[12%] A is an ionic compound that contains only the elements hydrogen, nitrogen, and oxygen. — Stoichiometry and Analytical Chemistry Chemistry Question
Problem Context
[12%] A is an ionic compound that contains only the elements hydrogen, nitrogen, and oxygen.
A 1.000-g sample of A is dissolved in 20 mL water and titrated with 0.5000 M NaOH solution, giving the data shown below. What is the molar mass of A?
Model Answer
The endpoint is at 25.0 mL added NaOH, and from the shape of the titration curve the compound is a monoprotic acid. So n = (0.0250 L)(0.5000 mol L-1) = 0.0125 mol. 1.000 g/0.0125 mol = 80.0 g mol-1.
When a 1.000-g sample of A is heated at 230 °C in an evacuated 1.50 L vessel, it decomposes into gaseous products, giving a final pressure of 784 mm Hg. How many moles of gas are formed in this reaction?
Model Answer
PV = nRT
(784 torr)(1.50 L) = n(62.4 L torr mol-1 K-1)(503.15 K)
n = 0.0375 mol
This is 3 mol gas per mol A, given the answer to part a.
If the gases produced from the decomposition of 1.000 g of A are instead first passed through a column packed with magnesium perchlorate (which strongly absorbs water vapor) and then collected at 25 °C and a pressure of 755 mm Hg, the total volume of gas is 308 mL. How many moles of gas are collected in this experiment?
Model Answer
PV = nRT
(755 torr)(0.308 L) = n(62.4 L torr mol-1 K-1)(298.15 K)
n = 0.0125 mol
This is 1 mol gas per mol A, so the decomposition must have produced 2 mol H2O per mol A.
What is the formula of A? Explain your reasoning.
Model Answer
Each mol of A produces two mol H2O when it decomposes, so the formula unit must contain at least four H's and two O's. The molar mass of the gas produced in part c. is thus 80 – 2(18) = 44. The titration in part a indicates that the acid has a pKa of 9.25; the only reasonable possibility is ammonium ion, NH4+. This means that the gas observed in part c. must contain at least one N. The only possible formulas of the gas in c. (M = 44, at least one N, only possible elements H, N, O) are NH30, H14NO, N2H16, N2O, and N3H2. The only chemically reasonable option in this list is N2O, nitrous oxide.
The overall formula of A is thus N2O + 2(H2O) = H4N2O3. Given the presence of ammonium ion, this translates to (NH4)NO3, ammonium nitrate.
Write Lewis structures for the cation and the anion present in A and for the product(s) of its decomposition at 230 °C. Your Lewis structures should include all bonds, lone pairs, and nonzero formal charges. You should show all significant resonance structures for each species.
Model Answer
Lewis structures are shown for the products: ammonium cation (NH4+), nitrate anion (NO3- with 3 resonance structures), water (H2O), and nitrous oxide (N2O with 2 resonance structures).