Answer to Question #270534 in Inorganic Chemistry for Kate

Question

2. The hardness of water (hardness count) is usually expressed as parts per million (by mass) of CaCO3. What is the molar concentration of Ca²+ ions in a water sample with a hardness count of 175 ppm?

Answer 1

1.75 × 10⁻³ M

Explanation:

Given:

Hardness = 175 mg CaCO₃/L

or mass of CaCO₃ in 1 L = 175 × 10⁻³ grams

Now,

Molar mass of CaCO₃ = 40 + 12 + ( 3 × 12 ) = 100 grams/mol

The concentration of CaCO₃ = number of moles / volume

or

= (175 × 10⁻³ / 100) / 1

or

= 1.75 × 10⁻³ M

Now,

CaCO₃ ⇒ Ca²⁺ + CO₃²⁻

1.75 × 10⁻³ M 1.75 × 10⁻³ M 1.75 × 10⁻³ M

thus,

the concentration of Ca²⁺ ions in water = 1.75 × 10⁻³ M