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I have a question asked in jee main 2020. 10.30 mg of o2 is dissolved into a litre of sea water of density 1.03 g/ml. The concentration of o2 in ppm is?
Well I know that ppm is generally possibly mostly done as mg/L. Now in this question doing this we get 10.30 as the answer but the answer is 10 which suggests that it's done in some other mass related unit. How to understand when to use mass or volume? Please help me out.
$1$ liter water weighs $1030$ g, and contains $10.3$ mg $O_2$. The mass concentration of $O_2$ in ppm is the ratio $0.0103 g/1030$ g = $10.0$ ppm.
ppm is not necessarily defined for 2 quantities of the same unit.
There is ppm (w/w), ppm (V/V), ppm (w/V), ppm(n/n).
Sometimes these variants use an explicit affix, like ppm (V/V)=ppmv.
Due to the ambiguity of the default interpretation, it is highly recommended not to use ppm / ppb / ppt anymore. If one has to use them and no explicit interpretation is provided, one has to stick to the most probable or common variant used in the given context.
I suppose in your case it can be both ppm (w/V) and ppm (w/w), probably the latter, but no guarantee for that.
