Calculating the pH and percentage deprotonation of a weak acid

[Alvin Lai 3J]

When you are using the ICE box to solve for x, when do you know to approximate for x? Does the Ka or Kb need to be *10^-4 or smaller? Thank you!

[Sabina House 2A]

Yes, the K value must be 10^-4 or smaller, but another way to check to make sure the approximation is valid is to check the percent ionization of the acid or base. If the value of x divided by the initial concentration of acid or base is less than 5%, then the approximation is valid. If the K value is close to 10^-4, I would always check this to make sure the approximation was valid.

[Victor Li 2A]

You can approximate x when the K value is less than 10^-4. However, it does not mean x=0; instead, it means that the x value is so small that it can be approximated as zero. As mentioned above, if you are not sure about using approximations, you can always use the 5 percent ionization rule to check the validity of the approximation.

[derickngo3d]

The K-value is less than 1.0 x 10^-3.

[Cecilia Cisneros 1F]

To be safe I would approximate when the K is < 10^-4, as Dr. Lavelle had mentioned in lecture. Once you have solved for x, you would have to check the % ionized and make sure your percentage is less than 5% in order to confirm that your approximation is valid. However, If your percentage is greater than 5% you must go back and solve for x using a quadratic equation.

[Jason Knight - 1F]

Calculating pH and percentage deprotonation of a weak acid requires the calculation of an equilibrium constant, such as Ka. Calculating the pH and percentage deprotonation of a strong acid is very simple as you compare molar concentrations from the reactants to products. I would be careful with approximating unless the K value is smaller than 10^-4 or greater than 10^4.

[Can Yilgor 2D]

Less than 10^-4 is safe territory for approximation!

[Shrey Pawar 2A]

Also, make sure that the percentage is not higher than 5% as the assumption for a value - x just equalling the initial value is not valid. Hope this helps.

[Simi Kapila_3E]

Yes, he said to be safe use when K<10^-4. However, if you do the percent protonation and find that it is greater than 5%, you cannot use the approximation even if the equilibrium constant is very small.