pH value (1 Viewer)

[uberturtle]

In an experiment to determine the concentration of a sodium hydroxie solution, a student found that 250ml of 0.2M ethanoic acid exactly reacted wih 10ml of the sodium hydroxide solution.

a)find the concentration of the sodium hydroxide solution.
b)Explain the pH of the solution at the equivalence point.


a)5mol/L
b) -log5= something, like a negative value.

how do i explain part b? do i just say its a super acid or something? :S

 

[Undermyskin]

a. n CH3COOH= 0.2*0.25=0.05 mol

[NaOH]=0.05/0.01= 5 M

Holy crap, this NaOH solution is damn concentrated. Sorry, I just want to check.

b. You can find the pH of the "solution" by find pH like that. They ask pH of CH3COOH+NaOH, not only that of NaOH

And even if you want to find the exact value of pH of NaOH, you lack another step: pH+pOH=14 ---> pH= 14-pOH. THis value give you something greater than 14 since pOH is a negative value.

b is a simply theory question based on the relationship between strong base and weak acid in a titration.

Answer: pH at the equivalence point is higher than 7. This indicates the slight basicity of the solution due to the components of this titration: strong base and weak acid.

I think that's all you need to write. If it takes more marks, then add:

CH3COOH (aq) + NaOH (aq) ---> CH3COONa (aq) +H2O (l)

The product salt in aqueous solution gives:

CH3COONa (aq) --> CH3COO- (aq) + Na+ (aq)

Na+ is a very weak acid and this its further activity is neglectable.

CH3COO- is a moderate base and acts as follow:

CH3COO- (aq) +H2O (l) <---> CH3COOH (aq) + OH- (aq)

This presence of OH- gives the solution that basicity and in turn a pH higher than 7.

I'm not sure how to find the pH of this solution but we need to know the ionization degree. However, do you think we can use that of CH3COOH for CH3COO- (8% or st)?

By the way, super acids like FSO3H is, like, thousand times stronger than concentrated sulfuric acid. (commonly 98% H2SO4)