Textbook Question
A compound whose empirical formula is XF3 consists of 65% F by mass. What is the atomic mass of X?
Ch.3 - Chemical Reactions and Reaction Stoichiometry
Chapter 3, Problem 52a
What is the molecular formula of each of the following compounds? (a) empirical formula HCO2, molar mass = 90.0 g/mol
Verified step by step guidance1
1. The first step is to calculate the molar mass of the empirical formula. The empirical formula is HCO2, which means it contains one atom of hydrogen (H), one atom of carbon (C), and two atoms of oxygen (O). The molar mass of hydrogen is approximately 1 g/mol, the molar mass of carbon is approximately 12 g/mol, and the molar mass of oxygen is approximately 16 g/mol. Therefore, the molar mass of the empirical formula is the sum of the molar masses of its constituent atoms.
2. Once you have the molar mass of the empirical formula, you can calculate the ratio of the molar mass of the compound to the molar mass of the empirical formula. This ratio will tell you how many empirical formula units make up the molecular formula of the compound.
3. If the ratio calculated in step 2 is a whole number, then the molecular formula is simply that number of empirical formula units. If the ratio is not a whole number, then you will need to multiply the subscripts in the empirical formula by the smallest whole number that will make the ratio a whole number.
4. After multiplying the subscripts in the empirical formula by the ratio calculated in step 3, you will have the molecular formula of the compound.
5. Remember that the molecular formula of a compound can be the same as its empirical formula, or it can be a multiple of the empirical formula. The molecular formula provides more information about the actual number of atoms in a molecule of the compound, while the empirical formula only provides information about the ratio of atoms in the compound.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Empirical Formula
The empirical formula represents the simplest whole-number ratio of atoms in a compound. For example, the empirical formula HCO2 indicates that for every one carbon atom, there are two oxygen atoms and one hydrogen atom. It does not provide information about the actual number of atoms in a molecule, which can be determined by the molecular formula.
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Empirical vs Molecular Formula
Molecular Formula
The molecular formula gives the actual number of each type of atom in a molecule of a compound. It is derived from the empirical formula by multiplying the subscripts by a whole number, known as the molecular weight factor, which is determined by dividing the compound's molar mass by the molar mass of the empirical formula.
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Molar Mass
Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in the molecular formula. In this case, knowing the molar mass (90.0 g/mol) allows us to find the molecular formula by comparing it to the molar mass of the empirical formula, which is essential for determining the correct molecular structure.
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