ATAR Notes: Forum

Archived Discussion => 2009 => End-of-year exams => Exam Discussion => Victoria => Chemistry => Topic started by: ladyy on March 26, 2010, 09:41:37 pm

Title: solution please :)
Post by: ladyy on March 26, 2010, 09:41:37 pm: Citric acid, C6H8O7, is an acid found in the juice of many fruit. The following analysis was carried out to
determine the concentration of citric acid in a sample of lemon juice.
A 25.00 mL sample of the lemon juice was diluted to 100.0 mL in a volumetric ß ask. A 20.00 mL aliquot of
this diluted solution was added to a 100 mL conical ß ask with two drops of indicator. A burette was Þ lled with
a solution of 0.142 M sodium hydroxide, NaOH, and the titration produced an average titre of 11.88 mL.
The equation for the reaction is

C6H8O7(aq) + 3NaOH(aq)+ Na3C6H5O7(aq) + 3H2O(l)
a. Calculate the concentration, in mol L-1, of C6H8O7 in the undiluted lemon juice.
Title: Re: solution please :)
Post by: Edmund on March 26, 2010, 09:58:32 pm: $n(NaOH)=C*V=0.142M*0.01188=0.001687mol$

$n(C_6H_8O_7 in 20mL aliquot)=0.001687*\frac{1}{3}=0.0005623mol$

$n(C_6H_8O_7 in 100mL)=0.0005623mol*5=0.0028116mol$

$C= \frac{n}{V} =\frac{0.0028116mol}{0.025L}=0.112M$
Title: Re: solution please :)
Post by: akira88 on March 26, 2010, 10:05:31 pm: Omg that's the exact same question I had on my SAC... where did you get that question from? :P
Title: Re: solution please :)
Post by: ladyy on April 18, 2010, 01:37:02 pm: its one of the VCAA sample prac exams