IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: Sweet_03 on October 07, 2009, 04:11:24 pm

Title: Chemistry ! Need Help Please :"(
Post by: Sweet_03 on October 07, 2009, 04:11:24 pm
Hello ,
i have this Chemistry thing that i cant do  :-\
We've done an experiment to determine the Vitamin C Concentration in Fruit Juice.

* We added 30cm3 of Potassium iodid and 10cm3 of sulphuric acid to a conical flask
* Then we added 25cm3 of Potassium iodate(V) and 25cm3 of Apple Juice
* Then we Titrate against Sodium thiosulphate , using starch as the indicator
= the result was 5.15 cm3

Concentration Of :
* iodate = 0.005
* Sodium Thiosulphate = 0.01

OK So the Qs were :
Q1 Calculate The number of moles Of iodate (V) ions Present in 25cm3 of Potassium iodate(V)
n of Moles = Mc / 1000
= 0.001 * 25 / 1000 = 2.5 * 10-5 Mol

Q2 Using the Equation
IO3- + 5I- + 6H+ --> 3I2 3H2O
Calculate the # of Moles Of iodine Produced
So  2.5 * 10-5 Multiply By 3
=  7.5 * 10-5 Mol

Q3 Using Your titre calculate the # of moles of thiosulphate that have reacted
n=cV
n= 0.01 * 5.15 = 0.0515 mol

Q4 Using This equation
I2 = S2O32- --> S4O62- + 2I-
Calculate the # Of moles of iodine that have reacted with the thiosulphate
It will be 0.0515 mol

Q5 Use Your ans. from part 2 & 4 to calculate the # of moles of iodine used to oxidise the Vitamin C

Q6 Calculate the # of moles of Vitamin C in 25cm3 of Apple Juice

Q7 Given that the molar mass of Vitamin C is 178 then Calculate the concentration of Vitamin C in the apple Juice in g dm-3

How Can i do the last Three Q :(:(
Title: Re: Chemistry ! Need Help Please :"(
Post by: Sweet_03 on October 07, 2009, 05:54:32 pm
Plz Help Me  :(
i wud reallly appreciate it
Title: Re: Chemistry ! Need Help Please :"(
Post by: nid404 on October 07, 2009, 06:28:47 pm
it'll take some time....i'll answer it tom....cuz i have to study ffor mechanics...it'll be up tom for sure
Title: Re: Chemistry ! Need Help Please :"(
Post by: nid404 on October 08, 2009, 09:06:30 am
can you tell me which paper it is from....

I'm kinda not sure abt the reaction...

if i know the 5th one, i can do the rest of them.....for that i think there needs to be another reaction
Title: Re: Chemistry ! Need Help Please :"(
Post by: Light on October 17, 2009, 04:27:39 am
the first ques was wrong at the first place.
no of moles=0.005 * 25/1000=1.25*10^-4.

Q2:ans in Q1 *3 =3.75*10^-4

Q3:0.01*5.15/1000=5.15*10^-5
Q4 answer as Q3.
actually very easy for Q5.for u just subtract answer in Q2 with Q4.cause the remaining not reacted with thiosulphate will oxidise vitamin c. Ans=3.235*10^-4.

Ans in Q6 same as Q5.

Q7:In 25 cm3 u have 3.235*10^-4 mol.In 1000 cm3 u have 0.01294 mol dm-3.*RMM(178)
Ans=2.30 gdm-3.



practical is easy if u get the flow.Light's Copyright.
Title: Re: Chemistry ! Need Help Please :"(
Post by: Sweet_03 on October 17, 2009, 05:36:19 pm
Thank You Very Much Light
i think i neeed more practice with these Qs  ::)