Now you get the chance to make a buffer solution. You get to decide what set of
ID: 1068130 • Letter: N
Question
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 pH ONCE so make sure you have done everything correctly. Your lab instructor will give you the values 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, NH_4^+, 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 pH meter good to three decimal places. We just want to make sure you use good analytical techniques like using pipits instead of graduated cylinders.) You get to decide which solutions will allow' you to make that buffer. You will have available to you bodies of 1.00 M solution of acetic acid, formic acid, ammonia, hydrochloric acid, sodium hydroxide, sodium format, sodium acetate and ammonium chloride. Use the following equilibrium constants: K_a for acetic acid = 1, 76 times 10^-5 K_a for formic acid = 1.77 times 10^-4 K_b for ammonia = 1.77 times 10^-5 Total concentration T he capacity of a buffer to tolerate addition of strong acid is related to the concentration of weak acid/conjugate base pair (or weak base/conjugate acid pair). Cone, of total acetate = Initial Cone, of sodium acetate + Initial Cone, of acetic acid OR Cone, of total formate - Initial Cone, of sodium formate + Initial Cone, of formic acid OR Cone, of total NH_3- Initial Cone, of ammonium chloride + Initial Cone, of ammonia PLEASE MAKE SURE YOU TAKE SOLID CHEMICALS OU T 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 glass 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 see your options. You also have bursts.Explanation / Answer
for
pH = 4.7
pKa = 4.75
Acetate = 0.140
so..
Acetic acid concentration = ?
pH = pKa + log(A-/HA)
4.7 = 4.75 + log(0.14 / HA)
0.14 / [HA] = 10^(4.70-4.75)
[HA] = 0.14/0.89125
[HA] = 0.15708 M of HA
You will need to add, to V = 1 liter (use a 1 liter flask)
mol of NaAc = MV = 0.14*1 = 0.14 mol of sodium acetate
mass of salt = 82.0343 *0.14 = 11.484 g of NaAc
mol of HAc = MV = 0.15708*1 = 0.15708 mol of acetic acid
mass of acid = 0.15708 *60 = 9.4248 g of acid
all in 1 liter
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