A buffer that contains 0.40M of a base, B, and 0.25M of its conjugate acid, B H^+, has a pH of 8

A buffer that contains 0.40M of a base, B, and 0.25M of its...

Key Concepts

Mole Balance and Concentration Adjustments

Accounting for the changes in moles due to the addition of a strong acid involves setting up a mole balance. This means calculating the moles of the buffer components before and after the reaction, and then adjusting for the final solution volume to find the new concentrations. These adjustments are necessary for accurately applying the Henderson–Hasselbalch equation to determine the new pH.

Reaction of Strong Acids with Buffer Components

When a strong acid like HCl is added to a buffer system containing a weak base and its conjugate acid, the added acid reacts primarily with the weak base, converting it into its conjugate acid. This reaction alters the relative concentrations of the buffer components, hence shifting the buffer equilibrium and leading to a change in pH.

Buffer Systems

A buffer system is a solution that contains both a weak acid and its conjugate base or a weak base and its conjugate acid. These solutions are capable of resisting significant changes in pH when small amounts of acid or base are added, due to the equilibrium established between the acid/base pair.

Henderson–Hasselbalch Equation

The Henderson–Hasselbalch equation provides a useful way to estimate the pH of a buffer solution. It relates the pH to the pKa (or pKb) and the logarithm of the ratio of the concentrations of the buffer components. This equation is fundamental in understanding how changes in the ratio of conjugate base to acid influence the pH.

Transcript

00:01 This problem is a bit of a long one, but i've written down a few things here.

00:07 So the ph is given as, of the buffer is given as 8 .88.

00:13 The base is point, the concentration is 0 .4 molar.

00:18 And the concentration of the conjugate acid is 0 .25 molar.

00:23 And the volume that they've given us is 0 .25 liters.

00:27 And we're going to use that information throughout this whole problem.

00:32 So, first we need to find the pca, and we're given the p8, the ph of 8 .8, and we're given the concentrations.

00:46 So we can plug these in, so that's the wrong number.

00:56 So 0 .4 molar over 0 .25 molar.

01:01 Okay? and so we rearrange the equation.

01:07 So we'll read pca equals 8 .88.

01:16 And when you take that log of that number, then it's subtracting 0 .204.

01:26 And so now we just subtract that, and we find that the pga equals 8 .04.

01:36 6 -76.

01:39 Okay? and we're going to use that later.

01:46 So we need to remember that.

01:48 Next, we need to figure out the initial moles so that we can figure out how many moles were added to it and what that will end up being.

02:07 So all we do to figure that out is we take the volume, times the concentration of both the acid and the base.

02:15 So for the acid, it's 0 .25 liters times 0 .25 moles per liter.

02:29 And as you can tell, the liters will cancel out there pretty easily like that.

02:35 And the answer we get is 0 .0625 moles.

02:41 Now for the base, all we do, is we take the initial volume, which is 0 .25 liters.

02:50 We multiply it by the concentration, which is 0 .4 moles per liter.

03:01 Again, the liters cancel out, and the answer we get is 0 .1 mole.

03:09 Okay? so, then we take these answers, and we move here.

03:15 When the hydrochloric acid is added, this is the equation.

03:21 And so essentially, what is happening is you add the acid and it goes into the base component of the buffer.

03:29 And so you subtract, as i've indicated here, this is the base, the b...

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