Question 10.Int.1: Nitryl fluoride is a reactive gas useful in rocket propellan...
Nitryl fluoride is a reactive gas useful in rocket propellants. Its mass percent composition is 21.55% N, 49.23% O, and 29.23% F. Its density is 2.7 g/L at 20°C and 1.00 atm pressure. Describe the nitryl fluoride molecule as completely as possibleـــــthat is, its formula, Lewis structure, molecular shape, and polarity.
Analyze
First, determine the empirical formula of nitryl fluoride from the composition data and the molar mass based on that formula. Next, determine the true molar mass from the vapor density data. Now the two results can be compared to establish the molecular formula. Then, write a plausible Lewis structure based on the molecular formula, and apply VSEPR theory to the Lewis structure to predict the molecular shape. Finally, assess the polarity of the molecule from the molecular shape and electronegativity values.
To determine the empirical formula, use the method of Example 3-5 (page 80). In 100.0 g of the compound,
mol N = 21.55 g N \times \frac{1 mol N }{14.007 g N } = 1.539 mol N
mol O = 49.23 g O \times \frac{1 mol O}{15.999 g O } = 3.077 mol O
mol F = 29.23 g F \times \frac{1 mol F }{18.998 g F } = 1.539 mol F
The empirical formula is
N _{1.539} O _{3.077} F _{1.539} = NO _2 F
The molar mass based on this formula is
14 + 32 + 19 = 65 g/mol
Use the method of Example 6-7 (page 208) to determine the molar mass of the gas.
\text { molar mass }=\frac{m R T}{P V}=\frac{d R T}{P}
= \frac{2.7 g / L \times 0.0821 L atm mol ^{-1} K ^{-1} \times 293 K }{1.00 atm }
= 65 g/mol
Because the two molar mass results are the same, the molecular and empirical formulas are the same: NO_2F.
Because N has the lowest electronegativity, it should be the lone central atom in the Lewis structure. There are two equivalent contributing structures to a resonance hybrid.
Three electron groups around the N atom produce a trigonal planar electron-group geometry. All the electron groups participate in bonding, so the molecular geometry is also trigonal planar. The predicted bond angles are 120°. (The experimentally determined \begin{matrix} F-N-O \end{matrix} bond angle is 118°.)
The molecule has a symmetrical shape, but because the electronegativity of F is different from that of O, we should expect the electron charge distribution in the molecule to be nonsymmetrical, leading to a small resultant dipole moment. NO_2 F is a polar molecule.
Assess
In the contributing structures to the resonance hybrid Lewis structure, the F atom has a formal charge of zero and the two O atoms have an average formal charge of \frac{1}{2}. As a result, we might expect the oxygen-atom region of the molecule to be the most negative in electron charge density and the fluorine region to be more neutral. This conclusion is confirmed by the electrostatic potential map for NO_2F. Notice also the positive charge density around the nitrogen nucleus, which is in accord with the fact that the nitrogen is the least electronegative of the three elements.
