Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: BlackCatHamlet on December 27, 2014, 02:56:48 PM
-
These types of questions I know are bound to come up in the exam I'm preparing for but I am unable to reach the answer that I am supposed to reach in doing this! Any help would be greatly appreciated!
A 10.0 g sample of an zinc alloy was dissolved in aqueous acid and
made up to 500 cm3 in an atomic absorption spectrophotometer, the diluted solution gave an absorbance of 0.350.
0.050 cm3 of a 5.00 g dm-3 standard solution of Zn2+ ions were added to 200 cm3 of the alloy solution. This increased the absorbance to 0.460. Calculate the concentration of zinc in the alloy in parts per million.
-
You have to show your attempts at solving the question to receive help. This is a forum policy.
Hint: this is so called method of standard addition.
-
Ah, alright - didn't know that!
My confusion is how to work out the added concentration of the Zn2+ ions...What I know is that x/(x+y) = 0.35/0.46 (x = conc. of zinc, y = conc. of zinc ions), I know that y can be worked out and I...think that after finding x the final answer can be reached (before conversion) through (x * 500) / 10 (i.e. total sample amount divided by sample size) but I can't reach the answer that is given as the right one. I've messed around with multiplying/dividing the various values (0.05, 5.00, 200/500 or 2/5) and the value I should get for y is achieved through (0.05 * 2 * 5 / 0.4) but without knowing where the *2 comes from I can't really go with that. I've also tried to work out the concentration through using the molar mass of the zinc ions (5 / molar mass) equalling 250/3269 then (moles / 0.05) which gets 1.52 which still doesn't let me reach the answer...
-
What I know is that x/(x+y) = 0.35/0.46 (x = conc. of zinc, y = conc. of zinc ions)
That's all - everything else is just dilutions.
What is the answer given as a correct one?
-
My answer was 198.86 ppm. What is the right answer?
-
yup, that's the right answer and I've managed to get it as well - was missing the C1V1 = C2V2 idea for finding out what y was!
Thanks for the help Borek!