buffer\'s ability to resist pHd ge will be destroyed (los) if you swamp Note tha
ID: 527864 • Letter: B
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buffer's ability to resist pHd ge will be destroyed (los) if you swamp Note that a the system. That is, ifyou add more acid than you have initial base present or you add more base than you have initial acid present, you force the weak acid equi completely to one side or the other. This shows up in the calculations as follows: negative number which indicates an excess OH of OH is present in the system. LA IH sa negative number which indicates an excess of H is present in the system In such a case, you can determine the final pH of the solution from the excess IH or excess [OHI as appropriate. F Dilution Calculations When you are mixing two or more solutions together, you cannot ignore di tion factors in the resulting mixture. In such cases, you need to first consider the dilution factor to determine the "initial" concentrations in the mixture before you consider changes in molarity as a result of any reaction occurring. Remember, for dilution calculations you use the simple relationship M, V where M, and VI are the original molarity and volume of a reagent added to the mixture and M, and V, are the final molarity and volume of the reagent in the mixture It is the M e for each reagent that needs to be used as the "ini- tial" concentration in any of the equilibrium calculations done on the resulting mixture. IV EXPERIMENTAL PROCEDURE Wear gloves during all parts of the procedure. The pH of solutions will be determined by using a pH meter with an attached elec trode. Extra care must be taken with the clectrode of the pH meter, as the tip is a very fragile glass bulb (and electrodes are very expensive). The tip of the electrode should be kept submerged in a 600 mL beaker filled with DI water when the meter is not in use. When testing solutions, the electrode needs to be immersed approximately inch in the solution being tested, up to the horizontal line marked on the electrode. The pH meter itself should be plugged in, turned on, and allowed to warm up for about 10 minutes. In general, pH meters need to be calibrated before they will give correct of pH. Using specific directions supplied by your instructor, you will calibrate your pH meter by sequentially immersing the electrode into two standardExplanation / Answer
Answer to part A
HCl is a strong acid. It readily dissociates as H+ and Cl- ions.
Hence [H+] = [HCl] = 0.1 M
pH = -log [H+]
= -log 0.1
= 1
Measured pH = 1.37
When this HCl is diluted, [H+] = [HCl] = 0.01 M
pH = -log [H+]
= -log 0.01
= 2
Measured pH = 2.14
Consider dissociation of a weak acid
HC2H3O2 <-------> H+ + C2H3O2-
Ka = [H+] [C2H3O2-] / [HC2H3O2]
[HC2H3O2] = 0.1 M
Initially only [HC2H3O2] is present. The change in the [H+] can be calculated as:
[H+]
[C2H3O2-]
[HC2H3O2]
Before dissociation
0
0
0.1 M
Dissociation
+x
+x
-x
At Equilibrium
x
+x
0.1-x
Since Ka is very small quantity 0.1-x = 0.1
1.8 X 10-5 = x (x) / 0.1
x2= 1.8 X 10-6
x = 1.8 X 10-6
= 0.00134 M
[H+]= 0.00134 M
pH = -log 0.00134 = 2.87
Measured pH = 3.04
When the weak acid is diluted, the calculations are as follows:
[HC2H3O2] = molarity X volume of acid / total volume of the solution
= 0.1 M X 1 mL / 10 mL
= 0.01 M
Initially only [HC2H3O2] is present. The change in the [H+] can be calculated using ICE table
[H+]
[C2H3O2-]
[HC2H3O2]
Before dissociation
0
0
0.01 M
Dissociation
+x
+x
-x
At Equilibrium
x
+x
0.01-x
Since Ka is very small quantity 0.01-x = 0.01
1.8 X 10-5 = x (x) / 0.01
x2= 1.8 X 10-7
x = 1.8 X 10-7
= 4.24 X 10-4 M
[H+]= 4.24 X 10-4 M
pH = -log 4.24 X 10-4 = 3.37
Measured pH = 3.38
The pH of acetic acid is higher than that of HCl. This is because acetic acid is a weak acid while HCl is a strong acid. We can see that the theoretical pH of HCl changes from 1 to 2 upon diluting it from 0.1 M to 0.01 M. While the theoretical pH of HC2H3O2 changes from 2.87 to 3.38 upon diluting it from 0.1 M to 0.01 M. Thus ideally the change in pH for HCl is 1 upon 10 times dilution while it is only 0.51 for HC2H3O2. Thus the pH change of HC2H3O2 was lower than that of HCl.
[H+]
[C2H3O2-]
[HC2H3O2]
Before dissociation
0
0
0.1 M
Dissociation
+x
+x
-x
At Equilibrium
x
+x
0.1-x
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