An EDTA solution was prepared by dissolving approximately 4 g of the disodium salt in approximately 1 L of water

An EDTA solution was prepared by dissolving approximately 4 g of the disodium salt in approximately 1 L of water. An average of 42.35 mL of this solution was required to titrate 50.00-mL aliquots of a standard that contained 0.7682 g of MgCO3 per liter. Titration of a 25.00-mL sample of mineral water at pH 10 required 18.81 mL of the EDTA solution. A 50.00-mL aliquot of the mineral water was rendered strongly alkaline to precipitate the magnesium at Mg(OH)2. Titration with a calcium-specific indicator required 31.54 mL of the EDTA solution. Calculate

(a) the molar concentration of the EDTA solution.

(b) the concentration of CaCO3 in the mineral water in ppm.

(c) the concentration of MgCO3 in the mineral water in ppm.

The Correct Answer and Explanation is :

To determine the concentrations of calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃) in the mineral water sample, we can follow these steps:

(a) Molar Concentration of the EDTA Solution

First, we standardize the EDTA solution using a magnesium carbonate (MgCO₃) standard.

1. Calculate the molarity of the MgCO₃ standard solution:

• Molar mass of MgCO₃: 24.305 (Mg) + 12.011 (C) + 3×15.999 (O) = 84.313 g/mol
• Concentration of MgCO₃ standard: 0.7682 g/L
• Moles of MgCO₃ per liter: 0.7682 g/L ÷ 84.313 g/mol ≈ 0.00911 mol/L

1. Determine the moles of MgCO₃ in a 50.00 mL aliquot:

• Volume in liters: 50.00 mL × (1 L / 1000 mL) = 0.05000 L
• Moles of MgCO₃: 0.00911 mol/L × 0.05000 L ≈ 0.0004555 mol

1. Since the reaction between Mg²⁺ and EDTA is 1:1, moles of EDTA required = moles of MgCO₃:

• Moles of EDTA: 0.0004555 mol

1. Calculate the molarity of the EDTA solution:

• Volume of EDTA solution used: 42.35 mL × (1 L / 1000 mL) = 0.04235 L
• Molarity of EDTA: 0.0004555 mol ÷ 0.04235 L ≈ 0.01075 M

(b) Concentration of CaCO₃ in the Mineral Water in ppm

1. Titrate a 25.00 mL sample of mineral water at pH 10:

• Volume of EDTA used: 18.81 mL

1. Calculate moles of EDTA used:

• Volume in liters: 18.81 mL × (1 L / 1000 mL) = 0.01881 L
• Moles of EDTA: 0.01075 M × 0.01881 L ≈ 0.0002022 mol

1. Assuming all hardness is due to Ca²⁺ (since Mg²⁺ is precipitated at pH 10), moles of Ca²⁺ = moles of EDTA:

• Moles of Ca²⁺: 0.0002022 mol

1. Calculate the mass of CaCO₃ equivalent:

• Molar mass of CaCO₃: 40.078 (Ca) + 12.011 (C) + 3×15.999 (O) = 100.086 g/mol
• Mass of CaCO₃: 0.0002022 mol × 100.086 g/mol ≈ 0.02023 g

1. Convert to ppm (mg/L):

• Since 25.00 mL of sample was used, to find the concentration in ppm:
• ppm CaCO₃: 0.02023 g × (1000 mg / 1 g) ÷ 0.025 L ≈ 809.2 ppm

(c) Concentration of MgCO₃ in the Mineral Water in ppm

1. Titrate a 50.00 mL aliquot of mineral water after precipitating Mg²⁺ as Mg(OH)₂:

• Volume of EDTA used: 31.54 mL

1. Calculate moles of EDTA used:

• Volume in liters: 31.54 mL × (1 L / 1000 mL) = 0.03154 L
• Moles of EDTA: 0.01075 M × 0.03154 L ≈ 0.0003392 mol

1. Assuming the titration now measures only Ca²⁺ (since Mg²⁺ was precipitated), moles of Ca²⁺ = moles of EDTA:

• Moles of Ca²⁺: 0.0003392 mol

1. Calculate the mass of CaCO₃ equivalent:

• Mass of CaCO₃: 0.0003392 mol × 100.086 g/mol ≈ 0.03394 g

1. Convert to ppm (mg/L):

• Since 50.00 mL of sample was used:
• ppm CaCO₃: 0.03394 g × (1000 mg / 1 g) ÷ 0.050 L ≈ 678.8 ppm

1. Determine the MgCO₃ concentration:

• Total hardness (from part b) is due to both Ca²⁺ and Mg²⁺.
• Hardness due to Mg²⁺: Total hardness – Ca²⁺ hardness
• Mg²⁺ hardness in ppm CaCO₃: 809.2 ppm – 678.8 ppm ≈ 130.4 ppm

1. Convert ppm CaCO₃ to ppm MgCO₃:

• Molar mass ratio: MgCO₃ / CaCO₃ = 84.313 g/mol / 100.086 g/mol ≈ 0.8425
• ppm MgCO₃: 130.4 ppm × 0.8425 ≈ 109.9 ppm

Summary:

• Molarity of EDTA solution: ≈ 0.01075 M
• Concentration of CaCO₃ in mineral water: ≈ 809.2 ppm
• Concentration of MgCO₃ in mineral water: ≈ 109.9 ppm

These calculations are based on the assumption that at pH 10, Mg²⁺ is precipitated as Mg(OH)₂, allowing for the selective titration of Ca²⁺. The total hardness is first determined, followed by the specific hardness due to Ca²⁺, and the difference is attributed to Mg²⁺. Converting these values to their respective carbonate forms provides the concentrations in ppm.