Neutralisation refers to the process whereby an acid reacts with a base in stoichiometric proportions to each other to form a salt and water. In this experiment, the neutralisation reaction can be summarized by the following equation:HCl (aq) + NH4OH (aq) NH4Cl (aq) + H2O (l)In this prac, the primary standard is Na2CO3. Primary standards are substances that possess certain properties (i.e. it is soluble), which enable it to be made up into a solution of a known concentration with distilled water to high degree of accuracy. A secondary standard on the other hand, is any solution that has an accurately known concentration. In this experiment, HCl acid is the secondary standard.
The standardising of HCl can be summarized by the following equation:2HCl (aq) + Na2CO3 (aq) 2NaCl(aq) + CO2 (g) + H2O (l)Since sodium carbonate is deliquescent, it is kept in a desiccator to prevent it from absorbing water vapour from the atmosphere. The equivalence point of a titration is the point where the reactants are present in stoichiometric proportions to each other. The end point of a titration is the stage at which the chosen indicator (methyl orange) changes colour to show that a reaction e.g.
neutralisation has occurred. In theory, the equivalence point should equal the end point.Materials:* 1x 10ml Pipette* 1x 20ml Pipette* 1x Burette* 2x 250ml Volumetric flask and stopper* 3x Conical/Titration flask* Sodium Carbonate (Na2CO3)* 1x Electronic balance* Distilled water* Hydrochloric Acid* Methyl orange indicator* 1x Clamp and stand* 10ml of cloudy ammonia* 1x Glass dropper* 1x 50ml beaker* 1x Glass funnel* 1x Plastic bottle and lid* 1x Spatula* 1x white tileProcedure:Part 1: Preparation of Na2CO3 Solution:1. The mass of the Na2CO3 required to prepare a 250ml solution of approximately 0.05M was calculated and recorded.
2. An empty beaker was weighed and the mass recorded in Table 1.1.3. The beaker was kept on the balance and the Na2CO3 added to the beaker until the required amount (as calculated in step 1) was reached. The mass of the beaker and Na2CO3 was recorded in Table 1.1.4.
The Na2CO3 was transferred into the volumetric flask. The ’empty’ beaker was then weighed again to ensure that all the Na2CO3 had been transferred.5.
Some distilled water was added to the volumetric flask to dissolve the Na2CO3.6. Once the Na2CO3 had dissolved, distilled water was added to the flask until the bottom of the meniscus reached the 250ml mark on the flask.Part 2: Standardisation of HCl:1. The correct rinsing procedures were used to rinse out all the glassware. (See Table 4.1).
2. 50ml of HCl acid was poured into the burette.3.
20ml of the Na2CO3 solution from part 1 was then transferred into a conical flask using a pipette.4. 2-3 drops of methyl orange indicator was added to the Na2CO3 solution.5. The HCl acid was titrated until the indicator change from yellow to ‘salmon pink’ (end point).
The volume of HCl used was then recorded in Table 2.2.6.
Steps 3-5 were repeated until two or three of the data obtained were within 0.1ml of each other.7. The results of the titration were then used to calculate the concentration of the HCl solution.Part 3: Analysis of ‘Cloudy Ammonia’:1.
The correct rinsing procedures were used to rinse out all the glassware. (See Table 4.1).2. A volumetric flask was filled with about two-thirds distilled water.
3. A 10.0ml sample of cloudy ammonia was transferred into the flask using a pipette. The flask was then filled up to the 250ml mark.4. 20.0ml of the diluted ammonia solution was then transferred into a titration flask and 2-3 drops of methyl orange indicator added.
5. The HCl acid was titrated with the ammonia sample until the indicator change from yellow to ‘salmon pink’ (end point). The volume of HCl used was recorded in Table 3.
2.6. Steps 4-5 were repeated until two or three of the data obtained were within 0.1ml of each other.8. The concentration of the original cloudy ammonia was then calculated.*N.
B. The burette was refilled with more HCl solution as needed.Results:Part 1: Preparation of Sodium Carbonate Solution:Calculations:The mass of Na2CO3 required to make up a solution of approx. 0.05M:Number of moles = Concentration Volume= 0.05 .
25= 0.0125 mol.Mass = number of moles molar mass? m(Na2CO3) === 1.325= 1.33 gTable 1.1: Mass of SubstancesSubstance:Mass: (g)Empty Beaker33.59Na2CO3 and Beaker34.92Na2CO31.
33Part 2: Standardisation of HCl:Table 2.1: Volume of NaCO3 SampleSample/ Titration Number:Volume (ml)120.0220.0320.
0Average volume of NaCO3 used: 20.0mlTable 2.2: Volume of HCl TitratedTitration Number:123Initial Reading (ml)0.017.133.0Final Reading (ml)17.
117.517.0Average Volume of Titrated HCl: 17.2mlCalculations:The equation: 2HCl (aq) + Na2CO3 (aq) 2NaCl(aq) + CO2 (g) + H2O (l)We know that the Na2CO3 solution is 0.05M and 20.0ml so:n(Na2CO3) = 0.05 0.
02= 1.0 10^-03 molesFrom the equation, the ratio of n(HCl) : n(Na2CO3) is 2:1? n(HCl) = 1.010^-03 2= 2.010^-03 molesConcentration (c) = Number of moles (n)Volume (L)? c(HCl) == 0.116MPart 3: Analysis of ‘Cloudy Ammonia’:Table 3.
1: Volume of ‘Cloudy Ammonia’ SampleSample/ Titration Number:Volume (ml)120.0220.0320.0Average volume of ‘Cloudy Ammonia’ used: 20.0mlTable 3.2: Volume of HCl TitratedTitration Number:123Initial Reading (ml)0.06.314.
2Final Reading (ml)7.914.222.2Volume (ml)7.97.98.
0Average Volume of HCl Titrated: 7.93mlCalculations:The equation: HCl (aq) + NH4OH (aq) NH4Cl (aq) + H2O (l)1. n(HCl) reacted = 0.116 0.00793= 9.
22510^-04 moles2. From the equation, the ratio of n(HCl) : n(NH4OH) is 1:1? n(NH4OH) in a 20.0ml sample = 9.
22510^-04 moles? n(NH3) in a 20.0ml sample = 9.22510^-04 moles3.
Concentration of (NH3) == 0.0461M? n(NH3) in a 250.0ml sample = 0.
0461 0.25= 0.01153 moles4. The dilution formula states that C1V1 = C2V2 or n1 = n2.? n(NH3) in a 10.0ml sample = n(NH3) in a 250.0ml sample= 0.01153 moles5.
c(NH3) in cloudy ammonia == 1.153M6. c(NH3) in cloudy ammonia =1.153M? n(NH3) in 1000ml =1.153 moles? m(NH4OH) in 1000 ml = 1.153 (14 + 4 + 16 + 1)= 40.36 gTherefore the concentration of ammonia as NH4OH in ‘cloudy ammonia’ is 40 g/L.
This is less than the manufacturer’s claim by 10 g/L. The reason for this difference may be because the bottle of ammonia used in the experiment had been opened for a long time and thus many of the ammonia had ‘escaped’ into the air as fumes.Discussion:1.
It is important to have the correct rinsing procedures as it minimises the chances of error. For example, if the burette was only rinsed with distilled water and not with the HCl acid afterwards in part 3, the solution in the burette will be diluted. Hence the consequent volume required to neutralise the ammonia will be higher than it should be (inaccurate).Another instance where incorrect rinsing procedures could affect the final result is the rinsing of the titration flask in part 3. The titration flask should only be rinsed out by distilled water and not by the ‘cloudy ammonia’ because the water left in the flask after rinsing would not affect the experiment.
(Dilution does not change the number of moles that is present in the ammonia solution.) Whereas if the flask is also rinsed with ammonia, the ammonia retained in the flask after rinsing would make the solution extra concentrated. This causes inaccuracies, as the volume of HCl needed to neutralise the ammonia would be less than normal.
Table 4.1 shows a list of the correct rinsing procedures for all the glassware used:Table 4.1: Rinsing Procedures of GlasswareEquipment:Rinsing Procedure:1. Volumetric flaskRinse only with distilled water.2. BuretteRinse with distilled water and then with the acid to be standardised or substance that will be transferred into the burette. (I.
e. HCl)3. PipetteRinse with distilled water and then with the substance that will be transferred into the pipette e.g. Na2CO3 in Part 2 and ‘cloudy ammonia’ in Part3.4. Titration FlaskRinse only with distilled water.
2. The ammonia must not be left unstoppered due to two major reasons. The first reason relates to the accuracy of the experiment. Ammonia is volatile; hence it will react easily with the surrounding air.
A stopper will keep losses of the ammonia to a minium, thus retaining the accurate number of moles of ammonia required in the titration process. Ammonia vapour may irritate the skin, eyes and respiratory system, thus the lab must be well ventilated, safety glasses worn and the ammonia stoppered to minimise the hazards.3. A few years ago, the bottles were labelled ‘2.5% w/w ammonia as NH3’, however the labels have now been changed to ‘5% w/w ammonia as NH4OH’. The change is probably due to the fact that the manufacturers have added water to their products. Although this causes the ammonia to be less concentrated, the new weight is heavier than the old weight by two times due to the fact that ammonia has been ionised by the water molecules.
This is best summarized in the equation:NH3 (aq) + H2O (l) NH4+(aq) + OH- (aq)It is possible that the manufacturers made such a change in their product for commercial purposes. To an ordinary consumer with no chemistry background, the change would probably mean to them that they are receiving more value for their money, whereas in actual fact, they are still getting the same amount of ammonia.Conclusion:In conclusion, it may be generalised that volumetric analysis is a fairly accurate method of analysing the amount of ammonia as NH4OH present in a sample of ‘cloudy ammonia’ and that the actual yield of ammonia will vary to the theoretical yield of the manufacturer due to various experimental inconsistencies and errors.