Weak Acid Calculations (1 Viewer)

[Zarathustra]

What is the formula to find the pH of weak acids when the dissociation is given - is this in the syllabus - or is it only strong acids (fully ionised)? e.g 0.1M/L of ethanoic acid that is 5% dissociated

 

[Affinity]

hmm don't know if weak acid one is in syllabus


anyway.. 0.1 M 5% dissociating will give

0.1 * 0.05 M H3O ions

so [H+] = 0.005 mol/L

that gives erh 2.3 pH

5% dissociation sounds a bit high for acetic acid

 

[xiao1985]

Originally posted by Affinity

5% dissociation sounds a bit high for acetic acid

yea... i fink normally 1% for acetic acid, 8% for citric... 100% for hyrochloric...

but then i heard from two sources that strong acid do NOT 100% dissociate, just the percentage of dissociation is soooo high dat it approaches 100%...

 

[Zarathustra]

Thnx,
In conq Chem it does say that acetic acid has 1% dissocaition and has a worked solution for finding it out, given pH (2.9) and concentration. If you put in 1% it comes out as a pH of about 3 - in an exam question can they say that the dissociation is any percentage for any acid or does it have to be true????

 

[Affinity]

follow what they give you, (they shouldn't be giving you false numbers)

it's actually about 1/250 for acetic at that conc. if I haven't forgot my chemistry (4 months since I read on it)

oh and, the extent of dissociation is related to concentration

 

[Xayma]

Originally posted by Affinity
oh and, the extent of dissociation is related to concentration

So the more concentrated it is the more/less dissociated?

 

[xiao1985]

uhm, i am guessin that more concentrate = less percentage dissociation... i dunno... =p blehz

 

[spice girl]

weak acid calcs are not in the syllabus, but if u're that interested, the dissociation is such that [H+][A-]/[HA] = Ka (a constant depending on the acid being considered)

 

[CM_Tutor]

As you dilute a weak acid, it becomes more ionised. For example, take acetic acid (CH3COOH) at 25 C:

0.100 M acetic acid has a pH of 2.88, and is 1.32 % ionised.

0.0100 M acetic acid has a pH of 3.38, and is 4.17 % ioinised.

0.00100 M acetic acid has a pH of 3.88, and is 13.2 % ionised.

If you think about it, this is exactly what you would expect to get by applying Le Chatelier's Principle to the equilibrium system:

CH3COOH + H2O <---> CH3COO- + H3O+

As you dilute (ie add water), the equilibrium shifts to the right, meaning that the proportion of acetic acid molecules that have undergone ionisation has increased.