High School

A 3.742 g sample of a compound containing only carbon and hydrogen was analyzed by combustion and found to contain 3.140 g of carbon and 0.602 g of hydrogen. Mass spectral analysis indicates that the molar mass for this compound is 100.2 g/mol. What is the molecular formula for this compound?

Answer :

Answer : The molecular formula for this compound is C7H14

To determine the molecular formula of the compound, we need to first calculate its empirical formula using the given mass percentages of carbon and hydrogen. The mass percent of carbon in the compound is: (3.140 g / 3.742 g) x 100% = 83.9%

The mass percent of hydrogen in the compound is: (0.602 g / 3.742 g) x 100% = 16.1%. Assuming a 100 g sample of the compound, we can calculate the masses of carbon and hydrogen in the sample: Mass of carbon = 83.9 g and Mass of hydrogen = 16.1 g

Next, we need to convert these masses to moles, using the atomic masses of carbon and hydrogen:1 mol C = 12.01 g, 1 mol H = 1.008 g. Moles of carbon = 83.9 g / 12.01 g/mol = 6.983 mol, Moles of hydrogen = 16.1 g / 1.008 g/mol = 15.95 mol. Dividing each mole value by the smallest mole value, we get the following mole ratio: C:H = 6.983 / 6.983 = 1.000 : 2.285

The empirical formula for the compound is therefore CH2. To determine the molecular formula, we need to find the molecular weight of the empirical formula, and then divide the given molar mass by this value to get the molecular formula multiplier. Molecular weight of CH2 = 12.01 + 2(1.008) = 14.026 g/mol, Molecular formula multiplier = 100.2 g/mol / 14.026 g/mol = 7.146. Multiplying the empirical formula by this multiplier, we get the molecular formula: C7H14

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