Here's how to solve this problem, along with explanations to help you understand the steps:

Understanding Alkalinity

Alkalinity is a measure of a water's ability to neutralize acids. In natural waters, it's primarily due to the presence of bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻) ions. Since the pH is close to 7, bicarbonate is the dominant alkalinity contributor.

Steps to Calculate Alkalinity as CaCO₃

1. Identify the Alkalinity-Contributing Ion: In this case, it's HCO₃⁻ (bicarbonate).

2. Calculate the Equivalent Weight of HCO₃⁻:

   • Molecular weight of HCO₃⁻ = 1 (H) + 12 (C) + 3 * 16 (O) = 61 g/mol
   • Since HCO₃⁻ has a charge of -1, its equivalent weight is equal to its molecular weight: 61 g/equivalent.

3. Calculate the Equivalent Weight of CaCO₃:

   • Molecular weight of CaCO₃ = 40 (Ca) + 12 (C) + 3 * 16 (O) = 100 g/mol
   • Since Ca²⁺ has a charge of +2 (and CO₃²⁻ has a charge of -2), CaCO₃ has