February 27, 2021, 02:28:52 PM
Forum Rules: Read This Before Posting


Topic: Ca2+ in water : acid/base?  (Read 312 times)

0 Members and 1 Guest are viewing this topic.

Offline xshadow

  • Full Member
  • ****
  • Posts: 316
  • Mole Snacks: +1/-0
Ca2+ in water : acid/base?
« on: October 04, 2020, 11:30:02 AM »
Hi

If I have to calculate  pH of Ca(OH)2 I get a basic pH...(using its Kps)

But I've stutied that a Ca2+ ion in a solution is coordinated by 6 (for example) molecules of water getting:

[Ca·(OH2)6]2+

Then my text says that  this ""complex"" is  acid and can loose H+ getting:

[Ca·(OH)6]4- + 6H+

But at this can it loose all the OH- ?
Because I've seen that Ca(OH)2 ----> Ca2+ + 2 OH-


Thanks!

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7955
  • Mole Snacks: +555/-92
  • Gender: Male
Re: Ca2+ in water : acid/base?
« Reply #1 on: October 04, 2020, 01:02:55 PM »
It is true that Ca(OH)2 is a strong base.
It is also true that the hydrated calcium cation is a very weak acid, which means that it forms a very small amount of H3O+ ions in the aqueous solution.
What happens when you mix a small amount of weak acid with a large amount of strong base?
AWK

Offline xshadow

  • Full Member
  • ****
  • Posts: 316
  • Mole Snacks: +1/-0
Re: Ca2+ in water : acid/base?
« Reply #2 on: October 04, 2020, 02:19:16 PM »
It is true that Ca(OH)2 is a strong base.
It is also true that the hydrated calcium cation is a very weak acid, which means that it forms a very small amount of H3O+ ions in the aqueous solution.
What happens when you mix a small amount of weak acid with a large amount of strong base?

It happens an acid-base reaction with a salt formation and  a residual amount of strong (base) in excess that will give the pH


My doubt now is:

If I put Ca(OH)2 in water It dissociates forming Ca2+ ions

Now those Ca2+ ions will be coordinated  by water molecule Ca(H2O)6 and it can lose H+ 

So is the same thing a Ca(OH)2 (the ionic compound)  and a [Ca(OH)6]4-??

In the second one  6 -OH-coordinate the Ca2+ ion

While in the first case  - Ca(OH)2-  I think  is a solid (ionic compound) with a different structure and ""bond type""


Thanks

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 26304
  • Mole Snacks: +1710/-402
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Ca2+ in water : acid/base?
« Reply #3 on: October 04, 2020, 02:45:14 PM »
There are many reactions that take place when you dissolve Ca(OH)2, but in the end what matters when calculating pH is _measured_ values of Ksp and/or Kb - they typically take everything into account. That's because we don't measure these values directly, we calculate them from measured equilibrium concentrations of ions involved, and they depend on all equilibria involved.

Sometimes it means the result is only approximate, but for most calculations it doesn't matter, there are many more sources of uncertainty, most of the more important.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline xshadow

  • Full Member
  • ****
  • Posts: 316
  • Mole Snacks: +1/-0
Re: Ca2+ in water : acid/base?
« Reply #4 on: October 05, 2020, 02:48:27 PM »
There are many reactions that take place when you dissolve Ca(OH)2, but in the end what matters when calculating pH is _measured_ values of Ksp and/or Kb - they typically take everything into account. That's because we don't measure these values directly, we calculate them from measured equilibrium concentrations of ions involved, and they depend on all equilibria involved.

Sometimes it means the result is only approximate, but for most calculations it doesn't matter, there are many more sources of uncertainty, most of the more important.

so when I see its K equilibrium all these possible reactions are already considered

thanks!

Sponsored Links