Final answer:
The molecular structure of a compound with a molar mass of about 42 g/mol containing 85.7% carbon and 14.3% hydrogen is trigonal planar. The molecular formula is C3H6, which corresponds to propene, and the trigonal planar structure arises from the double bond between two carbon atoms. The correct option is b.
Step-by-step explanation:
To determine the molecular structure of a compound with a molar mass of about 42 g/mol that contains 85.7% carbon and 14.3% hydrogen by mass, we first calculate the empirical formula.
Assuming we have 100 g of the compound, this would give us 85.7 g of carbon, approximately 7.14 moles (since the atomic mass of carbon is about 12 g/mol), and 14.3 g of hydrogen, which is about 14.3 moles (since the atomic mass of hydrogen is about 1 g/mol).
The ratio of carbon to hydrogen is essentially 1:2, which suggests a hydrocarbon with the empirical formula CH2. To find the molecular formula, we divide the molar mass of the compound (42 g/mol) by the molar mass of the empirical formula (approximately 14 g/mol, with 12 from carbon and 2 from hydrogen), which gives us 3. Th
erefore, the molecular formula is C3H6. This corresponds to the hydrocarbon known as propene, which has a trigonal planar structure around the carbon with the double bond.
The options given (linear, trigonal planar, tetrahedral, octahedral) refer to the electron-pair geometry around a central atom in a molecule and not the overall molecular structure. However, by knowing the molecular formula C3H6, the molecular geometry can be deduced.
Two carbons are bonded to each other with a double bond, forming a planar or flat molecule at that part, giving rise to the trigonal planar molecular structure. Option b) Trigonal Planar is the most appropriate answer. The correct option is b.