This equation simply states that the sum of the positive ions formed by the dissociation of the acid and water is equal to the sum of the negative ions produced by these reactions. We can generate a more useful version of this equation by remembering that we are trying to solve equilibrium problems for acids that are so weak we can't ignore the dissociation of water.
We can therefore assume that C is small compared with the initial concentration of the acid. By convention, the symbol used to represent the initial concentration of the acid is C a. If C is small compared with the initial concentration of the acid, then the concentration of HA when this reaction reaches equilibrium will be virtually the same as the initial concentration.
Substituting this approximation into the equation derived in this section gives an equation that can be used to calculate the pH of a solution of a very weak acid. Click here to check your answer to Practice Problem 3. Substituting this information into the acid-dissociation equilibrium constant expression gives the following result.
The concentration of the HA molecules at equilibrium is equal to the initial concentration of the acid minus the amount that dissociates: C. If C is small compared with the initial concentration of the acid, we get the following approximate equation. The first and second equations are nothing more than special cases of the third. When we can ignore the dissociation of the acid because there is no acid in the solution we get the first equation.
When we can ignore the dissociation of water, we get the second equation. When we can't ignore the dissociation of either the acid or water, we have to use the last equation. This discussion gives us a basis for deciding when we can ignore the dissociation of water. The only difference is the K w term, which is under the square root sign. As a rule: We can ignore the dissociation of water when K a C a for a weak acid is larger than 1. When K a C a is smaller than 1.
Calculate the pH of an 0. Click here to check your answer to Practice Problem 4. Click here to see a solution to Practice Problem 4. Practice Problem 1: Calculate the pH of 0. Click here to check your answer to Practice Problem 2 Click here to see a solution to Practice Problem 2. Practice Problem 4: Calculate the pH of an 0. Create a personalised ads profile. Select personalised ads.
Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.
Facebook Facebook Twitter Twitter. Updated January 29, Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. Weak Acid Definition and Examples in Chemistry. List of the Strong Acids and Key Facts. Polyprotic Acid Example Chemistry Problem. First, let's look at letter A and let's do this one in purple. Let's do the moment purple for letter.
This is not the answer for strong week or very weak. Now let's take a look at B in B. I see that every one of my H, c, l's and ages represented by the white and you'll see that it it's I've got an extra hydrogen on my water molecule. This isn't water. This is the H 30 plus plus I droney my on and each cl Everything's dissociated. So be represents a strong acid. Okay, now let's look at CND and we're gonna do these together to figure out which one is the week in which one is the very weak or weaker asset.
Remember that weak acids Onley partially associate. Excuse my writing their partially dissociate. So when I look at C and D, I can see that D has only one particle dissociated and, um, for see, I have one to three particles dissociated. So C is my weak acid and D is my very weak acid and that has to do with degree of dissociation.
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