Weird concentration, strength of acid, Le Chatelier's Principle Question (1 Viewer)

[mmmk]

A sample of vinegar of concentration of 1M has a pH of 3. A 25mL sample of this 1M vinegar requires the same volume of 0.5M sodium hydroxide solution to reach the equivalence point of titration, as does a 25mL sample of 1M HCl. Explain why the same volume of NaOh is required for each titration. Use Le Chaterlier's Principle in your explanation.

 

[xiao1985]

A sample of vinegar of concentration of 1M has a pH of 3. A 25mL sample of this 1M vinegar requires the same volume of 0.5M sodium hydroxide solution to reach the equivalence point of titration, as does a 25mL sample of 1M HCl. Explain why the same volume of NaOh is required for each titration. Use Le Chaterlier's Principle in your explanation.

equivalence point is when ALL acids react with ALL bases.

ethanoic acid gives a higher pH than HCl, due to incomplete ionisation nature of the acid, but 1M of ethanoic acid still can produce 1M of H+.

same goes for HCl... Therefore, same amount of base is needed to reach equivalence point.

 

[mmmk]

hmm, but i don't understand how this question is related to le chatelier's principle.

 

[Forbidden.]

equivalence point is when ALL acids react with ALL bases.

ethanoic acid gives a higher pH than HCl, due to incomplete ionisation nature of the acid, but 1M of ethanoic acid still can produce 1M of H+.

same goes for HCl... Therefore, same amount of base is needed to reach equivalence point.

To be more precise, the equivalence point is reached when the concentration of hydrogen (H⁺) ions is equal to the concentration of hydroxide ions (OH^-)

 

[mmmk]

my teacher said it's because adding base shifts equilibrium to the right. so even though acetic acid is weak, all of it eventually becomes ionised. and since both hcl and acetic acid are monoprotic, then the same amount is required.

thank you for helping btw.

 

[xiao1985]

To be more precise, the equivalence point is reached when the concentration of hydrogen (H⁺) ions is equal to the concentration of hydroxide ions (OH^-)

Hmm I am challenging that statement.

Use this reaction as an example:

CH3COOH + NaOH <----> CH3COONa + H2O

and then, at equivalence point, you may still have:

CH3COONa + H2O <----> CH3COOH + Na+ + OH-

meaning at equivalence point, you have more OH- than H+ in solution (hence making the solution more basic at equivalence point.

 

[HappyFeet]

Equivalance point in when all the acids and bases have reacted (on other words, equivalent).

The other important thing is the End Point and this is when the indicator changes colour.

 

[MaccaFacta]

Both HCl and CH3COOH are monoprotic acids i.e. when they react with a base, they can do so with one proton (H+) per molecule. The fundamental reaction is the same in both cases: acid + base = salt + water.

The application of Le Chat's principle is a bit of a strange idea to be tacked on, but mmmk is correct, 1 mole of a weak monoprotic acid will eventually produce 1 mole of H+ ions, as long as the pH is kept high enough.

One final thing, on "equivalence point". If a strong acid (SA) reacts with a strong base (SB) then the equivalence point will be at pH = 7. SA + weak base (WB) then equivalence point has pH<7. WA + SB has equivalence point pH >7. If you check out the Virtual Laboratory which is available from http://www.chemcollective.org/applets/vlab.php you can do a bunch of titration simulations and see how the pH curves vary (you'll need to plot the points yourself - its freeware and doesn't draw the graphs for you).