The hardness of water (hardness count )is usually expressed in parts per million(by mass) caco3, which is equivalent to milligrams of caco3 per litre of water. what is the molar concentration of ca2+ ions in a water sample with a hardness count of 175 mg caco3/l?

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