Transcript Hardness of Water Presentation

Hardness of Water
1st Step: The calcium ion coordinates with the indicator (Eriochrome Black T).
H2In- + Ca2+ ↔ CaIn- + 2H1+
2nd Step: The EDTA chelates the calcium ion and releases the indicator.
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
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Sample Calculations: 1st Standardization of EDTA solution.
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
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? Mol/L
10.00 mL
38.00 mL
It took 38.00 mL of EDTA solution to titrate 10.00 mL of standardized calcium
ion stock solution (1.000 g CaCO3/L solution). Determine the molarity of the
EDTA solution. (The molar mass of CaCO3 is 100.1 g/mol)
10.00 mL
CaCO3 sol’n
10-3
38.00 x
L
EDTA sol’n
1mol
1mol
CaCO3 EDTA
CaCO3
1000mL
100.1g 1mol
CaCO3 sol ' n CaCO3 CaCO3
1.000g
=
0.002629
mol EDTA
LEDTA 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 the three calculated values of your standards were
0.002629, 0.002711, and 0.002649. Calculate the relative precision in parts per
thousand (ppt).
1st find the mean: (0.002629 + 0.002711 + 0.002649)/3 = 0.002663
2nd: Find the absolute value of each trial’s deviation from the mean.
absolute deviation for trial n:
𝑛 = |Na2EDTA]mean − [Na2EDTA]n|
Trial 1: n = |0.002663 - 0.002629| = 0.000034
Trial 2: n = 0.000048
Trial 3: n = 0.000014
 (0.000034+0.000048+0.000014) 


3

 x1000  12.02 ppt
Estimated precision (ppt) 
0.002663
Calculating Water Hardness (parts per million)
Water hardness ppm
≈ mg CaCO3
≈ 10-3g
1 L CaCO3soln
1000g
= 10-6
Example: 25.00 mL of an unknown sample was titrated with 0.002663 M
Na2EDTA solution. If it took 18.25 mL of 0.002663 M Na2EDTA solution to reach
the endpoint, what was the “hardness of the water”?
EDTA(aq) + CaIn–(aq) + 2H+(aq) → H2In–(aq) + CaEDTA(aq)
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18.25 mL
25.00 mL
0.002663mol
L
EDTA
18.25 m L 0.002663 mol EDTA 1 mol CaCO3 100.1 g CaCO3 = 194.6 mgCaCO3
L EDTA
0.025 L
CaCO3 sol’n
or =
1 mol EDTA 1 mol CaCO3
LCaCO3
sol’n
18.25 mL Na 2 EDTAsol'n 0.002663mol Na 2 EDTA 1 mol CaCO3 100.1 g CaCO3
0.02500 L CaCO3 sol ' n
L Na 2 EDTA sol'n
1 mol Na 2 EDTA 1 mol CaCO3
Sample’s water hardness = 194.6 ppm
Calculate your average hardness and your experimental precision from the three trials
Eriochrome black T