Question

Two milligrams of a solid chemical are weighed and dissolved in 100 microlitres of water. What percent solution (w/v) was prepared?

Answer 1

Well, we have to assume ideal solution behavior, that is, that the final volume of the solution equals the volume of water used to make it. That may or may not be reasonable, depending on what the solid is.

But assuming that, then you have 2 mg of solute and 100 uL = 0.1 mL of solution, so the concentration is (2 mg)/(0.1 mL) = 2 x 10^1 g/L. You may wonder why I didn't write 20 g/L, and the answer is that it would seem you only know the mass of the solute to 1 significant digit, because you said "two" instead of "2.0" or "2.00" or something like that.