What is the ppm Fe in the diluted standard solution?

Question

A diluted iron standard is prepared by pipeting 2.00 mL of stock 40.0 ppm Fe solution into a 50.0 mL volumetric flask, adding the reagents, and diluting to the mark with water.

Answer 1

To solve this, you have to break the units of solution down. What does PPM actually mean? Parts per million, any google search will tell you that. What does it actually mean? Parts of solute per parts of solvent. In this case mg of Fe per 1000 grams of water. But because the specific gravity of water is one, grams of water is equal to ml of water. Which means we're actually working with mg of Fe / 1000mL of water. Which actually works out to be mg of Fe/ 1L of water. So....

(40mg/1000mL) x (2.00mL) = some number. (You can do the math)

The above equation will tell you the number of mg of Fe in 2.00mL of stock solution. Since you're taking this amount and putting it in another flask, then diluting, that means you'll have the same amount of Fe in the new flask, with a new (larger) amount of water. Since the volumetric flask has a volume of 50.0mL and you're filling it to the top, that will be your new volume. So you'd take whatever number you would get above, and divide it by 50.0mL, to get your answer.

(some number (in mg)) / (50.0mL) = another number

 

BUT WAIT!! Now you have mg/ml as your units. Which is cool, but it's not ppm, which is what the question asks for. So you have to convert.

(another number (in mg/mL)) x (1000ml/1L)

This should give you your final answer in mg/L or ppm in this case. If you have any questions, feel free to send me message on Wyzant. Keep in mind I do skype lessons, and the first lesson is free. And I'm hella cool, so there's that too.

Deuces.

Mat S.