What is the molarity of the standard solution used for iodimetric titration?

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Discussion Overview

The discussion revolves around determining the molarity of the standard iodine solution used in iodimetric titration, specifically in the context of analyzing the purity of anhydrous hydrazine through the titration of arsenic compounds. Participants explore the relationships between various chemical species involved in the titration process and the calculations necessary to find the molarity.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the procedure involving the titration of arsenic trioxide (As2O3) to determine the concentration of iodine in the standard solution.
  • Another participant questions the relevance of hydrazine to the titration process and highlights the difference in oxidation states between arsenous acid (H2AsO3−) and arsenic trioxide (As2O3).
  • Some participants express confusion regarding the chemical reactions and the identity of the analyte, with one suggesting that the reaction products may not be clearly understood.
  • A later reply proposes a method for calculating the moles of As2O3 and subsequently the moles of iodine (I3−) that reacted, leading to a calculation of the molarity of the iodine solution.
  • Another participant emphasizes the importance of considering the reaction conditions, specifically the role of sodium hydroxide in converting As2O3 to H2AsO3−.
  • Further calculations are presented by participants to derive the concentration of the iodine solution based on the moles of arsenous acid formed.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the chemical reactions involved and the calculations necessary to determine the molarity of the iodine solution. There is no consensus on the clarity of the relationships between the compounds or the correctness of the proposed calculations.

Contextual Notes

Participants note potential confusion stemming from the chemical identities and reactions involved, particularly regarding the conversion of As2O3 in sodium hydroxide and the implications for the titration process. The discussion reflects uncertainty about the roles of different species and the accuracy of the calculations presented.

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Homework Statement



Anhydrous liquid hydrazine is used as a rocket propellant. It is a highly hazardous, toxic, carcinogenic substance.
The %-purity of a sample, presumed to be neat anhydrous hydrazine, is to be determined by iodimetric titration.

First, the titrant iodine (+potassium iodide) solution (I2 + I− ⇌ I3−) was standardized.
61.24 mL of iodine solution was required to titrate 0.2802 g of As2O3 dissolved in a sodium hydroxide solution adjusted to near neutral pH.

H2AsO3− + I3− + 4H2O → HAsO42− + 3I− + 3H3O+

What is the molarity of the standard solution?



The Attempt at a Solution



I have to find the concentration of the standard which is iodine. I know that it took 61.24ml of that soln to titrant the analyte As2O3. So, my attempt is to take the mass of As2O3 that was used and convert that to moles using the molar mass.. then this moles will be the same as moles of iodine that reacted. from there i can find concentration...
BUT, what I don't get is the equation given... it started with H2AsO3... is that what the analyte is? or is it a different compound.. since the 2 after the As is missing
 
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You lost me at hydrazine (H4N2). What does arsenite ion have to do with this?

BTW. Arsenic in As(OH)3 or H2AsO3- is As+3 and Arsenic in As2O3 (O=As-O-As=O) it is +6. Yes, they are different compounds in different oxidation states.
 
I'm confused. But that is what the question is asking... maybe I mis-understood something. I don't know what is reacting with what
 
maybe the hydrazine, is not related to the question, I don't know :S

so is "As2O3 dissolved in a sodium hydroxide" => H2AsO3- ?
 
Apparently the arsenous acid is used to standardize the I3- solution. The standardized solution (I3-) is then used to titrate hydrazine, although that part of the procedure is not discussed in this problem.
 
Oh...

so, would this be the way to find the [ ] of the standard:

take the mass of As2O3 given (0.2802g) and divide by its molar mass to get the moles of it.

0.2802g *(1mol As2O3 / 197.839g) = 0.001416 mol As2O3 *2 = 0.00283mol As

0.00283 mol As = moles I3- reacted

0.00283mol I3- / 0.06124 L = 0.046 M I3- <--- that would be the concentration of the standard

is this way right?
 
Don't forget that "...As2O3 dissolved in a sodium hydroxide solution adjusted to near neutral pH" means,

As2O3 + 3OH- + H+ ------> 2 H2AsO3-
 
so after i find the moles of As2O3 times 2 = moles of H2AsO3-

0.2802g *(1mol As2O3 / 197.839g) = 0.001416 mol As2O3 *2
= 0.00283mol H2AsO3- = moles I3-0.00283mol I3- / 0.06124 L = 0.046 M I3- <--- that would be the concentration of the standard

like this?
 

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