Data Collection:
Table 1: Titration of the excess NaOH (0.084 mol dm-3) against HCl using Methyl Red Indicator
*Colourless solution turned a shade of orange upon endpoint.
Data Processing and Presentation:
- Find the number of moles of hydrochloric acid
The reaction between hydrochloric acid and sodium hydroxide is given in this equation:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
c(HCl) = 0.1 mol dm-3
v(HCl) = 7.9 cm3 → 0.0079 dm3
n(HCl) = c × v
n(HCl) = 0.10 × 0.0079
n(HCl) = 7.9 × 10-4 mol
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Find the initial number of moles of sodium hydroxide.
c(NaOH) = 0.0874 mol dm-3
v(NaOH) = 20.0 cm3 → 0.020 dm3
Initial n(NaOH) = c × v
Initial n(NaOH) = 0.02 × 0.0874
Initial n(NaOH) = 1.748 × 10-3 mol in each 20 cm3 aliquot
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Find the reacted number of moles of sodium hydroxide in 20 cm3
The molar ratio for NaOH and HCl is 1:1
Therefore excess n(NaOH) = 7.9 × 10-4 mol
n(NaOH) reacted = initial n(NaOH) – excess n(NaOH)
n(NaOH) reacted = 1.748 × 10-3 – 7.9 × 10-4
n(NaOH) reacted = 9.58 × 10-4 mol
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The mole of ammonium in 20 cm3 aliquot and 250 cm3 using the reaction between ammonium and sodium hydroxide:
NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)
Molar ratio for NH4+ to NaOH is 1:1
Therefore, the mol of ammonium would be equal to that of sodium hydroxide:
n(NH4+) in 20 cm3 = 9.58 × 10-4 mol
n(NH4+) in 250 cm3 = 9.58 × 10-4 ×
n(NH4+) in 250 ml = 0.011975 mol
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Find the nitrogen content by mass in ammonium ion (NH4+).
Molar ratio for NH4+ to N is 1:1
Therefore, n (N) = 0.011975 mol
m(N) in NH4+ = n × Mr
(g) of N in NH4+ = 0.011975 × 14
m(N) in NH4+= 0.16765 g
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The percentage by mass of nitrogen found in fertiliser:
% by mass of N = m(N) in the Sample x 100
Sample mass
% by mass of N = × 100%
% by mass of N = 13.87% ± 1.00% found in the sample Bunnings Green Up fertiliser
The difference between the theoretical value (manufacturer’s specification) and the experimentally found value: 13.87% - 12.3% = 1.57%
The percentage uncertainty: × 100 = 7.21%
The percentage difference: × 100 = 12.76%
Since the % difference is greater than the % uncertainty, it is suggested that systematic errors are present in this investigation.
Conclusion/Evaluation:
Fertiliser is a natural or synthetic chemical substance or mixture that is used to enrich soil in order to promote plant growth. It influences grass colour, ability to recover from stress, and helps prevent weed invasions and diseases. Plants require more than a dozen different chemical elements but nitrogen, phosphorus, and potassium are the three major nutrients needed by lawns; Nitrogen is the nutrient required most as it promotes growth and keeps the grass green, although too much nitrogen can have harmful effects on plants and crops, and on soil quality. In this investigation, the percentage by mass of nitrogen from the ammonium ion in the sample fertiliser is being determined. The manufacturer’s claim of the amount of nitrogen present in this type of fertiliser is 12.3%.
The ammonium that is present in fertilisers is a weak acid, thus it is hard to identify a sharp end point in direct titrations with the naked eye. Back titration is used instead. It is an analytical chemistry technique in which the unknown concentration of a reactant can be found by reacting it with an excess volume of another reactant of known concentration. The resulting mixture is then titrated back, taking into account the molarity of the excess which was added. Before titration, the sodium hydroxide was reacted with ammonium in the fertiliser to produce ammonia and water:
NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)
The basic solution needed for titration was reached through heating the solution to remove the ammonium ions. By heating it, the rate of reaction between the ammonium ions and hydroxide ions increases, consequently producing water and ammonia. Since ammonia is an extremely volatile gas, boiling the solution will result the vaporization rate, which is the desired outcome. The ammonium ions are also the limiting reagent thus once completely reacted, ammonia production ceases. This is indicated when the red litmus paper, which tests for the presence of a base, no longer turns blue meaning the ammonia has completely evaporated and no longer present in the solution.
By titrating the excess, unreacted hydroxide ions with standard hydrochloric acid, it allows us to calculate the amount of unreacted hydroxide ions since the molar ratio is equal, and from this the amount of reacted hydroxide ions with the fertiliser solution can be deduced. Subsequently, the amount of ammonium ions initially present in the 250 cm3 volumetric flask can be found and then the mass of nitrogen ions can be calculated. From this, the percentage by mass of nitrogen in the sample of fertiliser can be found. It was found that the percentage by mass of 1.209 g of fertiliser was 13.87% (± 1.00%). This is very close to the percentage of nitrogen present in the fertiliser as specified by the manufacturer - 12.3%. This discrepancy may be due to several errors and limitations encountered throughout the experiment.
The percentage difference of 12.76% is greater than the percentage uncertainty of 6.34% and as a consequence random errors alone cannot alone account for the difference. There must be systematic errors inherent to the investigation or the apparatus. Measurement errors could be the main source of uncertainty. One source of error could be that the exact mass of the fertiliser sample as measured on the electronic balance is not the same as in the volumetric flask, consequently affecting the final result. The loss due to the transfer process into the flask which could be reduced by sufficiently rinsing the apparatus used to transfer, weigh and grind the fertiliser sample − the funnel, weighing bottle, spatula and mortar and pestle − so that all sample weighed is transferred into the volumetric flask.
Another possible error is that the meniscus of solutions measured was not sitting exactly on the calibration line in the volumetric flask when preparing the standard solution. This may affect the results as the expected volume of the standard solution would differ from the actual volume measured. This can be overcome by allowing more than one individual to test whether or not the meniscus is positioned on the etched mark, making sure it’s at eye level, which will effectively limit this parallax error.
The interpretation of the orange colour determined at the end point could also have been another error. Subjective judgment when determining the shade of orange is another factor which may have affected the titre volume, and thus the final outcome. This problem was exacerbated by the fact that the methyl red indicator does not provide a sharp and easily distinguishable end point, as the colour change from yellow to orange. This may be overcome by using another indicator that allows for a clearer colour contrast.
Another possible limitation which may have impacted the results is that some ammonia may still have been present in the conical flask. Although the red litmus paper may have served its purpose, this may still occur as a result of not boiling the solution long enough and thus some ammonia may not have evaporated. The result of this is that the ammonia in the solution will react with the hydrochloric acid, and therefore more hydrochloric acid will be required for the equivalence point to be reached, affecting the titre volume. This can be easily overcome by ensuring that the fertiliser solution containing water and sodium hydroxide is boiled for long enough so that all ammonia can evaporate, while simultaneously ensuring that the solution remains at a constant 90mL.
The use of tap water may also have affected the results. It may not have been at a pH of 7 or may contain other contaminants such as salts which may ultimately interfere with the results. To overcome this, the most effective way is to use deionised water.
Conclusion:
It was found that the percentage by mass of nitrogen from the ammonium ion in Bunnings Green Up brand of lawn fertilizer was 13.9% (± 1.00%). This was very close to the percentage of nitrogen in lawn fertiliser as specified by the manufacturer − 12.3%.
Bibliography:
1. Unknown Author,
2. Chemistry for the International Baccalaureate, 2005, IBID Press
Chemistry Investigation