegar into a volumetric flask and add water until it reaches the mark

C  deliver a known volume of commercial vinegar from a burette into a measuring cylinder and add water until it reaches the mark

D  measure the volume of commercial vinegar from a pipette into a conical flask and then add a known volume of water from a burette


11.   Which one of the following is a suitable indicator for use in this titration of a weak, monoprotic acid with a strong base?

A    phenol red         B    thymol blue
C     phenolphthalein      D    bromophenol blue
Modify message



btw, thanks Izxnl. How do you know where the two steeps are? we did it in class but teahcer didnt exactly explain exactly where it occurs.

[RazzMeTazz]

In acid-base titrations, is it more common to place the base in the burrette or the acid? Or does it vary?

Thanks

[scarletmoon]

In acid-base titrations, is it more common to place the base in the burrette or the acid? Or does it vary?

Thanks

Not necessarily. If you're trying to find the concentration of a base for example then the acid of known concentration will be in the burette and your base will be in the conical flask

[KingDrogba]

Quick Q/clarification

The precipitation is performed in acidic conditions to convert Phosphate ions, PO4^3- into HPO4^2- ions and thus prevents the Ba^2+ ions forming a precipitate with the phosphate ions. Write the balanced ionic equation for the precipitation of Barium Phosphate....Am i over thinking this or is it just (Barium + Phosphate) in a balanced ionic equation and nothing else like i assumed straight away when i saw this

[RazzMeTazz]

Can someone please explain why the equivalence point between a weak base and a strong acid occurs at a pH range 3-7?

Thanks

[RazzMeTazz]

Not necessarily. If you're trying to find the concentration of a base for example then the acid of known concentration will be in the burette and your base will be in the conical flask

Oh okay so usually the standard solution is placed in the burette, and the solution with an unknown concentration is placed in the conical flask?

Thanks

[lzxnl]

sorry for the questions 

last one!!


Question 10 & 11 refer to the following information
A student wishes to determine the concentration of ethanoic acid in vinegar. The student titrates a 20.00 mL sample of a standard sodium hydroxide solution with a diluted vinegar solution from a burette. Four experiments were carried out and the following titration results were obtained, 22.16 mL, 22.30 mL, 22.35 mL, 22.35 mL.

10.   To make the solution of diluted vinegar, the student should

A  use a pipette to deliver a known volume of commercial vinegar into a volumetric flask and add water until it reaches the mark. i'd go with this one; pipettes are the most accurate

B  use a measuring cylinder to deliver a known volume of commercial vinegar into a volumetric flask and add water until it reaches the mark measuring cylinders aren't accurate

C  deliver a known volume of commercial vinegar from a burette into a measuring cylinder and add water until it reaches the mark measuring cylinder? those aren't good for this

D  measure the volume of commercial vinegar from a pipette into a conical flask and then add a known volume of water from a burette conical flask? Those don't tell you about the volume


11.   Which one of the following is a suitable indicator for use in this titration of a weak, monoprotic acid with a strong base?

A    phenol red         B    thymol blue
C     phenolphthalein      D    bromophenol blue
Modify message

After titration, you're left with a weak base (the conjugate of the weak acid) and a negligible acid (the conjugate of the strong base). This means pH > 7, so I'd go with phenolphthalein.


btw, thanks Izxnl. How do you know where the two steeps are? we did it in class but teahcer didnt exactly explain exactly where it occurs.

Use some program to plot the pH against volume added and you'll see the steep points plotted in front of you.

In acid-base titrations, is it more common to place the base in the burrette or the acid? Or does it vary?

Thanks

Not necessarily. If you're trying to find the concentration of a base for example then the acid of known concentration will be in the burette and your base will be in the conical flask

It doesn't actually matter. You can put the known quantity in the burette OR the conical flask, provided you clean them appropriately.

Quick Q/clarification

The precipitation is performed in acidic conditions to convert Phosphate ions, PO4^3- into HPO4^2- ions and thus prevents the Ba^2+ ions forming a precipitate with the phosphate ions. Write the balanced ionic equation for the precipitation of Barium Phosphate....Am i over thinking this or is it just (Barium + Phosphate) in a balanced ionic equation and nothing else like i assumed straight away when i saw this

It IS just balancing the 2+ barium ions with the 3- phosphate ions

Can someone please explain why the equivalence point between a weak base and a strong acid occurs at a pH range 3-7?

Thanks

Upon reacting a weak base with a strong acid, the conjugate of the weak base is a weak acid. For instance, the conjugate acid of ammonia (a weak base) is the ammonium ion, a weak acid with acidic properties. The conjugate of a strong acid, however, has no acid-base properties. If it did, then the reverse of the acidic dissociation would occur and the acid wouldn't be strong. For instance, HCl's conjugate base, chloride ion, has no acid-base properties (aqueous sodium chloride is neutral, for instance).
Hence, upon reaction, you're left with a weak acid and a negligible base. The negligible base doesn't do anything and the weak acid drops the pH. That's why you're left with an acidic solution. They say pH 3 because normally, it's hard to make a weak acid solution with pH much lower than 3. It'd have to be quite concentrated.

[RazzMeTazz]

Wow thanks for the great explanation lzxnl!

In Units 3&4 Chemistry, when you are naming and explaining errors in an experiment do you have to classify errors as belonging to one of these two groups: random or systematical.
For example if you were asked about the errors of a titration, would you be able to label a particular error as a 'parallax' error or would you have to call this a 'random' error?

Also , would it be correct to say that the reason the equivalence point for a weak acid + strong base titration occurs at a pH range of 7-11, is because one of the products formed is neither very acidic or basic, and the other is a weak base?

For e.g. A titration between CH3COOH (aq) and NaOH(aq) would form NaCH3COO (along with H2O) and due to the fact that the CH3COO- ion is a weak base, the equivalence point would occur at a pH range of 7-11?
Is that correct?
Thanks!

And final question: Would a standard solution be considered as a secondary standard?

 

[vobinhood]

Help!

A 2 L sample of a gaseous hydrocarbon is burnt in excess oxygen. The only products of the reaction are 8 L of CO2(g) and 10 L of H2O(g), all at 100°C and 1 atm pressure.

The formula of the hydrocarbon is:

A. CH

B. C2H4

C. C4H10

D. C8H10