Calculating pH with and without ionic strength

In summary, to calculate the pH of a 0.00196M solution of HCl, there are four scenarios to consider: without taking into account ionic strength, and taking into account ionic strength in a solution of just water or a solution of Na2SO4. The first scenario can be solved using the equation pH = -log([H+]). To include ionic strength in the calculation, an equation relating activity coefficient to ionic strength is needed. For the third scenario, HCl will dissociate fully, while in the fourth scenario, the dissociation will be affected by the presence of Na2SO4.
  • #1
Labchem3
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Homework Statement


There are four things to be calculated:

-pH of 0.00196M solution of HCl in water, without taking ionic strength into account,

-The same thing but taking ionic strength into account

-pH of 0.00196M solution of HCl in Na2SO4, without taking ionic strength into account,

-And the same thing but taking ionic strength into account.

Homework Equations


pH = -log([H+])

3. The Attempt at a Solution
So far I've done the first one only. HCl dissociates fully, so pH is - log10(0.00196) = 2.71.

But:
1. How to include "ionic strength" into pH calculation? I have no idea how to do it.
2. How to calculate pH of HCl in Na2SO4? How will it dissociate?
 
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  • #2
1. Do you know an equation relating activity coefficient to ionic strength?
2. Completely. But the ionic strength is different.
 

1. How do you calculate pH with ionic strength?

The pH of a solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log ([A-]/[HA]), where pKa is the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid. To account for ionic strength, an additional term, known as the Debye-Huckel equation, can be added to the Henderson-Hasselbalch equation.

2. What is the Debye-Huckel equation?

The Debye-Huckel equation takes into account the effect of ionic strength on the activity of ions in a solution. It is given by the equation: log([A-]/[HA]) = -0.51 (z+^2 * √(I))/ (1 + √(I)), where z+ is the charge of the cation, I is the ionic strength, and [A-] and [HA] are the concentrations of the conjugate base and acid, respectively.

3. How does ionic strength affect pH calculations?

Ionic strength can affect pH calculations by altering the activity of ions in a solution, which in turn affects the equilibrium between the conjugate base and acid. This can lead to a difference in the calculated pH compared to the actual pH of the solution.

4. Can pH be accurately calculated without taking into account ionic strength?

In most cases, pH can still be accurately calculated without taking ionic strength into account. However, for solutions with high ionic strength, such as concentrated salt solutions, the difference between the calculated and actual pH may be significant.

5. How can ionic strength be determined for a solution?

Ionic strength can be determined by measuring the concentration of all ions present in a solution and taking into account their respective charges. It can also be estimated by measuring the electrical conductivity of a solution, as the conductivity is directly proportional to the ionic strength.

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