Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Darkmatter1986 on January 25, 2015, 07:04:16 PM
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Hey guys,
I hope this is the correct place to post this question, if not I apologize!
If someone can help me out by providing the necessary steps and or point me in the right direction on how to answer this question that would be a great help. Im a bit lost on where to even begin.
"Soluble phosphorus (P) in water is regarded as a nutrient. Typically, P in wastewater exists in the form of HPO42-. It should be removed from wastewater before it is discharged into natural water systems. In wastewater treatment plants, precipitation is a frequently used method to remove P from wastewater. In the precipitation, a coagulant (e.g. FeCl3) is added to transform soluble P to insoluble chemicals (precipitates), as shown below.
FeCl3 + HPO42- = FePO4 (down arrow) + H+ + 3Cl-
These precipitates can be easily removed from water through gravity sedimentation. As a result, the effluent is P-free.
If a wastewater has 8 mg/L HPO42-, what amount (mg/L) of FeCl3 should be added to completely remove it?"
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Welcome, Darkmatter1986: to the Chemical Forums. According to the Forum Rules{click} (http://www.chemicalforums.com/index.php?topic=65859.0), we want to see your attempt. You may not have had a chance to read the rules yet, but we do want you to follow them. For starters, you should try to setup a word problem like this. Did you realize, the only pertinent part of the question you wrote is this:
FeCl3 + HPO42- = FePO4 (down arrow) + H+ + 3Cl-
These precipitates can be easily removed from water through gravity sedimentation. As a result, the effluent is P-free.
If a wastewater has 8 mg/L HPO42-, what amount (mg/L) of FeCl3 should be added to completely remove it?"
You've been given one reactant in mg/L, and you want another reactant in mol/L. But how are they related to each other? You have an equation, a balanced chemical equation, but what else do you think you'll need to relate them to each other?
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I apologize for not having attempted the problem first. Part of my problem is that I'm not even sure where to start. I appreciate your help in getting me moving here.
I'm guessing you first need to convert everything to the same units?
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And those units are? What are the units (understood) in a balanced chemical reaction? We don't actually reach into a bottle on a shelf, pick up one FeCl3 molecule, slap on one HPO42- molecule, and watch one iron phosphate molecule fall to our feet while a proton and a chloride anion fly over our heads, do we?
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Would it be g/mol?
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Ok I think I may have figured out the process. I ended up coming up with 12.96mg of FeCl3
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You might want to show us the steps you did to get your answer
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8mg/L HPO4 = 0.008 g/L
0.008 g HPO4 = 0.00008 mol
HPO4 = 96g/mol
FeCl3 = 162 g/mol
so 0.00008 x 162 g/mol = 0.01296g
=12.96mg FeCl3 required
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Could you tell us how you derived this
0.008 g HPO4 = 0.00008 mol
Just want to make sure you understand the concept
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Could you tell us how you derived this
0.008 g HPO4 = 0.00008 mol
Just want to make sure you understand the concept
No its fine, I actually wanted to make sure I got it right. :) Chemistry has never been my thing in the past, but I do find I'm enjoying it a bit more than I have before.
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OK so
Could you tell us how you derived this
0.008 g HPO4 = 0.00008 mol