Question

Calculate the oxygen ppm concentration of a 3.50 kg sample of lake water that comtains 2.34 mg of dissolved oxygen

Answer 1

There are two very easy formula to remember for ppm. If I'm dealing with solutions I remember
1 ppm = 1 mg/L

If I'm dealing with solids (or at least with kg), I use
1 ppm = 1 mg/kg.

So ppm for this is (2.34 mg/3.50 kg) = ?
That sure makes it simple doesn't it? What do I do if the problem is not in mg and kg. Convert what I have to mg and kg and go from there. (Or convert to mg and L and go from there.).

Answer 2

I got 1.5 x 10^12
Is that correct?

Answer 3

I don't know what kind of calculator you're using but I suggest you get a different one. ;-). Just in my head I can see that 2/3.5 is nowhere near 1.5E12.
2/4 is about 0.5 and 2/3 is about 0.7 so the answer MUST be somewhere in that vicinity.

Answer 4

My calculator is perfect. In fact it is the graphing calculator i have.
I did this before and got 0.7
But i was told it was wrong

Answer 5

Its also because in ppm you multiply by 10^6

Answer 6

I've always wanted a graphing calculator but never had one.
The answer is not 0.7 ppm which may be why you were told it wasn't right.
2.34 mg/3.5 kg = 0.66857 ppm which I would round to 0.669 ppm. My response above does not contain anything about multiplying by 10^6 because you don't if you use what I posted. That 1E6 is built in to the mg/kg formula. If you are intent on using the 1E6 it is done this way.
(g solute/g solution)*1E6 = ppm
So 2.34 mg is 0.00234 g.
and 3.50 kg is 3.50E3 g.
Then (0.00234/3.50E3)*1E6 = guess what? 0.669 ppm.
Your problem is simple. You took my formula, added your own modification to it (incorrectly), and came up with the wrong answer.