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PHYSICAL CHEMISTRY LAB MANUALEXPERIMENT: 1 CONDUCTOMETRIC TITRATION OF A STRONG ACID WITH A STONG BASE AIM: To determine the concentration of a strong acid by a conductometric titration using a strong base INTRODUCTION: The conductivity of an electrolytic solution depends on 1. ionic concentration and 2. Ionic mobility. The changes in the number and nature of the ions during an acid base titration, which affect the total conductance of the solution, can be utilized for the determination of end point. Now let us consider an acid base titration in which 50.0 ml of 0.1 N hydrochloric acid is titrated with 1.0N sodium hydroxide solution. In aqueous solution a strong acid can be considered to be completely in ionized form. HCl + H 2 O H 3 O + + ClAlthough both chloride and hydrogen ions contribute to the total conductance of the solution, hydrogen ion has a major share due to its high mobility (349.8 mhos.cm 2.mol-1) when compared to chloride ion (76.4 mhos.cm 2.mol-1). When a small portion of a strong alkali
such as sodium hydroxide is added to the solution, the OH - ion of the alkali reacts with the hydrogen ion forming water. Further, a sodium ion is introduced in its place. It means addition of sodium hydroxide results in a replacement of high mobile proton with a low mobile sodium ion (50.1 mhos.cm 2.mol -1). This results in a steep decrease of conductance until the equivalence point is reached.
PHYSICAL CHEMISTRY LAB MANUAL
Corrected conductance, milli mhos
x VOLUME OF SODIUM HYDROXIDE, ml
After the equivalence point the excess of sodium hydroxide causes the conductance to rise a little bit slowly when compared to the decre ase before the equivalence point. Both sodium ions and hydroxyl ions
contribute to the conductance after the equivalence point. The major contribution, however, is due to OH-, which has an ionic mobility of 198.6. mhos.cm 2.mol-1. Therefore the titration curve contains two As conductance is sensitive to
lines intersecting at the end point.
dilution, a volume correction needs to be applied to the meter readings. The volume of the analyte solution goes on increasing with the addition of sodium hydroxide solution during the titration.. If C is the measured conductance, V is the initial volume of the analyte solution and v is the volume of sodium hydroxide added, then the corrected conductance is given by, Corrected conductance, C' = C. V+v V
The corrected conductance in milli ohms should be plotted against volume of the sodium hydroxide for detecting the end point of the titration. SOLUTIONS: 1. Sodium hydroxide solution: Prepare 250 ml of approximately 1.0 N Sodium hydroxide by dissolving around 10 gms of analytical grade sodium hydroxide (gm.equivalent weight=40.0)
PHYSICAL CHEMISTRY LAB MANUALin carbon dioxide free distilled/ deionized water or conductivity water. Use a rough balance for weighing sodium hydroxide. 2. Oxalic Acid (gm.equivalent weight = 63.03) solution:
Prepare 100 ml of 1.0 N oxalic acid solution by dissolving exactly 6.303 gms of analytical grade oxalic acid in distilled water and make it up to the mark. Calculate the strength of the oxalic acid solution using the actual weight of the substance transferred in to the flask if it is not exactly 6.303 gms. 3. Hydrochloric acid solution: Prepare 100.0 ml of approximately 1.0 N hydrochloric acid stock solution by diluting concentrated hydrochloric acid (11.4N). PROCEDURE: y Standardize sodium hydroxide solution using standard oxalic acid solution with phenolphthalein as indicator. y Also standardize the hydrochloric acid using the standard sodium hydroxide solution with phenolphthalein as ind icator. y Prepare 100 ml of 0.1N hydrochloric acid solution by the exact dilution of the stock solution. y Switch on the conductivity meter for at least one hour before taking any measurements. y Standardize the conductivity meter (using internal standard) following the instructions given in the Instruction Manual supplied by the manufacturer along with the Instrument. y Clean the conductivity cell thoroughly with distilled water and then with conductivity water. y Prepare the experimental solution by taking 25.0 ml each of 0.1N hydrochloric acid and conductivity water with a burette in to a 100 ml beaker. Mix the contents of the beakers thoroughly with a glass rod. y Insert the conductivity cell in to the experimental solution and note down the meter reading after sel ecting an appropriate range using the range switch of the meter.
PHYSICAL CHEMISTRY LAB MANUALy Fill the burette with the standardized solution of sodium hydroxide and titrate the experimental solution by adding 0.2 0.5 ml portions. y Note down the meter readings each time after thoroughly stirring the contents of the beaker. y Tabulate the results in the Table format shown below
Concentration of the titrant (Sodium hydroxide)= ________ N
Total initial volume of the Analyte solution, V= ________ ml
S.No Volume of Sodium Observed . 1 2 3 4 etc.hydroxide added, v ml Conductance in milli mhos,
Corrected conductance, C= C X (V+v)/V milli mhos
0.0 1.0 2.0 3.0
Draw a graph of corrected conductance versus volume of sodium hydroxide added.
Join the points linearly before and after the equivalence point and extend them to get the intersection point.
Note down the volume of the sodium hydroxide x corresponding to the intersection point (end point).
Calculate the concentration of hydrochloric acid using the formula V1.N1=V2.N2, where V1 is the volume of hydrochloric acid taken = 25.0 ml, N 1 is the concentration of H Cl to be determined, V 2 is the volume of sodium hydroxide at the end point = x ml and N2 is the strength of the standardized sodium hydroxide
PHYSICAL CHEMISTRY LAB MANUALEXPERIMENT: 2
CONDUCTOMETRIC TITRATION OF A MIXTURE OF STRONG ACID AND WEAK ACID WITH A STRONG BASE
AIM: To determine the concentrations of strong acid and weak acid in a mixture by a conductometric titration using a strong base.
When a mixture of strong acid and weak acid is taken, the strong acid exists almost completely in the ionic form as shown below. HCl + H 2 O H 3 O+ + ClWhereas as the weak acid such as acetic acid mostly exists in the un dissociated molecular form. In the presence of the strong acid, due to common ion effect, the dissociation is further suppressed and the molecule almost exists in the un dissociated molecular form. CH 3 COOH H 2 O CH3 COO- + H3 O+ Therefore, the initial conductivity of a mixture of strong acid and weak acid is only due to the ions of the strong acid. The initial
conductivity of the solution is therefore, high, due to the high mobility and abundance of the hydrogen ions in solution. When sodium hydroxide is added during the titration, these hydrogen ions are replaced with less mobile sodium ions resulting in a rapid decrease of conductance. This happens until all the strong acid is neutralized. Further addition of sodium hydroxide results in the formation of
Corrected conductance, milli mhos
x y VOLUME OF SODIUM HYDROXIDE, ml
PHYSICAL CHEMISTRY LAB MANUALsodium acetate due to the neutralization of acetic acid. As sodium acetate exists in ionic form, its formatio An raises the conductance of the solution slightly due to the low ionic mobilities of both Na + (50.1 mhos.cm 2.mol-1) and CH 3COO- (40.9 mhos.cm 2.mol-1) ions. This continues until all the acetic acid is neutralized. The excess alkali
added over and above this again raises the conductance but now with different and high rate due to the relatively high mobility of hydroxyl ions. The titration curve therefore contains three linear portions with two intersection points. The first intersection point x corresponds to the neutralization of the strong acid and the second at y corres ponds to the total of strong and weak acids. From the values of x and y the concentrations of both strong and weak acids in a mixture can be determined. SOLUTIONS: 1. Sodium hydroxide solution: Prepare 250 ml of approximately 1.0 N Sodium hydroxide by dissolving around 10 gms of analytical grade sodium hydroxide (gm.equivalent weight=40.0) in carbon dioxide free distilled/ deionized water or conductivity water. Use a rough balance for weighing sodium hydroxide. 2. Oxalic Acid (gm.equivalent weight = 63.03) solution: Prepare
100 ml of 1.0 N oxalic acid solution by dissolving exactly 6.303 gms of analytical grade oxalic acid in distilled water and make it up to the mark. Calculate the strength of the oxalic acid solution using the actual weight of the substance transferred in to the flask if it is not exactly 6.303 gms. 3. Hydrochloric acid solution: Prepare 100.0 ml of approximately 1.0 N hydrochloric acid stock solution by diluting concentrated hydrochloric acid (11.4N). 4. Acetic acid solution: Prepare 100 ml of stock solution of
approximately 1.0 N acetic acid using conductivity water. This
PHYSICAL CHEMISTRY LAB MANUALcan be done by the dilution of approximately 6.0 ml (use measuring jar) of glacial acetic acid (17.4N) to 100 ml using conductivity water.
PROCEDURE: 1. Standardize sodium hydroxide solution using standard oxalic acid solution with phenolphthalein as indicator. 2. Standardize the stock solutions of hydrochloric and acetic acid using the standardized sodium hydroxide solution. 3. Prepare 0.1 N solutions of both hydrochloric and acetic acid by exact dilution of the stock solutions. 4. Switch on the conductivity meter for at least one hour before taking any measurements. 5. Standardize the conductivity meter (using internal standard) following the instructions given in the Instruction Manual supplied by the manufacturer alo