[Darkmatter1986]

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 + HPO₄^2-  = 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?"

[Arkcon]

Welcome, Darkmatter1986: to the Chemical Forums.  According to the Forum Rules{click}, 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 + HPO₄^2-  = 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?

[Darkmatter1986]

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?

[Arkcon]

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 HPO₄^2- molecule, and watch one iron phosphate molecule fall to our feet while a proton and a chloride anion fly over our heads, do we?

[Darkmatter1986]

Would it be g/mol?

[Darkmatter1986]

Ok I think I may have figured out the process. I ended up coming up with 12.96mg of FeCl₃

[billnotgatez]

You might want to show us the steps you did to get your answer

[Darkmatter1986]

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

[billnotgatez]

Could you tell us how you derived this

0.008 g HPO4 = 0.00008 mol

Just want to make sure you understand the concept

[Darkmatter1986]

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.

[billnotgatez]

OK so
Could you tell us how you derived this

0.008 g HPO4 = 0.00008 mol