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

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.).

I got 1.5 x 10^12

Is that correct?

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.

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

Its also because in ppm you multiply by 10^6

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.

To calculate the oxygen ppm (parts per million) concentration of a sample of lake water, you need to use the following formula:

Oxygen ppm = (mass of oxygen in the water / mass of water sample) * 1,000,000

Given the information you provided, the mass of oxygen in the water is 2.34 mg, and the mass of the water sample is 3.50 kg. However, we need to convert the mass of oxygen to the same unit as the mass of the water sample, which is kilograms (kg).

Converting 2.34 mg to kilograms:

2.34 mg = 2.34 * 10^(-6) kg

Now, we can calculate the oxygen ppm concentration:

Oxygen ppm = (2.34 * 10^(-6) kg / 3.50 kg) * 1,000,000

Simplifying the equation:

Oxygen ppm = (2.34 * 10^(-6)) / (3.50) * 1,000,000

Oxygen ppm ≈ 668.571 ppm

Therefore, the oxygen concentration in the lake water sample is approximately 668.571 ppm.