Consider the molecule NI3 . a. Write the total number of valence electrons in the NI3 molecule AND draw its Lewis structure. Be sure to clearly mark any lone pairs of electrons. b. What is the molecular shape (molecular geometry) of NI3 ? c. Briefly explain how to determine the polarity of the NI3 molecule and whether it is polar or nonpolar. 2. Propane, a common fuel, has a molecular formula of C3 H8 . a. How many hydrogen atoms are in a 85 gram sample of propane? Show all your work and be sure to include units throughout the calculation. b. Write a balanced chemical equation for the complete combustion of propane in molecular oxygen gas. You do not need to show the physical states.
The Correct Answer and Explanation is
1. NI₃ (Nitrogen Triiodide)
a. Valence Electrons & Lewis Structure
- Valence Electrons:
- Nitrogen (N): 5 valence electrons
- Iodine (I): 7 valence electrons × 3 = 21
- Total = 5 + 21 = 26 valence electrons
Lewis Structure:
- Nitrogen is the central atom.
- Each iodine is bonded to nitrogen by a single bond (3 bonds total).
- After bonding, 6 electrons are used (3 bonds × 2 electrons).
- Remaining electrons: 26 − 6 = 20 electrons.
- Distribute the remaining 20 electrons to complete the octets of the I atoms (6 electrons each × 3 I = 18 electrons), and 2 electrons (1 lone pair) on nitrogen.
..
:I:
|
.. :I–N–I: ..
..
Each I has 3 lone pairs, N has 1 lone pair.
b. Molecular Shape (Geometry)
- Electron groups around N: 3 bonding + 1 lone pair = 4 regions.
- According to VSEPR theory, this corresponds to a tetrahedral electron geometry, but the molecular geometry (shape) is trigonal pyramidal.
c. Polarity of NI₃
- NI₃ is polar.
- Though the three I atoms are the same, the lone pair on nitrogen makes the molecule asymmetrical, resulting in a net dipole moment.
- The shape (trigonal pyramidal) causes the bond dipoles to not cancel out.
2. Propane (C₃H₈)
a. Hydrogen Atoms in 85g of Propane
- Molar mass of C₃H₈:
= (3 × 12.01) + (8 × 1.008) = 36.03 + 8.064 = 44.094 g/mol - Moles of propane in 85 g:
= 85 g ÷ 44.094 g/mol ≈ 1.928 mol - Hydrogen atoms per molecule: 8 H atoms
→ 8 mol H atoms per 1 mol propane - Moles of H atoms:
= 1.928 mol propane × 8 mol H/mol propane = 15.424 mol H - Number of atoms:
= 15.424 mol × 6.022×10236.022 \times 10^{23} atoms/mol
≈ 9.29 × 10²⁴ H atoms
b. Balanced Combustion Equation
C3H8+5O2→3CO2+4H2OC_3H_8 + 5O_2 → 3CO_2 + 4H_2O
Explanation
Understanding molecular structure and stoichiometry is fundamental in chemistry. NI₃, nitrogen triiodide, is a covalently bonded molecule with nitrogen at the center and three iodine atoms surrounding it. Nitrogen has five valence electrons and forms three single bonds with iodine atoms, each contributing one electron. The resulting molecule uses 26 valence electrons, with lone pairs completing the octets on all atoms. The geometry of NI₃ is trigonal pyramidal due to three bonded pairs and one lone pair on nitrogen, which pushes the bonded atoms downward. This shape makes NI₃ polar because the molecule lacks symmetry and has a net dipole moment, mainly due to the lone pair on nitrogen and the electronegativity difference between N and I.
For propane (C₃H₈), calculating the number of hydrogen atoms in a given mass involves multiple steps. First, the molar mass is determined, then the number of moles of propane in 85 grams is calculated. Since each propane molecule contains eight hydrogen atoms, we multiply the number of propane moles by 8, and finally convert that to actual atoms using Avogadro’s number. This results in approximately 9.29×10249.29 \times 10^{24} hydrogen atoms in 85 grams of propane.
Finally, the combustion reaction of propane involves reacting with oxygen to produce carbon dioxide and water. Balancing the chemical equation ensures that matter is conserved—each side has the same number of atoms of each element. The complete combustion reaction is written as:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, which confirms the conversion of hydrocarbon fuel into typical combustion products.
