Final answer:
To find the molecular formula from percent composition, calculate the empirical formula using atomic masses and then use the true molar mass to determine the molecular formula.
Step-by-step explanation:
To determine the molecular formula of an unknown compound using its percent composition and molar mass, we need to follow specific steps. We first calculate the empirical formula based on the percent composition and then use the molar mass to determine the molecular formula.
The empirical formula represents the simplest whole-number ratio of elements in a compound. To find this, divide the percent composition by the atomic mass of each element to find the ratio of moles of each element present, and then simplify to the smallest whole numbers. If the molar mass of the compound is given, divide it by the molar mass of the empirical formula to find the ratio. Multiply the subscripts in the empirical formula by this ratio to get the molecular formula.
Let's take the given percent composition: 47.0% potassium (K), 14.5% carbon (C), and 38.5% oxygen (O), with a molar mass of 166.214 g/mol. First, we would calculate the moles of K, C, and O based on their atomic masses (K=39.10, C=12.01, O=16.00 g/mol). Then, we'd determine the simplest mole ratio to get the empirical formula. Finally, we'd divide the true molar mass by the molar mass of the empirical formula to get the multiplier for the molecular formula.