Calculating the pH and percentage deprotonation of a weak acid
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Calculating the pH and percentage deprotonation of a weak acid
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!
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Re: Calculating the pH and percentage deprotonation of a weak acid
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.
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Re: Calculating the pH and percentage deprotonation of a weak acid
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.
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Re: Calculating the pH and percentage deprotonation of a weak acid
The K-value is less than 1.0 x 10^-3.
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Re: Calculating the pH and percentage deprotonation of a weak acid
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.
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Re: Calculating the pH and percentage deprotonation of a weak acid
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.
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Re: Calculating the pH and percentage deprotonation of a weak acid
Less than 10^-4 is safe territory for approximation!
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Re: Calculating the pH and percentage deprotonation of a weak acid
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.
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Re: Calculating the pH and percentage deprotonation of a weak acid
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.
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