A teacher wants to prepare a 0.010 ppm solution of arsenic pentoxide in the laboratory for calibrating a water quality testing system. If the teacher uses 500.0 mL of water (which has a mass of 500.0 g), what mass of arsenic pentoxide should the teacher mix to get the required solution?

Ppm is a unit of concentration that means number of grams of solute per million grams of solution.

Then, 0.010 ppm is 0.010 grams of arsenic pentoxide per million grams of solution.

And to find the number of grams in 500.0 grams of solution you can state the proportion with the unknown:

0.010 g arsenic pentoxide / 1000000 g solution = x / 500.0 g solution.

Here we are taking 500.0 grams of water equal to 500.0 grams of solution because we expect that the number of grams of arsenic pentoxide is much smaller than 500.0 g which implies that 500.0 g + x ≈ 500.0 g

Now you can solve for x:

x = 500.0 g solution * [ 0.010 g arsenic pentoxide / 1000000 g solution ]

x = 0.000005 g arsenic pentoxide.

Answer: 0.000005 g