What does molarity mean in case of weak electrolytes?

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Discussion Overview

The discussion revolves around the concept of molarity in the context of weak electrolytes, specifically acetic acid in water. Participants explore how to calculate molarity considering dissociation and the behavior of weak acids in solution.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether molarity should be calculated based on the total moles of acetic acid added or only the moles that dissociate in solution.
  • Another participant asserts that molarity is defined as the number of moles dissolved in solution, suggesting that it would be 5 moles per liter.
  • A different viewpoint introduces the concept of formal concentration, stating that while the analytical concentration of acetic acid is 5 M, the equilibrium concentrations of its dissociated forms will differ, estimating them to be approximately 4.990 M for HAcetate and 9.34×10-3 M for both Acetate- and H+.
  • One participant emphasizes that molarity is always the quotient of moles per total volume, indicating that the final molarity would be less than 5 moles per liter.
  • A later reply suggests that assuming 1 L of water and 5 moles of acetic acid, the final molarity would be approximately 3.96 M.

Areas of Agreement / Disagreement

Participants express differing views on how to define and calculate molarity for weak electrolytes, with no consensus reached on the correct approach.

Contextual Notes

Participants highlight the distinction between formal concentration and equilibrium concentrations, indicating that assumptions about dissociation and volume may affect calculations.

Who May Find This Useful

This discussion may be useful for students and professionals interested in chemistry, particularly those studying solutions, weak electrolytes, and molarity calculations.

AnkurGarg
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I am really confused about it.Suppose we add 5 moles of acetic acid in 1 litre of water..The molarity of solution will be -(5 moles)/volume of solution?Or it will be -(the number of moles of acetic acid dissociated/dissolved)/volume of solution
 
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I believe molarity is defined as the number of moles dissolved or otherwise well mixed into solution. Vinegar and water are quite miscible so far I know, so it would be 5 moles/ liter.
 
Molarity of what?

Formal concentration (also called analytical concentration) of acetic acid - 5 M.

Equilibrium concentrations of HAcetate, Acetate-, H+ will be different, you can estimate them to be approximately 4.990 M, 9.34×10-3 M and 9.34×10-3 M respectively. One thing you can be sure about is that [HAcetate] + [Acetate-] = 5 M (this is mass balance of acetic acid).
 
No, molarity always is the quotient of moles per total volume, so 5 moles of acetic acid and one liter of water give a molarity less than 5 moles/l.
 
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Good point.

Assuming 1 L of water and 5 moles of acetic acid final solution would be 3.96 M.
 
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