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Analysis of the Nitrogen Content of Lawn Fertiliser

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CHEMISTRY INVESTIGATION Analysis of the Nitrogen Content of Lawn Fertiliser Research Question: What is the percentage by mass of nitrogen (from the ammonium ion NH4+) in Bunnings Green Up brand of lawn fertilizer compared to the manufacturer's specification of 12.3%? Aim: To determine the percentage by mass of available nitrogen, present as the ammonium ion, in Bunnings Green Up brand of lawn fertiliser, then comparing it with the one specified by the manufacturer. Hypothesis: Since the ammonium ion (NH4+) present in fertilisers is a weak acid, it is hard to obtain an exact end point. Hence, back titration can be used to determine the percentage by mass composition of nitrogen in the fertiliser. To deduce the amount of nitrogen in ammonia, the NH4+ ion is reacted with an excess of NaOH, and the amount of unreacted NaOH found by titration with HCl is used to deduce the amount of NH4+ ion present by subtracting it from the initial amount of NaOH to find the amount that reacted with it. From this, the percentage by mass of nitrogen present in the fertiliser can be found and subsequently compared to the manufacturer's specification. It is hypothesized that the percentage by mass of nitrogen present in the fertiliser will be similar or close to that specified by the manufacturer (12.3%). ...read more.


reacted = 9.58 � 10-4 mol 4. 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 5. 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 6. 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. ...read more.


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%. ...read more.

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