shaka23h

10+ Year Member
Dec 27, 2007
42
0
Status
Pre-Medical
Ok this is from my analytical chemistry class. Eventhough its not a MCAT problem the concepts involved are relevent to MCAT? I've tried this problem several times but I think I'm missing the Dimensional analysis part of this problem. Can anyone help me with this?

Thanks

Jason


Modern industrial society depends heavily on the availability and quality of water for industrial as well as drinking purposes. The water department of each municipality is responsible for monitoring the quality of the water, and conducts periodic analyses to see that the water meets certain minimum standards. One of the components of interest to industrial users is the amount of CaCO3 present in the water. When large amounts of calcium carbonate are present in boiler waters, the following reaction takes place when the water is heated,
Ca(HCO3)2
CaCO3 + H2O + CO2

This reaction causes calcium carbonate to deposit on the plumbing, thereby clogging the plumbing. This process is called “scaling”. EDTA is often added to boiler waters to complex the calcium and prevent the scaling. How many pounds of EDTA (372.25 g/mol) should be added to a 60,000 gallon boiler system to give a two-fold excess of EDTA over calcium (twice as many moles of EDTA as moles of Ca) if the hardness of the water is determined to be 118 ppm CaCO3 (100.09 g/mol)? [1 gallon is 3.78 liter; 1 ppm calcium carbonate is 1 mg calcium carbonate per liter; 454 grams per pound]
 

Kaustikos

Archerize It
10+ Year Member
Jan 18, 2008
12,205
4,164
Always Bespin
Ok this is from my analytical chemistry class. Eventhough its not a MCAT problem the concepts involved are relevent to MCAT? I've tried this problem several times but I think I'm missing the Dimensional analysis part of this problem. Can anyone help me with this?

Thanks

Jason


Modern industrial society depends heavily on the availability and quality of water for industrial as well as drinking purposes. The water department of each municipality is responsible for monitoring the quality of the water, and conducts periodic analyses to see that the water meets certain minimum standards. One of the components of interest to industrial users is the amount of CaCO3 present in the water. When large amounts of calcium carbonate are present in boiler waters, the following reaction takes place when the water is heated,
Ca(HCO3)2
CaCO3 + H2O + CO2

This reaction causes calcium carbonate to deposit on the plumbing, thereby clogging the plumbing. This process is called “scaling”. EDTA is often added to boiler waters to complex the calcium and prevent the scaling. How many pounds of EDTA (372.25 g/mol) should be added to a 60,000 gallon boiler system to give a two-fold excess of EDTA over calcium (twice as many moles of EDTA as moles of Ca) if the hardness of the water is determined to be 118 ppm CaCO3 (100.09 g/mol)? [1 gallon is 3.78 liter; 1 ppm calcium carbonate is 1 mg calcium carbonate per liter; 454 grams per pound]
I'm confused. Does the calcium carbonate form with H2O and CO2 to form the bicarbonate which solidifies in the plumbing or does the bicarbonate turn into those forms?

Anyways, here's how I did it:
118 ppm = 118 mg/L * 226,800L [60,000g *3.78 L/g] = 26.7 x 10^6 mg

26.7 x 10^6 mg = 26,762 g CaCO3 (Again, I do not fully understand what the formula is doing and what EDTA is really chelating, the bicarbonate (which, upon research is the aqeous portion) or the carbonate)
26,762 g * 1 mole/100.9g = 265.24 moles * 2 moles EDTA/1 mole CaCO3
This equals 530.5 moles EDTA * 372.25 g /mole EDTA * 1lb/454g = 435 lbs


Can anyone explain this process? Wiki fails me and so does anyone else.
Does the Calcium carbonate become bicarbonate and solidify or does the boiling water move the reaction towards the formation of carbonate and then solidify? Also, which does the EDTA chelate?
 
OP
S

shaka23h

10+ Year Member
Dec 27, 2007
42
0
Status
Pre-Medical
I'm confused. Does the calcium carbonate form with H2O and CO2 to form the bicarbonate which solidifies in the plumbing or does the bicarbonate turn into those forms?

Anyways, here's how I did it:
118 ppm = 118 mg/L * 226,800L [60,000g *3.78 L/g] = 26.7 x 10^6 mg

26.7 x 10^6 mg = 26,762 g CaCO3 (Again, I do not fully understand what the formula is doing and what EDTA is really chelating, the bicarbonate (which, upon research is the aqeous portion) or the carbonate)
26,762 g * 1 mole/100.9g = 265.24 moles * 2 moles EDTA/1 mole CaCO3
This equals 530.5 moles EDTA * 372.25 g /mole EDTA * 1lb/454g = 435 lbs


Can anyone explain this process? Wiki fails me and so does anyone else.
Does the Calcium carbonate become bicarbonate and solidify or does the boiling water move the reaction towards the formation of carbonate and then solidify? Also, which does the EDTA chelate?




This did give me the correct answer though! thanks so much.. I don't think I'll have to worry about the equation it just shows that the SR starts as 1:1 but now it goes to 2:1!!

thanks so much!
 
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