I suppose I know what the question is TRYING to ask about, but it is wrong in many aspects.
You are probably expected to take Archimedes law into account. Your solid is submerged in the air and its apparent mass is smaller by the weight of the displaced air. You know the mass of the solid (do you? - read on), you have its density, that's enough to calculate solid volume. Use air density at 24 deg C to calculate mass of displaced air - tha's the difference bewteen real and apparent mass of the solid weighted. That's what you are IMHO expected to do.
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But then... Solid should be not dissolved in 500.00 mL, it should be FILLED UP to 500.00 mL. It also lloks like the water has a temperature of 24 deg C, so the solution will contract on cooling to 20 deg - which will change the concentration. Do you have to take that into account? Probably not, but if you are asked to be so precise when weighting, than why are you to ignore other sources of errors? And finally answer given is - once again IMHO - incorrect. 50.506g is not half a mole of KNO3, as it is calculated using wrong molar mass (should be 101.1032g). Where is the sense of asking imprecise question about superprecise weighting?
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