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# Parts per million (ppm) question Watch

1. I am really confused on how to answer this chemistry AS level question

Q: 1Kg of a solution contains 0.100 mol of calcium ions, Ca 2+

What is the concentration of the calcium ions by mass in parts per million (ppm)?

[Assume the relative atomic mass of calcium is 40]

I have tried looking up the method to answer these types of questions in CGP and edexcel student book, but i still don't understand how to actually work out the answer to these ppm questions?
2. Assume the relative molecular mass of water to be 18. That means that you have 1000 / 18 = 55.5 moles of water.
Hence conc in ppm = 0.1 x 10^6 /55.5 = 1802 ppm

Ignore the above, it`s conc by amount

m(Ca2+) / m(H2O) = n(Ca2+)A(Ca2+) / m(H20) = 0.1x40 / 1000

Now, 0.1x40 / 1000 = x ppm = x /10^6

That gives x = 4000
3. So ppm means how many units of a solute do you have in 1,000,000 units of the solvent.
So you work out the Mass of Ca2+ ions that you have in the solution...
So you have 1000g of solution, but as you need the mass of solute in 1,000,000 grams you need to multiply the answer by 1000.
4. (Original post by 16ARTER)
I am really confused on how to answer this chemistry AS level question

Q: 1Kg of a solution contains 0.100 mol of calcium ions, Ca 2+

What is the concentration of the calcium ions by mass in parts per million (ppm)?

[Assume the relative atomic mass of calcium is 40]

I have tried looking up the method to answer these types of questions in CGP and edexcel student book, but i still don't understand how to actually work out the answer to these ppm questions?
Work out the mass of calcium in 1 dm3 of solution.

You then have grams of calcium per 1000g of solution. However you want grams of calcium per 1,000,000, so multiply by 1000.

EDIT: hmmmmm, looks like the xmas pudding has slowed me up...
5. Thanks everyone, it makes sense now! I just didn't get the general formula

Thanks again

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Updated: December 29, 2010
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