The concentration of sodium hydroxide in waste water from an alumina refinery was found by titrating 20.00 mL aliquots of the water against 0.150 M hydrochloric acid, using phenolphthalein as indicator. The average titre of several titrations was 11.40 mL.

 

a) Why is an indicator used?

 

What can I say to make a well response answer? I'm guessing I should use the words "end-point" and "equivalence point" and how they have to be relatively equal for the experimenter to obtain accurate results. But how can I explain in words why this is the case? Here is the answer I have written:

"An indicator is used in the titration so that when the titration reaches a point where all the reactants have reacted with each other in their correct mole ratios (i.e. the equivalence point), a colour change is produced straight after. This is termed the endpoint and if the indicator had been chosen wisely for experiment then it should have a sharp endpoint. This allows..."

 

However how can I make it better? How would you answer this question?

 

b) Write an equation for the reaction that occurred.

 

[latex]NaOH(aq) + HCl(aq)[/latex] ---> [latex] NaCl(aq) + H2O(l)[/latex]

 

c) What was the molarity of sodium hydroxide in the waste water?

[latex]c_{1}V_{1}=c_{2}V_{2}[/latex]

[latex]c_{1}*0.02000=0.150*0.01140[/latex]

[latex]c_{1}*0.02000=0.00171[/latex]

[latex]c_{1}=0.855 M[/latex]

 

d) What mass of sodium hydroxide would be present in 100 L of the waste water?

 

[latex]c(NaOH)=0.0855 M[/latex]

[latex]V(NaOH)=100 L[/latex]

 

[latex]n=cV[/latex]

[latex]n(NaOH)=0.0855*100=8.55 mol[/latex]

[latex]m=n*M[/latex]

[latex]m(NaOH)=8.55*(22.9898+15.9994+1.00797) = 341.9758035 = 342 g[/latex]

that's ok, but i'd just write: the indicator is used to tell when the solution is neutralized.

that's ok, but i'd just write: the indicator is used to tell when the solution is neutralized.

 

or: the indicator is used to tell when the sodium hydroxide has been completely neutralized.