[IceFeast]

Hello,

I am a a Biology student, and I don't really know about computing how much potassium dichromate to weigh in the analytical balance. Because we need this following measurements: 10, 30, 50, 70, 90 mg of chromium VI ONLY to mix with 1 kg of soil.

If we weigh for example, 10 mg of potassium dichromate, does that mean that we also weigh 10 mg of chromium VI?

please help me compute for this. I would really really appreciate it. And also, if we measure these in the analytical balance, does the balance offer accuracy of the data?

[Arkcon]

That's one possible way.  You might make a solution, at maximum concentration or higher, and mix varying lesser volumes of this stock solution with your kilo of soil, mix well, at let it dry.  You could also make a stock mixture of a large amount of chromium in soil, then dilute this mixture of soil with fresh soil.  Whichever method you choose, you have mix thoroughly, because you have a small mount of chromium in a kilo of soil.

[IceFeast]

arkcon, thank you so much.

"You might make a solution, at maximum concentration or higher, and mix varying lesser volumes of this stock solution with your kilo of soil" - what do you mean by this? im so sorry im not really good at chemistry.

"You could also make a stock mixture of a large amount of chromium in soil, then dilute this mixture of soil with fresh soil." - and this, how can we dilute the mixture?

[IceFeast]

ive been thinking about weighing .2828 g of potassium dichromate to get 10 mg of chromium VI. Is this correct?

[Arkcon]

ive been thinking about weighing .2828 g of potassium dichromate to get 10 mg of chromium VI. Is this correct?

No, that is too much dichromate.  Can you see why?  Did you really expect to get milligrams of Cr VI from grams of potassium dichromate?  Do you now see why we make the most concentrated on first, and then serially dilute to get lower concentrations?

[IceFeast]

my bad with the conversion. 0.02829 g of potassium dichromate.

[Arkcon]

my bad with the conversion. 0.02829 g of potassium dichromate.

Do you routinely mass out to 5 or 6 decimal places?  I have from time to time, and it is difficult, the balance bounces a bit, especially if the environment isn't too stable.  You are probably expected, for the purpose of this assignment, to mass out the biggest one, and serially dilute.  But you can do the math both ways, and submit the plan to a person of authority for the project, and see what they'd like.

[IceFeast]

you are kind of rude but thanks anyway.

[Pradeep]

If you want to be biologist, you should be good at chemistry.


Try to understand the calculating procedure. It is very easy arrow maths.


298.1846 of K₂Cr₂O₇ contains  103.992 g of Cr

Therefore,

To take 103.992 g of Cr you should take  298.1846 of K₂Cr₂O₇

To take 1 g of Cr  you should take 298.1846/103.992 g = 2.86738 g

To take 1 mg of Cr  2.83738 mg

To take 10mg of Cr    28.3738 mg you should weigh

Double check my figures!!!!!!!!!!!!!!!!!

[IceFeast]

sir, i need 10 mg of Cr not g. i took the advice of Arkcon to just make a stock solution and get an mL equal to 10 mg of Cr.

so i need to weigh 2.829 g of potassium dichromate and then mix it with 1000 mL of distilled water. Then, to get 10 mg or Cr, i just need to get 10 mL of the prepared stock solution.

[sjb]

sir, i need 10 mg of Cr not g. i took the advice of Arkcon to just make a stock solution and get an mL equal to 10 mg of Cr.

so i need to weigh 2.829 g of potassium dichromate and then mix it with 1000 mL of distilled water. Then, to get 10 mg or Cr, i just need to get 10 mL of the prepared stock solution.

Almost, but not quite. Don't forget the volume of the salt.

[AWK]

need to weigh 2.829 g of potassium dichromate and then mix it with 1000 mL of distilled water

Water is added up to 1000 ml of solution.

[Pradeep]

Yeah sure Icefeast. I just wanted to show how to calculate. You can't accurately weigh 28 mg. So weigh .28 or 2.8 g and make a stock solution and dilute it.

[IceFeast]

thanks, you guys.