A compound used as a fertilizer contains only the elements C, H, N, and O. — Stoichiometry and Colligative Properties Chemistry Question
Problem Context
A compound used as a fertilizer contains only the elements C, H, N, and O.
Combustion of 1.000 g of the fertilizer in an oxygen atmosphere produces 0.5637 g CO2, 0.6924 g H2O, and 0.3589 g N2. What are the mass percentages of C, H, and N in the fertilizer?
Model Answer
0.5637 g CO2 (12.01 g C/44.01 g CO2) = 0.1538 g C, 15.38% C
0.6924 g H2O (2.016 g H/18.016 g H2O) = 0.07748 g H, 7.75% H
0.3589 g N2 = 35.89% N
Give the empirical formula of the fertilizer.
Model Answer
%O = 100% – (15.38% + 7.75% + 35.89%) = 40.98% O
So in 100 g fertilizer there would be:
15.38 g C/(12.01 g mol -1) = 1.28 mol C
7.748 g H/(1.008 g mol -1) = 7.69 mol H
35.89 g N/(14.01 g mol -1) = 2.56 mol N
40.98 g O/(16.00 g mol -1) = 2.56 mol O
Dividing by 1.28 mol gives an empirical formula CH6N2O2.
A solution of 1.000 g of the fertilizer dissolved in 20.00 g water has a freezing point of –2.38 ºC. What is the apparent molar mass of the fertilizer? Combined with the result in (b), what is the implication of this molar mass? (For water, the freezing point depression constant Kf = 1.86 ºC/m.)
Model Answer
2.38 ºC/(1.86 ºC/m) = 1.28 m solution
(1.28 mol/1000g H2O)•(20 g H2O) = 0.0256 mol solute
1.000 g solute/0.0256 mol solute = 39.1 g mol -1.
From the empirical formula, the formula mass is 78.1 g mol -1! This says that the molar mass appears to be half the formula mass; the only way this is possible is if each mol of fertilizer gives rise to two moles of particles. Thus, the fertilizer is likely an ionic compound.
Propose a structure for the fertilizer compound.
Model Answer
The most likely cation given the formula is ammonium ion, which leaves an ion with the formula CH2NO2. Several chemically plausible structures can be written, but the only one where the anion is not so basic that it would deprotonate the ammonium ion is carbamate, NH2CO2–. (The only other chemically reasonable alternative is hydrazinium formate, (NH2NH3+)(HCOO–).) Ammonium carbamate is a common fertilizer.
Fertilizer =