DCP:
NaOH added to Fertiliser = 25mL ± 1mL ± 4%.
Concentration of Sodium Hydroxide:
NaOH + HCl ⇒ NaCl + H₂O
Concentration (NaOH) *volume (NaOH) = concentration (HCl) + Volume (HCl).
Concentration NaOH = 0.0175/0.01 = 1.75M.
Error: 1.75 ± 1.6% = 1.75 ± 0.028M = 1.7 ± 0.03M. NH₃
Formula of the reaction:
NH₄X + NaOH ⇒ NaX + NH₃ + H₂O.
Other possibilities with a non-mono-atomic X would face the same problem, being that the ratio of moles of Nitrogen to those of Sodium Hydroxide will still be maintained at 1:1.
Moles of NaOH un-used in initial reaction = cv = 1.75*0.01 = 0.0175 moles.
Moles in total to start with = cv = 1.75*0.025 = 0.0438 moles.
Used moles = 0.0438 – 0.0175 = 0.026 moles.
Mass = moles*mass of 1 mole = 0.026*14 = 0.364g.
Percentage by mass of Nitrogen in the Fertiliser = (0.364/2.085)*100 = 17.5%
Error = 1.6% + 2.4% + 0.6% + 4% = 8.6% ⇒ 0.364*(8.6/100) = 0.031g.
⇒ (0.031/2.085)*100 = ± 1.5%.
CE:
The Percentage by Mass of Nitrogen in the Fertiliser was 17.5% ± 1.5%. It was stated that the Fertiliser was 21.2% Nitrogen which is 2.1% off our result. This can be partially explained.
It is important to note that the starting weight of Fertiliser used in each case was varied. This means that there was no opportunity for a “rough” titration because we would only do a titration for a particular weight once. This required great precision and allowed a decent margin of error if not taken with patience. By simply making sure that the weight was 2.00g. each time, we could have largely simplified the process and made it a lot more accurate.
We made sure in all cases that the NH₄ was no-longer present in the boiling solution by holding wet PH paper over it or carefully smelling it. But by the end of the procedure, it is very simple to attempt to make the process faster and then forget to ensure that the NH₄ is entirely used up. The PH paper helped to make it more accurate, because it would have been exceedingly difficult for us to make sure that all the NH₄ had reacted simply by smell. In either case, we can't be 100% sure that it all reacted with the Sodium Hydroxide, which would have later changed our titration results.
