A water sample contains 0.00505 grams of arsenic in 12.0 liters. What is the arsenic concentration in ppm?

Answer 1

That's easy if you use the metric system

Here's what we know:
> 12 liters of water weighs 12 kg (12,000 g)
> the water contains 0.00505 g of As

So divide the amount of water by the amount of As to get ppm:

1,000,000/12,000*0.00505=0.42 ppm

ppm refers to parts per million, so its just like parts per hundred more commonly known as percent.