A 250.0 mL buffer solution initially contains 0.025 mol of HCHO2 and 0.025 mol of NaCHO2 . In or

A 250.0 mL buffer solution initially contains 0.025 mol of...

Key Concepts

Acid Dissociation Constant (pKa)

The pKa is a quantitative measure of the strength of an acid in solution, representing the pH at which the acid is half dissociated. It is a critical parameter in buffer systems since the pKa of the weak acid determines the pH around which the buffer is most effective, and it is used in the Henderson–Hasselbalch equation to relate pH to the acid/base ratio.

Stoichiometric Calculations

Stoichiometry in the context of buffer adjustments involves calculating the change in moles of the weak acid and conjugate base upon the addition of a strong acid or base. By determining the number of moles of reagent needed to convert a portion of the weak acid to its conjugate base (or vice versa), one can predict the new concentration ratio and, subsequently, the new pH of the buffer solution.

Buffer Solution

A buffer solution is a mixture typically composed of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH upon the addition of small amounts of acid or base. The components are present in sufficient quantities to react with any added acid or base, thereby maintaining the overall pH of the solution.

Henderson–Hasselbalch Equation

The Henderson–Hasselbalch equation provides a mathematical relationship between the pH of a buffer solution, the pKa of the weak acid, and the ratio of the concentrations of the conjugate base to the acid. It is expressed as pH = pKa + log([Conjugate Base]/[Acid]). This equation is essential for calculating how adjustments in the amounts of acid or base will alter the pH of a buffer.

Acid–Base Neutralization Reaction

In a buffer system, the addition of a strong base such as NaOH or a strong acid like HCl results in a neutralization reaction with the weak acid or conjugate base, respectively. When a strong base is added, it reacts with the weak acid to produce additional conjugate base, thus shifting the ratio in the Henderson–Hasselbalch equation and increasing the pH of the buffer.

Transcript

00:01 So in this video we're going to be answering question 118 from chapter 17, which says a 250 milliter buffer solution is initially contained 0 .025 moles of formic acid and 0 .025 moles of sodium formate.

00:16 In order to adjust the buffer ph to 4 .10, should you add sodium hydroxide or hydrochloric acid to the mixture? what mass of the correct reagent should you add? so right off the back list, we know that this is a buffer solution.

00:32 So we can use the henderson -hasselback equation, which tells us that ph is equal to pga plus log of the ratio of base to acid.

00:40 So we can solve for that ratio of base to acid and then compare the current ratio to the desired ratio when we have that desired ph of 4 .10 and use that to determine whether or not we want to add acid or base to the solution.

00:57 So we need to solve the henderson -hauselbach equation for that ratio of base to acids.

01:02 So we go ahead and move the p .c .a.

01:04 To the left -hand side of the equation and flip the fraction in the logarithm, which makes it negative.

01:09 And we have ph -mix -a -equ -a -equ -a -equ -a -h -h -over -a -minas.

01:14 Then we take 10 to the negative of both sides of the equation.

01:17 So we have 10 to the p -k -a -m -h equals a -m -m -m -1.

01:24 And then we just go ahead and flip the.

01:26 That fraction since we want the ratio of base to acid.

01:29 And then we swap the ph and pga in the exponent because we flip that fraction.

01:35 So we have 10 to the ph minus pga equals the concentration of base divided by the concentration of acid.

01:43 And then since concentration is just amount divided by the total volume, since we're dividing by the total volume in both the numerator and the denominator, it's the same thing as the number of moles of base divided by the number of moles of acid.

01:56 So we don't need to worry about working with concentrations and volumes...

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