Question

Toxic chemicals in drinking water usually are reported and a safety level specified in units of parts per million (ppm) by mass. What is the molar concentration of arsenic in a water sample that has 1ppm arsenic (As)?

Answer 1

1 ppm = 1 mg/L and although that isn't by mass at 1 ppm by mass and by volume is essentially the same.
So you have 1 mg/L
Convert 1 mg to mols As.