8.4: Procedure
- Page ID
- 537579
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)For this experiment, you will be assigned to a small group that will be given the task of preparing a buffer solution of a specified pH and total buffer concentration. Your group will then calculate what quantities of reagents are needed to make the solution. Finally, your group will prepare the buffer solution and then measure its pH using a pH meter. Record the pH, the total buffer concentration, and the volume of the buffer solution assigned to your group.
pH = ____________ [buffer] = ____________ volume = ____________
You will have the following chemicals available to prepare your solution.
1.0 M HC2H3O2 solid NaC2H3O2
1.0 M NH3 solid NH4Cl
You do not need to use all these reagents to prepare your buffer solution. Which conjugate pair would be best to create a buffer for the desired pH? Each member of the group should do the calculations. (You may want to do your initial calculations on another piece of paper and then transfer them to your lab report.) When everyone has an answer, compare answers to see if you agree. If you do not agree, check to find any errors. Clearly and logically show your final calculations in the space below. Remember the [concentration acid] + [concentration base] = [concentration buffer].
_____ mL 1.0 M HC2H3O2 _____ g NaC2H3O2 _____ mL 1.0 M NH3 _____ g NH4Cl
After you have calculated the appropriate amounts of acid and conjugate base, prepare the buffer solution using the appropriate amounts of each reagent. Carefully add the appropriate quantity of each reagent to one beaker. Add a small quantity of deionized water to the beaker and stir to dissolve. Transfer the solution to a volumetric flask of appropriate size. Rinse the beaker and the stirring rod with small amounts of deionized water and transfer these to the volumetric flask. You may need to repeat the rinsing process several times. Fill the volumetric flask to the mark with deionized water. The final few milliliters of water are best added with a small dropping pipet. IF YOU FILL THE FLASK PAST THE MARK, YOU WILL HAVE TO DISCARD THE SOLUTION AND BEGIN AGAIN! Cover the mouth of the volumetric flask with Parafilm and, with your thumb over the Parafilm, invert the flask several times to thoroughly mix the solution.
Follow the steps on the pH meter handout instructions to standardize and use your pH meter. Always rinse the electrode with DI water and gently pat dry with a Kimwipe® between solutions. Put approximately 30 mL of each standardized solution in a 50 mL beaker. Using the standardized pH meter, measure the pH of your group’s buffer solution.
Measured pH of buffer = __________
Calculate the percent error for your buffer solution.
% error = 
% error = __________
The actual pH values of buffer solutions often differ slightly from the calculated values since the derivation of the Henderson-Hasselbalch equation involves several approximations. For this reason the final adjustment of the pH of a buffer solution is often made by adding a small amount of some strong acid or base. For your buffer solution, would you have to add HCl or NaOH to make the final pH adjustment?