A buffer consists of 0.20-M propanoic acid (Ka=. 1.4 ×10^-5 ) and 0.30-M sodium propanoate. (a)

A buffer consists of 0.20-M propanoic acid (Ka=. 1.4 ×10^-5...

Key Concepts

Acid-Base Neutralization in Buffers

In the context of buffers, when a strong acid (or base) is added, the weak base (or acid) component reacts with it, which changes the ratio of the conjugate base to acid. This reaction alters the pH gradually rather than drastically, which is the defining property of buffer systems.

Molarity and Mole Calculations

Understanding the relationship between molarity, volume, and moles is crucial for buffer problems. Calculating the initial moles of acid and conjugate base and then adjusting these values after the addition of a titrant is fundamental to determining the new buffer pH after the reaction.

Buffer Solution

A buffer solution is composed of a weak acid and its conjugate base (or a weak base and its conjugate acid) and resists changes in pH upon the addition of small amounts of strong acid or strong base. It does so because the components react with the added acid or base to minimize pH fluctuations.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a simplified way to calculate the pH of a buffer solution. It relates the pH to the pKa of the acid and the ratio of the concentrations (or moles) of the conjugate base to the acid, making it an essential tool for buffer calculations.

Transcript

00:01 Take a look at a buffer solution and calculating the ph based on some given information.

00:06 It's a propanoic acid and propanoate buffer solution.

00:13 We're going to use the henderson -hasselbach equation.

00:16 It basically looks like this.

00:17 It says we can calculate the ph of a buffer solution.

00:20 If we know the pca of the acid, we know the ratio of the conjugate base to the acid that's used in creating the buffer solution.

00:32 They have all of that information here.

00:34 The only problem is they provided for us the ca value rather than the pca value for the acid.

00:41 But we know this, that the pca is minus the log of the ca value.

00:48 So if we utilize our calculator here and take the log of 1 .4 times 10 and negative 5, we get negative 4 .085, but we need to take the negative of that value, and the pca then is 4 .85 here for that propanoic acid.

01:09 So let's plug in the information that we have now to calculate the ph of this buffer solution.

01:15 4 .85 for the pca and the concentration of the base they told us is 0 .30 molar, the acid 0 .20 molar.

01:25 All right.

01:28 So this is basically a 3 to 2 ratio.

01:32 So if we take the log of 3 over 2, that should help us find sort of the adjustment factor we need to determine the ph of this buffer.

01:46 So when we take that log, we get a value of 0 .18.

01:49 If we add that to the pca, we end up getting a ph of our buffer solution of 5 .03.

01:58 So that's part a of the question to determine the ph of the buffer solution.

02:03 Makes sense that it's a little greater than the pca value because we have a concentration in the buffer solution of a little more base than we have acid, so that's going to make for a little bit higher ph buffer solution than the pca value that we have.

02:25 Now we add a very small concentration of, or small volume rather, of concentrated acid, 1 -millimeter of 0 .1 -molar, hydrochloric acid to 10 millyte.

02:37 Liters of the buffer solution.

02:39 We're asked to calculate how the ph changes here.

02:43 And since we're dealing with very small volumes, it's oftentimes a little bit easier to think about this in terms of instead of moles per liter, formularity, millimoles per milliliter.

02:57 And so these two concentrations, first the acid at 0 .20molar can be thought of as 0 .20 millimoles per milliliter.

03:08 Same thing with the base, 0 .30 millimoles per mill liter.

03:12 And if we take a 10 -mill sample of the buffer solution, as we're told that we do in part b, that means that we have 2 -molmols of acid, 3 -mills of base.

03:27 To that, we are going to add 0 .10 millimoles of basically hydrogen ion from the hydrochloric acid.

03:37 And so that's going to impact both the concentration of the acid and base components in my buffer.

03:44 For the acid component, we're now going to have an additional 0 .10 millingoles of hydrogen ion to increase its concentration to 2 .1 molar, or, well, we'll say 2 .1 millimoles of acid that we now have available.

04:05 For the base, we're going to react away 0 .1 millimoles as it reacts with the hydrogen ion, so it's going to decrease its concentration to 2 .9 millimoles.

04:20 And so we're going to compare the ratio now instead of the 3 to 2 ratio, 2 .9 to 2 .1, and see how that impacts.

04:29 Again, the same pca value...

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