Experiment One: Standardization Standardization of Hydrochloric Acid
Objective To determine the concentration of hydrochloric acid (HCL) (by measuring the volumes of it) using sodium carbonate (Na 2CO3) as the primary standard in volumetric analysis, analysis, using the method of acid-base titration.
Theoretical Principles behind Titration In this acid-base titration, we are trying to determine the concentration of hydrochloric acid by using a base sodium carbonate. It involves reacting a known volume of a solution (HCL) with a standard solution (Na 2CO3). The whole experiment was divided into two parts; firstly, we need to prepare a standard solution of sodium carbonate, while second, titrate sodium carbonate solution against hydrochloric acid solution. A. Preparation of Standard Sodium Carbonate Solution The standard solution is obtained by dissolving a known mass of anhydrous sodium carbonate powder in deionized water. Later than, continues add deionized water into a 250 cm3 volumetric flask until the meniscus just touches the graduation mark. The equation is following: Na2CO3 (s) + H2O (l) --→ 2Na+(aq) + CO32-(aq) + H3O +(aq) + OH-(aq) The mass and the relative molecular mass of anhydrous sodium carbonate are known. Thus we can calculate the number of moles of sodium carbonate added to the standard solution. The expression is following: Number of moles of Na2CO3 = actual mass added /relative molecular mass Then Then,, we can can calc calcul ulat ate e the the mora morali lity ty of sodi sodium um carb carbon onat ate. e. The The expr expres essi sion on is following: Concentration of Na2CO3 = no. of moles of Na2CO3 / volume of solution
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B. Determination of the concentration of hydrochloric acid by acid-base titration The concentration of hydrochloric acid will be obtained by reacting the standard sodium carbonate solution with hydrochloric acid solution. Sodium carbonate solution is titrated against hydrochloric acid. We need to add dew drops of methyl orange indicator to show the equivalence point which shows the neutralization has completed and the exact amount that has been consumed. After that, record the volume of hydrochloric acid used in the titration. The expression is following: Na2CO3 (aq) + 2HCl (aq) ─→ 2NaCl (aq) + CO2 (g) + H2O (l) As the mole ratio of the compounds is known, we can finally calculate the concentration of hydrochloric acid. The expression is following: Number of moles of HCl = 2 x Number of moles of Na2CO3 Concentration of HCl = no. of moles of HCl / volume of solution
Chemicals Anhydrous sodium carbonate Hydrochloric acid
1.062 g About 150 cm3
Methyl orange indicator
8 drops
Apparatus •
10 cm3 pipette
1 piece
•
50 cm3 burette
1 piece
•
100 cm3 beaker
•
250 cm3 beaker
•
100 cm3 volumetric flask
•
100 cm3 conical flask
•
filter funnel
•
glass rod
1 piece
•
stand and clamp
1 piece
•
white tile
•
washing bottle
•
pipette filler
2 pieces 1 pieces 1 piece 4 pieces 1 piece
1 piece 1 piece 1 piece
2
*
Procedure
A. Preparation of Standard Sodium Carbonate Solution A1. The required mass (1.06g) of anhydrous sodium carbonate powder was calculated. A2. A 100 cm3 beaker was weighed by electronic balance and then set zero. A3. Anhydrous sodium carbonate powder was added to the beaker from the container until an approximately mass had reached. A4. Small amount of deionized water was added to the beaker and stirred with glass rod until all the solid had dissolved completely. A5. The sodium carbonate solution was poured to a 100 cm3 volumetric flask using a filter funnel. A6. The beaker was washed by deionized water and the washed solution was added into the volumetric flask. This step was repeated for 3 times. A7. The filter funnel and glass rod were washed by deionized water and the washed solution was added into the volumetric flask. This step was repeated for 3 times. A8. Deionized water was added to the volumetric flask until the meniscus just touched the graduated mark. A9. The volumetric flask was stopped and then was turned upside down and shook several times. B. Using the Standard Sodium Carbonate Solution to titrate hydrochloric acid B1. The following titration set-up was constructed as shown in the diagram. B2. 10 cm3 standard sodium carbonate solution was transferred to a clean 100 cm 3 conical flask by using a 10 cm 3 pipette. B3. 2 drops of methyl orange indicator were added to the conical flask. B4. The initial burette reading was recorded. B5. 10 cm3 standard sodium carbonate solution was titrated against hydrochloric acid while continuously swirling until the color of solution was just changed from yellow to orange. B6. The final burette reading was recorded. B7. Steps 2 to 6 in this titration process were repeated for 3 more times.
Collected Data, Observation and Relevant Results
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2nd
3rd
4th
Final burette reading (cm 3)
29.35
22.60
40.85
Initial burette reading (cm 3)
11.00
04.15
22.60
Volume of hydrochloric acid reacted(cm 3)
18.35
18.45
18.25
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Calculations A. Preparation of Standard Sodium Carbonate Solution Data collected from Data book, Relative molecular masses: C=12.0 Na=23.0
O=16.0
Mass of anhydrous sodium carbonate used =1.062g Number of mole of Na 2CO3 (s) used = actual mass added/ relative molecular mass = 1.062 / (23.0×2 + 12.0 + 16.0×3) = 0.0100 mol (corr. to 3 sig. fig.) Concentration of Na2CO3 (aq) = no. of mole / volume = [1.062 / (23.0×2 + 12.0 + 16.0×3)] / 0.1 =0.100 mol dm- 3 (corr. to 3 sig. fig.) Concentration of standard solution of sodium carbonate is 0.100 mol dm - 3
∴
B. Using the Standard Sodium Carbonate Solution to titrate hydrochloric acid Average volume of hydrochloric acid reacted = 18.35 cm 3 Equation of the reaction : Na2CO3 (aq) + 2HCl (aq) ─→ 2NaCl (aq) + H2O (l) + CO2 (g) According to the equation, 1mole of Na 2CO3 react with 2 moles of HCl and the mole ratio of Na2CO3 to HCl is 1:2. Number of mole of HCl solution required = no of mole of Na 2CO3 × 2 = 1.062 / (23.0×2 + 12.0 + 16.0×3) × 2 = 0.0200 mol (corr. to 3 sig. fig.) Concentration of HCl solution = no. of mole / Volume = 0.020037735 / 0.018.35 x10/100 = 0.109 mol dm- 3(corr. to 3 sig. fig.) Concentration of HCl solution is 0.109 mol dm - 3
∴
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Discussion Safety measures Generally, the chemicals used in this experiment were quite safe to use. However, there was still being l some hazards. Thus, safety goggle should be worn for protection. Hydrochloric acid is corrosive and we should handle the chemicals with care. Precautions 1.
All the apparatus of the experiment had to be washed correctly before used.
2.
Anhydrous sodium carbonate was transferred to the weighing bottle from desiccators to prevent the impurities and water vapor from contaminating the solid.
3.
The chemicals might be adhered to the inner surface of the containers. After the rinsing was done, we needed to wash it again with deionized water clear fully to completely clean it.
4.
When transferring solutions, it must be checked that there should not be any air bubble trapped in the pipette or burette.
5.
When taking readings, the bottom of the meniscus should just touch the line and we should keep our eyes on the same level as the water level.
6.
The burette was set up vertically.
7.
The tip of the burette should be brought closer to the solution in the conical flask, to prevent excessive slashing of solution all over the sides of the conical flask.
8.
A white tile placed under the conical flask, to facilitate detection of end-point.
Source of experimental errors -Errors due to the manipulation of apparatus Although everything was done with care, it was unavoidable to have errors in the manipulation of apparatus. There might be some contamination of chemicals during the experiment. The chemicals might be adhered to the inner surface of the containers and not reacted. Also, the reading taking might not be very accurate as we were easily read the scale wrongly such as weighting and burette. The reading takings and calculations were rounded up to the second decimal place only. Moreover, traces of chemicals involved in the experiment might be left in the containers previously very easily since it involved the titration of water. Finally, there were limitations about the precision of the apparatus. Therefore, errors resulted and more significant errors would occur when the low concentration of chemicals involved. 6
-Errors in the determination of proper end-point
Actually, these factors were the most significant in affecting the accuracy and correctness if the experiment. The colour change of indicator was not dominant as at the end point, the color of changed from yellow to orange was similar. It was easy for us to add excess drop of the acid although the end-point was reached. -Errors due to reactants or products Anhydrous sodium carbonate (Na 2CO3) was a very good primary standard because it was pure, stable and relatively non-hygroscopic. But there might be absorption of water in air in the anhydrous sodium carbonate since it took out from the desiccator for a long time. Therefore, the weight could not be ensure and not accurate. The calculations would be affected.
Second, sodium carbonate solution was titrated against hydrochloric acid solution and formed the equation below: Na2CO3 (aq) + 2HCl (aq) ─→ 2NaCl (aq) + H2O (l) + CO2 (g) CO2 (aq) + H2O (l) ─→ H2CO3 (aq) We know that carbon dioxide was produced during the neutralization and carbon dioxide was dissoluble in water which would affect the acidity of the reacting mixture. Since the pH valve was slightly decreased, the end point would be affected. Thus, the result was not accurate. Conclusion In this experiment, the concentration of hydrochloric acid (HCL) was determined which was nearly 0,109M and the average volume of hydrochloric acid reacted was18.35 cm3.
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