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### Topic: analytical chem (calculations help)  (Read 2473 times)

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#### newbie!

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##### analytical chem (calculations help)
« on: December 05, 2011, 07:20:10 AM »
help on the following questions pls.

You determine lead in an unknown sediment sample using an analytical instrument (e.g. emission spectrometry or mass spectrometry).
a.   For a calibration curve, make up 50 mL of a 10 mg/kg stock solution of Pb from Pb(NO3)2; the solution should contain 1% HNO3:
Calculate the amount of the lead salt that you have to dissolve;
how much HNO3 (65%) do you have to add (in g)?

b.   Calibration standards need to be prepared from the stock solution described above, of the following concentrations in ppb (parts per billion as µg per kg solution):                       0 - 0.1 - 0.3 - 0.5 - 1 - 5 - 10 - 100 – 500; each 50 mL of the standards are required; how much of the 10 mg/kg stock solution do you need per concentration?

c.   Your calibration measurements are as follows:
C (ppb)   counts
0   2
0.1   3
0.3   10
0.5   18
1   31
5   160
10   298
100   2670
500   15890
Produce a calibration graph (in Excel) with regression function

d.   You have extracted 1.234 g of sediment in 10 mL of aqua regia. 1 mL of the extract was diluted to 10 mL and measured, to give an answer signal of 24 counts. Calculate the lead concentration in the original sample.

is it just me or these really badly worded?
for a) do i need to write a balanced reaction to start with?

#### Borek

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##### Re: analytical chem (calculations help)
« Reply #1 on: December 05, 2011, 08:48:07 AM »
I think I understood everything without problems, so wording is not that bad.

No need for any balanced reactions, it is just a solution preparation - nothing very fancy.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### newbie!

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##### Re: analytical chem (calculations help)
« Reply #2 on: December 08, 2011, 12:58:27 PM »
well would u like to  point me in the right direction then as i dont have a clue.

i've calculated the % of Pb in Pb(NO3)2 to be 35.52%
i want 50ml of a solution of conc. 10ml/kg = 10ppm
conc. of what is 10ppm? Pb? the overall conc. of the solution should be 10ppm. and the solution is .? i dont understand?
my starting materials are Pb(NO3)2 and HNO3 yes?
it says my solution should contain 1% HNO3 but i am provided with HNO3 (65%)? or is that simply stating how the % of Pb in Pb(NO3) 2 (because with the 35% i calculated for Pb that would make 100% so it would make sense)

can someone just help me answer:
a)For a calibration curve, make up 50 mL of a 10 mg/kg stock solution of Pb from Pb(NO3)2; the solution should contain 1% HNO3:
Calculate the amount of the lead salt that you have to dissolve; how much HNO3 (65%) do you have to add (in g)?

• Mr. pH