How many moles of MnO4- were added?

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Discussion Overview

The discussion revolves around a titration problem involving the determination of moles of MnO4- ions added to a sample containing iron (II). Participants explore the calculations necessary to answer the initial question and related queries about the sample's composition.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about how to calculate the moles of MnO4- added, indicating a lack of clarity from their professor.
  • Another participant suggests that the question could be rephrased to ask about moles of KMnO4, which they believe is essentially the same.
  • A participant provides a calculation for the moles of MnO4- based on the concentration and volume of KMnO4 used, arriving at approximately 2x10-4 moles.
  • Further calculations are presented for the moles of iron (II), grams of iron (II), and the percent of iron in the sample, with one participant questioning the origin of a factor used in the calculations.
  • Another participant notes that rounding in the calculations could lead to a loss of accuracy in the final result.

Areas of Agreement / Disagreement

Participants generally agree on the calculations presented, but there is some uncertainty regarding the factor used in the moles of iron (II) calculation and the implications of rounding on accuracy. No consensus is reached on the best approach to clarify the initial question.

Contextual Notes

Some calculations lack detailed explanations for certain factors, such as the multiplication by 5 in the moles of iron (II) calculation. The discussion does not resolve whether this factor is justified or how it relates to the titration process.

MrPoison
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Hello,

I have no idea how to solve this...

The initial mass of an unknown sample containing iron (II) is 0.512 g.
12.6 mL of 0.01522 M KMnO4 is required to titrate the unknown to the endpoint.

a) How many moles of MnO4- were added?
b) How many moles of iron (II) must be present in the sample?
c) How many grams of iron (II) must be present in the sample?
d) What is the percent of iron present in the sample?


I would know b, c, d, if I would know how to solve a. b, c, d, questions are simple, but I totally don't understand how to solve a. I wrote the rest so people who don't know similar exercise could get something more out of that topic.
I know I shouldn't ask such a basics here, but I spent like an hour on YouTube and internet with no success. My professor is very unclear on every topic.

Thank you so much for helping.
 
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Hi MrPoison, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif


Would it be any easier for you if the question was:

a) How many moles of KMnO4[/color] were added?[/size]

because I think that's essentially what it's asking. :smile:
 
Last edited by a moderator:
I have no idea, because that's what the question from the professor is. I literally rewrote it from the lab assignment.Is that right?

a) How many moles of MnO4- were added?
0.01522 moles/1 L x 0.0126L ≈ 0.0002 = 2x10-4 moles

b) How many moles of iron (II) must be present in the sample?
2x10-4 x 5 = 0.001

c) How many grams of iron (II) must be present in the sample?
0.001 moles of Fe2+ x 56g Fe/1 mole Fe = 0.056g Fe

d) What is the percent of iron present in the sample?
0.056g Fe/0.512 g unknown sample x 100 = 10.9%
 
Last edited:
MrPoison said:
I have no idea, because that's what the question from the professor is. I literally rewrote it from the lab assignment.


Is that right?

a) How many moles of MnO4- were added?
0.01522 moles/1 L x 0.0126L ≈ 0.0002 = 2x10-4 moles
That's right. I wondered whether you were being tripped up by it asking for the Mn04- ions rather than KMnO4.
b) How many moles of iron (II) must be present in the sample?
2x10-4 x 5 = 0.001
That 5 seems to have sprung from nowhere, without explanation. You'd need to account for it if you want marks for this part.

There are plenty of examples of oxidizing Fe(II) on the web for you to follow, e.g.,
http://faculty.uml.edu/james_hall/84124/16.htm[/color]

Good luck with your studies!
 
NascentOxygen said:
That's right.

It would be perfect, if not for the fact OP rounded it down, losing accuracy of the final result.
 

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