What is the Hydronium Ion Concentration in a Magnesium Hydroxide Solution?

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Discussion Overview

The discussion revolves around calculating the hydronium ion concentration in a magnesium hydroxide solution, specifically addressing the dissolution of magnesium hydroxide (Mg(OH)2) in water and the resulting pH and pOH values. The context is primarily homework-related, focusing on the application of chemical equilibrium principles and calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant presents a method for calculating the hydronium ion concentration, deriving a value of 5 * 10^-12 M based on their calculations of hydroxide ion concentration and pH.
  • Another participant questions the correctness of the initial result and asks for the molar concentration of Mg(OH)2.
  • A participant calculates the molarity of Mg(OH)2 as 0.24 M based on the mass and volume provided.
  • Further calculations by another participant suggest that the hydroxide ion concentration should be 4.11 * 10^-3 M, leading to a pOH of 2.08 and a corresponding hydronium ion concentration of 1.2 * 10^-12 M.
  • There is a correction regarding the interpretation of moles versus concentration, indicating a misunderstanding in the previous calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct hydronium ion concentration, with differing calculations and interpretations of the results presented. Multiple competing views remain regarding the methodology and final values.

Contextual Notes

There are unresolved issues regarding the assumptions made in the calculations, particularly concerning the complete dissolution of magnesium hydroxide and the interpretation of molarity versus moles in the context of the solution.

a.a
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Homework Statement



If 0.12 g of magnesium hydroxide Mg(OH)2 is dissolved in sufficient water to make 500. mL of solution, what is the hydronium ion concentration in the solution? (Assume that all of the magnesium hydroxide dissolves.)

Homework Equations



pH = -log [OH-]
pOH = -log [H3O+]

The Attempt at a Solution



I got 5 * 10^-12:

Mg(OH)2 --> Mg2+ + 2OH-
[OH-] = 2.058 * 10^-3 M

pOH = 2.6852
then pH = 14 - pOH = 11.31

[H3O+] = 10^-11.30
= 5*10^-12

is this the right method, is there an easier way?
 
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Approach is correct, but the result is wrong. What is molar concentration of Mg(OH)2?
 
Last edited:
M = .12/.5 = .24
 
got it.. thanks though
 
moles Mg(OH)2 = 2.058*10^-3

Mg(OH)2 --> Mg2+ + 2OH-
1 mol Mg(OH)2 ---> 2 mol OH-
2.058*10^-3 mol Mg(OH)2---> 2 * 2.058*10^-3 = 4.11*10^-3


thus [OH-] = 4.11*10^-3 M
pOH = 2.08
then pH = 14 - pOH = 11.9

[H3O+] = 10^-11.9
= 1.2*10^-12

:) is this rite?
 
a.a said:
2.058*10^-3 mol Mg(OH)2---> 2 * 2.058*10^-3 = 4.11*10^-3

This is number of moles, not concentration.
 

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