Use the following information to answer these three questions: Initial EDTA Volu
ID: 879677 • Letter: U
Question
Use the following information to answer these three questions:
Initial EDTA Volume (mL)
Final EDTA Volume (mL)
Total volume of EDTA used (mL)
Trial 1
10 mL
7.5
2.5
Trial 2
11 mL
6.5
4.5
Trial 3
13 mL
10
3
Average Volume of EDTA Used (mL) : 3.33
Average volume of EDTA used (mL)
Concentration Ca2+ ions per liter of water (mol/L)
Water hardness (ppm CaCO3)
3.33 mL
0.00333 mol/L
333.2 ppm
Concentration: Moles of Ca^2+ / L = molarity of EDTA * ml of EDTA / ml of water sample
Moles of Ca^2+ = 0.01 * 3.33 ml / 10.0 ml = 0.00333 mol / L
Water hardness: Ppm CaCO3 =(moles of Ca^2+/1L)*(1 mol CaCO3/1mol Ca^2+)*(100.06g/1 mol CaCO3) *(1000 mg /1g)
= 333.2 ppm
QUESTIONS:
Based on the analysis of your local water, would you classify its hardness as soft, moderate, hard, or very hard? Explain your answer.
Approximately how much calcium would you ingest by drinking eight 8-oz glasses of your local water? Hint: 1 oz (fluid ounce) = 29.57 mL.
Assume an average minimum daily requirement for calcium is 1,150 mg. Calculate what percentage of your daily requirements could be met by drinking 1.0 L of your local water.
Initial EDTA Volume (mL)
Final EDTA Volume (mL)
Total volume of EDTA used (mL)
Trial 1
10 mL
7.5
2.5
Trial 2
11 mL
6.5
4.5
Trial 3
13 mL
10
3
Average Volume of EDTA Used (mL) : 3.33
Explanation / Answer
Water hardness is classified by the U.S. Department of Interior and the Water Quality Association as follows:
Classification
mg/l or ppm
grains/gal
Soft
0 - 17.1
0 - 1
Slightly hard
17.1 - 60
1 - 3.5
Moderately hard
60 - 120
3.5 - 7.0
Hard
120 - 180
7.0 - 10.5
Very Hard
180 & over
10.5 & over
From the table in the question the local water has 333.2 ppm of Calcium ions.
According to the water classification table the local water can be considered as very hard.
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By drinking eight 8-oz (1 oz = 29.57 mL ) glasses of local water one will be drinking
= 8 x 8 x 29.57 = 1,892.48 mL local water
Local water hardness = 333.2 ppm = 333.2 mg/L
According to hardness, so if in 1L, 333.2 mg of calcium is present then 1,892.48 mL water will have
1892.48 x 333.2 /1000 = 630.57 mg of Calcium will be ingested by drinking eight 8-oz glasses of local water.
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If average daily requirement = 1150 mg and
If a person drinks 1L of local water then they will be ingesting 333.2 mg of Calcium. This will be equal to
333.2 x 100 / 1150 = 28.97% percentage of one’s daily requirements could be met by drinking 1.0 L of your local water.
Classification
mg/l or ppm
grains/gal
Soft
0 - 17.1
0 - 1
Slightly hard
17.1 - 60
1 - 3.5
Moderately hard
60 - 120
3.5 - 7.0
Hard
120 - 180
7.0 - 10.5
Very Hard
180 & over
10.5 & over