Help calculating the amount to pipet (mL) for aliquots
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Help calculating the amount to pipet (mL) for aliquots

[From: ] [author: ] [Date: 11-05-18] [Hit: ]
05, 0.10, 0.15, 0.......
The only information I was given was Standard iron contains 0.2500g/L of pure iron. Pipet 25.00mL of this standard solution into a 500mL volumetric flask and dilute up to mark with distilled water.

Pipet aliquots of this solution containing approx. 0.00, 0.05, 0.10, 0.15, 0.20 and 0.25 mg of iron and transfer each into one 50 ml volumetric flask.

How do I calculate the amount to pipet for each of the mg of iron? Please show steps. Showing how to do 0.15 would be enough, I can do the rest myself (I just need to learn how to do it first)


You start with the information that your standard solution contains 0.2500g/litre Fe

That is 250mg Fe per litre

You have removed 25mL of this solution
You have removed 25/1000* 250 = 6.25mg of Fe

You have diluted this 25mL or 6.25mg Fe to 500mL
Therefore each mL of the final diluted solution contains 6.25/500 = 0.0125mg Fe per mL

You want :
0.05mg Fe =0.05/ 0.0.0125 = 4mL
0.10 mg Fe = 0.10/0.0125 = 8mL
0.15mg Fe = 0.15/0.0125 = 12mL
0.20mg Fe = 0.20/0.0125 = 16mL
0.25mg Fe = 0.25/0.0125 = 20mL

The concentration of iron in the diluted standard solution is:

C1V1 = C2V2
0.2500g/L X 25.00 mL = C2 X 500 mL
C2 = 0.0125 g/L

Converting this into mg/mL gives:

0.0125 g/L X 1L/1000 mL X 1000 mg/1g = 0.0125 mg/mL

Starting with the largest concentration, 0.25 mg / 0.0125 mg/mL = 20 mL. So, if you pipet 20 mL of your diluted iron solution into the 50 mL flask, it will contain 0.25 mg.

Do the others the same way. Divide the mass you want by the concentration (0.0125 mg/mL) to give you the volume you need to pipet.
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