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Topic: Ksp of CaSO4  (Read 7548 times)

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Offline BHAVESH

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Ksp of CaSO4
« on: January 29, 2020, 06:43:36 AM »
Hi Everyone,
I found an equation from Google about Ksp of CaSO4 which is as follows

   Ksp CaSO4  = 10^[1/(-0.0222XLog(Ionic Strength of Water)+2.2954)2-0.2478]

I just wanted to know how these numeric figures have been derived in above equation.

Thanks....
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Offline mjc123

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Re: Ksp of CaSO4
« Reply #1 on: January 29, 2020, 07:02:42 AM »
Did it give a reference? Can you look up that reference?
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Offline Corribus

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Re: Ksp of CaSO4
« Reply #2 on: January 29, 2020, 09:46:51 AM »
Just by eye it looks like a empirical equation, determined by fitting to a series of experimental data points. If that's the case, the terms and coefficients probably don't have any specific scientific meaning.
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Offline BHAVESH

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Re: Ksp of CaSO4
« Reply #3 on: January 31, 2020, 02:58:34 AM »
They have just mentioned - (Dupont 1992)
They have mentioned - The solubility products for sparingly soluble salts are based on expressions derived from published data on solubility as a function of the ionic strength of the solution at a given temperature. The equation for calculating Ksp for CaSO4 at 25 Degree C.
And when I checked in Dupont document for Ksp, they have given a graph, it seems it must have been derived from that.

So, still the question remains is that how to calculate Ksp of CaSO4, BaSO4, SrSO4 at any given temp.? - Can I apply Van't Hoff equation here as well???
« Last Edit: January 31, 2020, 03:11:31 AM by BHAVESH »
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Offline Corribus

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Re: Ksp of CaSO4
« Reply #4 on: January 31, 2020, 08:28:38 AM »
Quote from: BHAVESH on January 31, 2020, 02:58:34 AM
So, still the question remains is that how to calculate Ksp of CaSO4, BaSO4, SrSO4 at any given temp.? - Can I apply Van't Hoff equation here as well???
Sure... bearing in mind sources of error.
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Offline BHAVESH

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Re: Ksp of CaSO4
« Reply #5 on: February 01, 2020, 06:24:59 AM »
Hi Corribus,
I didn't understand about sources of error. Can you please explain.
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Offline Corribus

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Re: Ksp of CaSO4
« Reply #6 on: February 01, 2020, 09:37:47 AM »
For one, the accuracy and presumed temperature independence of thermodynamic quantities (e.g., standard enthalpy change).
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Offline BHAVESH

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Re: Ksp of CaSO4
« Reply #7 on: February 02, 2020, 01:24:09 AM »
Ksp of CaSO4 @ 25°C is 4.93 X 10-5. When calculated using Van't Hoff Equation @ 40°C, surprisingly reduction in solubility is 4.52 X 10-17 using standard enthalpy of CaSO4 (-1433 KJ mol-1) at 25°C.

Such a drastic drop in solubility!!!
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Offline Borek

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Re: Ksp of CaSO4
« Reply #8 on: February 02, 2020, 04:30:29 AM »
All that means is that you are doing something wrong.
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Offline Corribus

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Re: Ksp of CaSO4
« Reply #9 on: February 02, 2020, 10:05:14 AM »
Check units in general but especially on your literature value for standard enthalpy of calcium sulfate. That seems way too high (like, by several orders of magnitude).
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Offline mjc123

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Re: Ksp of CaSO4
« Reply #10 on: February 05, 2020, 04:59:59 AM »
What is the value of ΔH that you must use in the van't Hoff equation? Is that what you have used?
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Offline BHAVESH

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Re: Ksp of CaSO4
« Reply #11 on: February 07, 2020, 06:46:21 AM »
Yes I used -1433 KJ mol-1. This I took from Wikipedia Data Standard Enthalpy. Can you please help me correct my mistake?
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Offline BHAVESH

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Re: Ksp of CaSO4
« Reply #12 on: February 07, 2020, 06:49:50 AM »
However, it has been observed that when Temp. increases, solubility of CaSO4 increases. In this case, I feel that I am doing some GROSS MISTAKE in calculating solubility using Van't Hoff Equation.
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Offline mjc123

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Re: Ksp of CaSO4
« Reply #13 on: February 07, 2020, 07:14:11 AM »
-1433 kJ/mol is the enthalpy of formation of solid CaSO4. Is that what you need to use in the van't Hoff equation for the solubility equilibrium?
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Offline BHAVESH

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Re: Ksp of CaSO4
« Reply #14 on: February 07, 2020, 11:55:00 PM »
yes, this is because I want to calculate the prediction of CaSO4 precipitation at given concentration in water. I mean when water gets concentrated by means of either evaporation or by reverse osmosis process, CaSO4 tends to precipitate depending upon concentration of individual ions and comes out as solid from water.
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