- 2 0 年前最佳解答
Aim: Volumetric analysis of the concentration of cloudy ammonia in a sample claimed to have 50g/L of ammonia as NH4OH.
In chemistry, titration is commonly used as a method of determining the concentration of a base or an acid. This is done by neutralisation with the concentration of one of the acid or base known accurately. This solution is called the standard solution. A primary standard (in this experiment, Na2CO3) is a substance of a known high degree of purity that undergoes one invariable reaction with the other reactant of interest, which is the secondary standard. Standard solution (hydrochloric acid in this experiment) is hence a secondary standard. In this experiment, anhydrous sodium carbonate (NaCO3) was made into a solution with known molarity. Due to this reason, sodium hydroxide, NaOH is not a suitable chemical to use as primary standard as it absorbs moisture from the atmosphere, making its weight uncertain, and also it reacts with carbon dioxide in the atmosphere to form sodium carbonate, thus there would be doubts regarding its purity. Anhydrous sodium carbonate is a suitable primary standard as it has a high degree of purity, and has relative high molar mass to minimise the error when measured. However, it was kept in a desiccator as it is efflorescent, which means that it may react with water to form the hydrated form of Na2CO3 10H2O.
Known volume of sodium carbonate solution was reacted with hydrochloric acid until the reaction had just completed. This is represented in the equation: Na2CO3 (aq)+ 2HCl (aq) 2NaCl(aq) + H2O (l) + CO2 (g) . The volume of the hydrochloric acid recorded, hence the molarity of the acid was found, that is the acid was standardised. The hydrochloric acid with known concentration could be used to neutralise. Similar process was used to neutralise the cloudy ammonia: HCl (aq) + NH4OH (aq) NH4Cl (aq). + H2O (l) .
Methyl orange indicator was used as in neutralisation; the solutions and products are usually colourless, making it difficult to determine when the reaction had fully completed. When the reaction has just finished the methyl orange indicator would change from yellow to salmon pink. At the point where the reaction had just completed, is called the equivalence point, the determination of this point is critical for titration. However, in real life, it is impossible to determine exactly when the solution has reached the equivalence point. The point at which the colour of the indicator changes is called the end point, however, at this point the solution is either slightly excess is added as the indicators themselves are usually weak acids or bases. Therefore the end point is an approximation to the equivalence point.
300mL conical flasks
Anhydrous Sodium Carbonate
250mL Volumetric Flask
Methyl Orange Indicator
Cloudy Ammonia Sample
Preperation of Sodium Carbonate Solution
1.1.33 gram of sodium carbonate was weighed and was put into the 100mL beaker.
2.Some distilled water was added to the beaker until all sodium carbonate was dissolved.
3.This solution was transferred to the 250mL volumetric flask through the funnel.
4.The funnel and the beaker were washed several times with distilled water and those washings were added to the volumetric flask.
5.Distilled water was added until approximately half of the volumetric flask was filled up.
6.The volumetric flask was swirled to ensure that all sodium carbonate was dissolved.
7.The volumetric flask was filled to the mark 250mL mark by distilled water, and was swirled thoroughly, to ensure that the concentration in uniform throughout solution.
Standardisation of Hydrochloric Acid
8.The burette was rinsed with a small portion of hydrochloric acid, and was then filled up by hydrochloric acid.
9.The burette was setup on the burette stand.
10.The 20mL pipette was rinsed with some sodium carbonate solution.
11.20,00mL of sodium carbonate was pipetted into a 300mL conical flask.
12.3 drops of methyl orange indicator was added to the sodium carbonate solution in the flask.
13.A white tile was placed under the conical flask, allowing observer to determine the end point at greater ease.
14.The volume reading in the burette was recorded, and the tap of the burette was turned on, allowing only drops of acid to drip whilst the conical flask was swirled thoroughly, until the sodium carbonate solution became salmon pink.
15.The volume reading of hydrochloric acid in the burette was taken at the end of titration, and was recorded.
16.Step 14 to 15 was repeated for three times and results were taken.
Analysis of Cloudy Ammonia
17.A 250mL volumetric flask was filled approximately two thirds with distilled water.
18.The 10mL pipette was rinsed with a portion of cloudy ammonia sample, and 10.0mL of the sample was pipetted into the 250mL volumetric flask.
19.The volumetric flask was filled to the 250mL mark by distilled water.
20.The volumetric flask was swirled thoroughly, and 20.0mL of this solution was pipetted into a 300mL conical flask.
21.Three drops of methyl orange indicator was added to the solution in the conical flask and a white tile was placed under the conical flask.
22.The volume reading in the burette was recorded, and the tap of the burette was turned on, allowing only drops of acid to drip whilst the conical flask was swirled thoroughly, until the sodium carbonate solution became salmon pink.
23.The volume reading of hydrochloric acid in the burette was taken at the end of titration, and was recorded.
24.Steps 22 and 23 were repeated for three times, and all the results were collected and recorded.
Preparing Sodium Carbonate solution:
100mL Beaker + sodium carbonate 28.89
The concentration in molar of sodium carbonate can be calculated:
n Sodium Carbonate = = = 0.0125mol
Molarity = = =0.0502 mol/L = 0.0502M
Standardisation of Hydrochloric Acid:
First AttemptSecond AttemptThird Attempt
Reading of HCl in burette before titration (mL)169.904.20
Reading of HCl in burette after titration (mL)36.1031.7024.25
Volume of HCl used (mL)20.1021.8020.30
From chemical equation:
Na2CO3 (aq)+ 2HCl (aq) 2NaCl(aq) + H2O (l) + CO2 (g)
The mole ratio of sodium carbonate to hydrochloric acid is 1:2.
Therefore, n Hydrochloric acid = 2 n Sodium Carbonate
n Hydrochloric acid = 2 C V
= 2 0.0502 0.02 (20mL of Na2CO3 was used)
In the table, the second attempt is not considered as the colour of the solution was pinker than desired. The average volume is then = = 0.0202 L
Molarity = = = 0.0994M 0.1M
Analysis of Cloudy Ammonia:
First AttemptSecond AttemptThird Attempt
Reading of HCl in burette before titration (mL)5.1214.5523.40
Reading of HCl in burette after titration (mL)13.823.1531.85
Volume of HCl used (mL)8.688.608.45
The average volume of hydrochloric acid used is
From the chemical equation: HCl (aq) + NH4OH (aq) NH4Cl (aq). + H2O (l) ..The mole ratio of ammonia to hydrochloric acid is 1:1
Therefore, n Ammonia = n Hydrochloric acid = CV
= 0.1 0.00858
= 0. 00853 mol (in the 20mL solution of conical flask)
= 0. 0426M (the concentration in 250mL volumetric flask)
n = CV
= 0.0426 0.25
= 0. 0107 mol (No. of moles of NH4OH in 250mL volumetric flask)
= 1. 07 M (concentration of the original cloudy ammonia sample)
1.07M = 1.07 mol /Litre
Mass (g) = n Molar Mass
= 1.07 (14.01 + 1.01 4 + 16.00 + 1.01)
1.07mol / Litre = 37. 52 g / L參考資料： 我的化學報告..NH4OH在清潔季禮的濃度
- wowsocheapLv 61 0 年前