You are asked to determine the Fe^3+ concentration of a 50 mL unknown water samp

Question

You are asked to determine the Fe^3+ concentration of a 50 mL unknown water sample. You decide to use an EDTA titration buffered to pH = 10 For Fe^3+. and EDTA, log K_f = 25.1 and alpha_y^4- pH y 10.0 = 0.30. You add 25 mL of .06M EDTA and find that after addition pFe^3+ = 1.57 What is the this initial [Fe^3+] in the water sample? 0.02 0.07 0.04 0.40 2. Which of the following statements (there may be more than one) is correct for making a good buffer from the triprotic acid phosphoric acid (H_3PO_4) to simulate human blood plasma at pH =

Explanation / Answer

Free Fe3+ ion is solution is represented by pFe3+ and is equal to -log[Fe3+].

Free Fe3+ after addition of 25mL of 0.06M is,

1.57 = log[Fe3+]

therefore, [Fe3+] in 75 mL of solution = 10(-1.57) = 0.0269 M = 0.002 moles of Fe3+

EDTA added = 25 mL of 0.06 M = 0.0015 mols of EDTA

Hence, Fe3+ is present in water solution = 0.002 moles + 0.0015 moles of EDTA (as equal amount of Fe3+ reacts)

= 0.0035 moles

0.0035 moles in 50 mL = 0.0035/0.05 = 0.07

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