Chemical equilibrium concentrations

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Discussion Overview

The discussion revolves around determining equilibrium concentrations for the dissolution of magnesium hydroxide (Mg(OH)2) based on a given equilibrium constant (K). Participants explore whether the information provided is sufficient to solve the problem and discuss the implications of the chemical equation and equilibrium expressions.

Discussion Character

  • Homework-related
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions if it is possible to find equilibrium concentrations given only the equilibrium constant and the chemical equation.
  • Another participant seeks clarification on whether the correct formula for magnesium hydroxide is Mg(OH-) or Mg(OH)2 and discusses the dependence of ion concentrations when a solid dissolves.
  • A participant proposes using an ICE table with an assumed concentration of 1 for Mg(OH)2 to find other equilibrium concentrations, raising concerns about the validity of this approach.
  • Another participant asserts that the activity of solid magnesium hydroxide is assumed to be 1 and suggests that there is no need for an ICE table if solid is present.
  • One participant provides a calculation for the concentrations of Mg2+ and OH- based on an equilibrium expression but questions the validity of their results due to the numbers being "absurd."
  • A later reply indicates that the initial reaction equation and the value of K may not be consistent with chemical reality, suggesting a potential error in the problem setup.
  • Another participant corrects the reaction direction, stating that the correct dissolution reaction should be Mg(OH)2 <===> Mg2+ + 2OH-, which would yield more reasonable concentration values.
  • One participant mentions that their sources indicate the solubility product constant (pKsp) for Mg(OH)2 should be around 10-10 or 10-11, suggesting that the provided K value of 10-6 appears too large.

Areas of Agreement / Disagreement

Participants express disagreement regarding the correct formulation of the equilibrium expression and the direction of the reaction. There is no consensus on the validity of the calculations or the sufficiency of the given information to solve the problem.

Contextual Notes

Participants note potential inconsistencies in the chemical equation and the equilibrium constant provided, which may affect the calculations. There are unresolved questions regarding the assumptions made in the equilibrium expressions and the implications of solid activity.

cj2222
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I have a problem in which It asks to find the equilibrium concentrations but it only give the value of the equilibrium constant and the chemical equation. Is this even possible to solve or does it not have enough information?

Here is the problem

Determine the equilibrium concentrations for the reaction below. K = 2.6 x 10-6.

Mg2+ + 2OH- <====> Mg(OH-) (solid)
 
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Mg(OH-) or Mg(OH)2?

Write expression for the equilibrium constant. Is there any dependence between concentrations of the ions produced when solid dissolves?
 
Last edited by a moderator:
Can you use 1 as the equilibrium concentration of Mg(OH)2 and use this on the ICE table to find the other equilibrium concentrations or would this give you the wrong answer?

because k = 1/[Mg2+][OH-]

or would the answer be

1/(x)(2x)^2 = 2.6 x 10^-6

where x = the equilibrium concentration of Mg2+
 
Last edited:
1 is OK, but not as a concentration - magnesium hydroxide is a solid, so its activity is assumed to be 1. There is no need for ICE table - as long as there is solid present, its activity is 1. Just 1.

Your k is wrong - please write balanced reaction of magnesium dissolution first, then use it to construct reaction quotient.
 
Last edited by a moderator:
Mg2+ + 2OH- <===> Mg(OH)2k = 1/[Mg2+][OH-]2

The concentrations of OH- is going to be 2 times the concentration of Mg2+

so 2.6 x 10-6 = 1/(x)(2x)2so x = 45.81

the concentration of Mg2+ is 45.81

the concentration of OH- is 91.62

Is this correct or not because I can't think of any other way to do it?
 
You are on the right track, but numbers you get are absurd (BTW, you should list units in your answer).

IMHO that's because your initial reaction equation and value given for k are not consistent with chemical reality, which is probably not your fault.
 
Last edited by a moderator:
yeah, the reaction is supposed to be reversed, k is correct.

The reaction is supposed to be


Mg(OH)2 <===> Mg2+ + 2OH-


which would give much smaller and more reasonable concentrations.
 
My sources give pKsp for Mg(OH)2 as something between 10 and 11, which means k should be around 10-10 or 10-11. 10-6 looks way too large.
 

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