Back Titration Of Metal Carbonate

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Discussion Overview

The discussion revolves around a back titration problem involving the identification of a metal in a metal carbonate. Participants explore the calculations related to moles of reactants and products, the stoichiometry of the reactions, and the determination of molar mass, while grappling with uncertainties in their calculations and assumptions about the metal involved.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant calculates the moles of HCl added and the moles that reacted with NaOH, leading to a determination of moles of HCl that reacted with the metal carbonate.
  • Another participant suggests ignoring the metal and focusing on calculating the number of moles of the carbonate anion.
  • A participant questions the mole ratio used in the reaction, proposing a reaction equation involving the carbonate anion and HCl.
  • Concerns are raised about the dilution factor being overlooked in earlier calculations.
  • One participant revises their calculations after considering dilution but continues to struggle with the resulting molar mass, indicating potential errors in their approach.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the problem, particularly regarding the role of the metal and the correct stoichiometry. There is no consensus on the correct method or final calculations, as participants continue to refine their understanding and calculations.

Contextual Notes

Participants note potential errors in calculations, such as overlooking dilution and miscalculating molar masses. The discussion reflects uncertainty in the stoichiometric relationships and assumptions about the metal involved in the carbonate.

Who May Find This Useful

This discussion may be useful for students or individuals working on analytical chemistry problems, particularly those involving titration methods and stoichiometric calculations.

Lancelot59
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I'm having an issue solving a back titration problem. We need to identify the metal in a metal carbonate. It can either be a group 1A or 2A

1.8500 grams of the carbonate are diluted to .2500L and then 0.02000L is taken and mixed with 0.01500L of 0.1789M HCl in a flask. After shaking the mixture around 0.01756 L of 0.1013M NaOH was used to titrate the remaining HCl.

I started this by first determining how many moles of HCl were added to the flask, and how much titrated with the NaOH:

0.01500L*0.1789M = 0.0026835 mol HCl total in flask
0.01756 L NaOH * 0.1013M x 1mol HCl/1mol NaOH = 0.001778828 mol HCl titrated.

So 0.000904672 mols of the HCl reacted with the metal carbonate. From here I would go to determine how many mols of the carbonate there are, but I'm not sure how to do that. Writing the equation could turn out two different ways because I don't know how the metals are grouped. Just picking one and running with it gave me a molar mass of about 3400...not good.

This is what I think the equation should be:
M2CO3 + 2HCL -> H2CO3 + 2MCL2

How can I solve this?
 
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Ignore metal - it is just a spectator. Calculate number of moles of carbonate anion.

--
 
What do I use for a mole ratio though? If I just ignore the metal I came up with:

CO3-2 + 2HCL --> H2CO3 + 2Cl-

So using that setup:

0.000452336 mol HCL x 1mol CO3-2/2mol HCL = 0.000226168 mol CO3-2

Then since g=mols*molar mass --> molar mass = g/mols. The molar mass turns out to be 8179.760178. A bit much...
 
Perhaps there is something more, but seems like you forgot about dilution.

--
 
Right...oops. /facepalm

So there's 0.000226168 mol in 0.02000L, so 0.113084M x 0.250L = 0.028271 mols of the anion in the main sample bucket. So using that my molar mass is 65.4ish... well it's getting better at least. I'll do some more math and edit it in.

No, still not working. I dropped a 0. The molar mass I got was 327ish. I have no clue what's going wrong.
 
Last edited:

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