System selected Acetic pH: 3.600 Total common ion concentration 0.150 M What siz
ID: 560110 • Letter: S
Question
System selected Acetic
pH: 3.600
Total common ion concentration 0.150 M
What size of glassware should I use to get up to 4 significant figures. 500mL flasks are the biggest we have and we cannot use more than 5.0g of solid reagent.
Experiment 25 Buffer Protocol (60 points) Now you get the chance to make a buffer solution. You get to decide what set of chemicals to use and how to combine them. You are only measuring the plH ONCE so make sure you have done everything correctly. Your lab instructor will give you the valucs of X and Y Your goal is to prepare 100.00 mL of a buffer with a pH-X. When you prepare this buffer you need to have a total concentration of the species other that H' (the common ion, NHL, Ac or HCOO) of Y M (as discussed in lecture on 4/5.) (Your X and Y have three significant figures although we don't have a pII meter good to three decimal places. We just want to make sure you nse gond analytical techniques like using pipets instead of graduated cylinders.) You get to decide which solutions will allow you to make that buffer. You will have available to you bottles of 1.00 M solution of acetic acid, formic acid, ammonia, hydrochloric acid, sodium hydroxide, sodiurm formate, sodium acetate and ammonium chloride. Use the following equilibrium constants Ka for acetic acid1.76 x 10 Ka for formic acid = 1.77 x 10-4 Kb for ammonia = 1.77 x 10 Total concentration weak acid/conjugate base pair (or weak base/conjugate acid pair). Conc of total acetate Initial Conc, of sodium acetateInitial Conc. of acetic acid Conc. of total formale Initial Conc. of sodium formate +Initial Conc. of formic acid Conc, of total NH3 Initial Cone, of ammonium chloridc +Initial Conc. of ammonia The capacity of a buffer to tolerate addition of strong acid is related to the concentration of OR OR PLEASE MAKE SURE YOU TAKE SOLID CHEMICALS OUT OF THE BOTTLES WITH CLEAN, dry spatulas. You should place a small amount in a clean dry beaker. The chemicals should not leave the hood. Do not return unused portions to the bottles. Look at the samples in beakers to get a rough idea of what 0.5 g of each of the solids looks like so that you can better gauge how much solid you need to take with you. Also remember to transfer solids into beakers outside of the balance. These solids can damage the balance. Also note that we have giass pipets that are graduated since some of the volumes you might need won't be pretty whole numbers. The lab instructor can bring in some of these so that you can sce your options. You also have burets. 97Explanation / Answer
if we need
pH = 3.60
then, for this acid, pKa = 4.75
pH = pKa + log(acetate/Acetic acid)
3.60 = 4.75 + log(A-/HA)
10^(3.60 - 4.75 ) = (A-/HA)
0.07079 = (A-/HA)
A- = 0.07079 *HA
also, we know total acid + acetate = 0.15
A- + HA = 0.150 M
0.07079 *HA + HA = 0.150 M
HA = 0.150/(1+0.07079) = 0.14018
then
A- = 0.07079 *HA =0.07079* 0.14018 = 0.009923
the basis:
500 mL buffer:
mol of A- required= M*V = 0.009923*0.5 = 0.00496 mol of acetate required
mol of HA required = MV = (0.14018)*0.5 = 0.07009 mol of acetic acid
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