Hardness of Water 1 st Step The calcium

Sample Calculations: 1 st Standardization of EDTA solution. EDTA(aq) + Ca. In–(aq) + 2

The mean (average) of your three trials will be used for your calculations. Example:

Calculating Water Hardness (parts per million) Water hardness ppm ≈ mg Ca. CO 3

Slides: 5

Hardness of Water 1 st Step: The calcium ion coordinates with the indicator (Eriochrome Black T). H 2 In- + Ca 2+ ↔ Ca. In- + 2 H 1+ 2 nd Step: The EDTA chelates the calcium ion and releases the indicator. EDTA(aq) + Ca. In–(aq) + 2 H+(aq) → H 2 In–(aq) + Ca. EDTA(aq) pink blue

Sample Calculations: 1 st Standardization of EDTA solution. EDTA(aq) + Ca. In–(aq) + 2 H+(aq) → H 2 In–(aq) + Ca. EDTA(aq) pink blue ? Mol/L 10. 00 m. L 38. 00 m. L It took 38. 00 m. L of EDTA solution to titrate 10. 00 m. L of standardized calcium ion stock solution (1. 000 g Ca. CO 3/L solution). Determine the molarity of the EDTA solution. (The molar mass of Ca. CO 3 is 100. 1 g/mol) 10. 00 m. L Ca. CO 3 sol’n 0. 002629 38. 00 x 10 -3 L EDTA sol’n The mean (average) of your three trials will be used for your calculations.

The mean (average) of your three trials will be used for your calculations. Example: Suppose three calculated values of your standards were 0. 002629, 0. 002711, and 0. 002649. Calculate the relative precision in parts per thousand (ppt). 1 st find the mean: (0. 002629 + 0. 002711 + 0. 002649)/3 = 0. 002663 2 nd: Find the absolute value of each trial’s deviation from the mean. absolute deviation for trial n: �� = |Na 2 EDTA]mean − [Na 2 EDTA]n| Trial 1: n = |0. 002663 - 0. 002629| = 0. 000034 Trial 2: n = 0. 000048 Estimated precision (ppt) Trial 3: n = 0. 000014

Calculating Water Hardness (parts per million) Water hardness ppm ≈ mg Ca. CO 3 ≈ 10 -3 g 1 L Ca. CO 3 soln 1000 g = 10 -6 Example: 25. 00 m. L of an unknown sample was titrated with 0. 002663 M Na 2 EDTA solution. If it took 18. 25 m. L of 0. 002663 M Na 2 EDTA solution to reach the endpoint, what was the “hardness of the water”? EDTA(aq) + Ca. In–(aq) + 2 H+(aq) → H 2 In–(aq) + Ca. EDTA(aq) pink blue 18. 25 m. L 25. 00 m. L 0. 002663 mol L EDTA 18. 25 m L 0. 025 L Ca. CO 3 sol’n Sample’s water hardness = 194. 6 ppm Calculate your average hardness from the three trials = 194. 6 mg. Ca. CO 3 LCa. CO 3 sol’n

Eriochrome black T