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 1 ppm arsenic (As)

Answer 1

The molar concentration of water sample that has 1 ppm arsenic is 1.33 * 10⁻⁵ M

Step-by-step explanation:

1 parts per million (ppm) means 1 part of solute in a million parts of solution.

Since 1 million, 10⁶ mg = 1 kg, 1 ppm = 1 mg/kg

The density of water = 1.0 kg/L, therefore, the volume of 1 kg of water = 1 L

Hence, 1 ppm = 1 mg/kg = 1 mg/L

molar mass of arsenic = 75 g/mol

1 mg of arsenic = 10⁻³ g or 0.001 g

number of moles of arsenic in 0.001 g of arsenic = 0.001 g / 75 g/mol = 1.33 * 10⁻⁵ moles

Molar concentration = 1.33 * 10⁻⁵ mol / 1 L = 1.33 * 10⁻⁵ M

Therefore, the molar concentration of water sample that has 1 ppm arsenic is 1.33 * 10⁻⁵ M