Final answer:
The compound with a molar mass of about 70 g/mol, containing 19.7% nitrogen and 80.3% fluorine, is likely NF3. Its Lewis structure includes nitrogen in the center, surrounded by three fluorine atoms, each with three lone pairs, and nitrogen holds one lone pair. The formal charges on all atoms in NF3 are zero.
Step-by-step explanation:
To determine the chemical formula of a compound with a molar mass of about 70 g/mol that contains 19.7% nitrogen (N) and 80.3% fluorine (F) by mass, we need to look at the possible options given in the question.
The molar mass of nitrogen (N) is approximately 14 g/mol, and that of fluorine (F) is 19 g/mol. Therefore, using the percentage composition and the molar masses of the individual elements, we can determine the empirical formula of the compound.
Given that the compound is 19.7% N, we have (19.7 g N/14 g/mol), which gives us approximately 1.4 moles of N. For fluorine, which makes up 80.3% of the compound, we have (80.3 g F/19 g/mol), which gives us approximately 4.23 moles of F. The closest whole number ratio would be 1 mole of N to 3 moles of F, making the empirical formula NF3.
The molar mass of NF3 is (14 + 3(19)) g/mol, which is 71 g/mol, very close to the given molar mass of about 70 g/mol. Considering this, the compound is most likely NF3, which is choice (a) from the options provided.
The Lewis structure of NF3 would show the nitrogen atom at the center with three fluorine atoms arranged symmetrically around it, each bonded to nitrogen with a single bond. In addition, there should be one lone pair of electrons on the nitrogen atom. Each of the fluorine atoms will also have three lone pairs of electrons.
Regarding the formal charges, we follow the formula: Formal charge = (Valence electrons) - (Non-bonding electrons) - 0.5*(Bonding electrons). Nitrogen in NF3 has five valence electrons, and since there are a lone pair and three bonds, its formal charge is zero.
Each fluorine has seven valence electrons, and as each forms one bond with nitrogen and has three lone pairs, their formal charges are also zero.