Finding pH using ICE chart/quadratic formula on the last step

In summary, the monoprotic acid will have the form HA --> H + A and the pH will be equal to the Ka for the acid minus the x value that you found to be 4.10 x 10-2 M.
  • #1
IntegrateMe
217
1
Calculate the pH of a 0.0253 M aq solution of a monoprotic acid. The Ka for the acid is 1.79 x 10-2

So a monoprotic acid will have the form HA --> H + A, correct?

I 0.0253 0 0
C -x x x
E (0.0253-x) x x

EDIT: I just realized the formatting is all messed up on my chart, but it shouldn't be too difficult to discern what I'm doing.

So we have:

1.79 x 10-2 = x2/(0.0253 - x)
0 = -x2 - 1.79x10-2x + 4.53x10-4

After using the quadratic formula, we find the acceptable value of x (the one that is positive) to be:

4.10 x 10-2 M

Now my question is: how do I find the pH? I thought I could use -log10(4.10x10-2), but apparently that's wrong.

Any help? Thank you!
 
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  • #2
IntegrateMe said:
Calculate the pH of a 0.0253 M aq solution of a monoprotic acid. The Ka for the acid is 1.79 x 10-2

So a monoprotic acid will have the form HA --> H + A, correct?

I 0.0253 0 0
C -x x x
E (0.0253-x) x x

EDIT: I just realized the formatting is all messed up on my chart, but it shouldn't be too difficult to discern what I'm doing.

So we have:

1.79 x 10-2 = x2/(0.0253 - x)
0 = -x2 - 1.79x10-2x + 4.53x10-4

After using the quadratic formula, we find the acceptable value of x (the one that is positive) to be:

4.10 x 10-2 M

Now my question is: how do I find the pH? I thought I could use -log10(4.10x10-2), but apparently that's wrong.

Any help? Thank you!

Your method and your quadratic equation I think are OK so there must be some mistake somewhere in arithmetic.

I don't know how you know it is wrong. But I think a way to know without anyone telling you (in other words better than 'apparently', rather the sort of checkup thinking you should habitually do) is that your [H+] result is higher than the total concentration of acid that is there, which is impossible.

I went though a calc. and if you get pH 1.85 same as me it is probably not coincidence, if you get different check a bit but I also am error-prone.
 
Last edited:

Related to Finding pH using ICE chart/quadratic formula on the last step

1. What is an ICE chart and how is it used to find pH?

An ICE chart is a tool used in chemistry to organize and track the changes in concentrations of reactants and products during a chemical reaction. It stands for Initial, Change, and Equilibrium, and is used to determine the equilibrium concentrations of species in a reaction. To find pH using an ICE chart, the initial concentrations of all species involved in the reaction are listed, then the changes in concentration are calculated based on the reaction stoichiometry. Finally, the equilibrium concentrations are determined by subtracting the changes from the initial concentrations. These equilibrium concentrations are then used to find the pH using the Henderson-Hasselbalch equation or the quadratic formula.

2. Why is the quadratic formula used to find pH in some cases?

In some cases, the equilibrium concentrations calculated using an ICE chart may result in a quadratic equation when plugged into the Henderson-Hasselbalch equation. This is because the Henderson-Hasselbalch equation assumes that the concentrations of the acid and conjugate base are much greater than the concentration of H+, which is not always the case. When this happens, the quadratic formula is used to solve for the equilibrium concentration of H+, which is then used to find the pH.

3. What is the Henderson-Hasselbalch equation and how is it used?

The Henderson-Hasselbalch equation is a mathematical expression that relates the pH of a solution to the pKa of an acid and the ratio of the concentrations of the acid and its conjugate base. It is written as pH = pKa + log([conjugate base]/[acid]). This equation is used to calculate the pH of a solution in equilibrium, given the pKa and concentrations of the acid and conjugate base.

4. Can the ICE chart and quadratic formula be used for all acid-base reactions?

The ICE chart and quadratic formula can be used for most acid-base reactions, but there are some exceptions. For example, if the acid or base is a weak polyprotic acid or base, the calculations become more complex and may require additional steps. Additionally, if the acid or base is very strong, the equilibrium concentrations of the acid and conjugate base may be negligible, making the quadratic formula unnecessary.

5. How can I check if my calculations for pH using an ICE chart and the quadratic formula are correct?

To check your calculations, you can plug your equilibrium concentrations into the Henderson-Hasselbalch equation and see if it gives you the same pH value as your original calculation. You can also double check your math and make sure all units are correct. It can also be helpful to compare your results to known values or use a pH meter to measure the pH of the solution.

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