# Empirical formulas and molecular formulas concept questions and answers

1. Q: What is an empirical formula? A: An empirical formula represents the simplest whole-number ratio of atoms of each element in a compound.

2. Q: What is a molecular formula? A: A molecular formula represents the actual number of atoms of each element in a molecule of the compound.
3. Q: How do empirical formulas and molecular formulas differ? A: While an empirical formula shows the simplest whole-number ratio of atoms in a compound, the molecular formula shows the actual number of atoms of each element in a molecule of the compound.
4. Q: How do you determine the empirical formula of a compound? A: The empirical formula of a compound can be determined from the percent composition of each element in the compound. These percentages are used to calculate the ratio of moles of each element, which can then be simplified to the smallest whole numbers to find the empirical formula.
5. Q: How do you determine the molecular formula of a compound? A: To determine the molecular formula, you must know the molar mass of the compound and its empirical formula. The ratio of the molar mass of the compound to the molar mass of the empirical formula (which is essentially the molar mass of the simplest ratio of elements) can be used to find the number of empirical formula units per molecule, which gives the molecular formula.
6. Q: Can the empirical formula and molecular formula of a compound be the same? A: Yes, for some compounds the empirical formula and the molecular formula can be the same. This occurs when the ratio of atoms in the molecule is the same as the simplest whole-number ratio. An example is water (H2O) where the molecular and empirical formulas are the same.
7. Q: Why might a chemist be interested in knowing both the empirical and molecular formulas of a compound? A: The empirical formula can provide information about the ratio of atoms in the compound, which can help in understanding its composition, while the molecular formula provides information about the actual structure of the molecule, which can be used to infer its chemical and physical properties.
8. Q: How is the percent composition of a compound related to its empirical formula? A: The percent composition of a compound is used to calculate the ratio of moles of each element in the compound, which can then be simplified to give the empirical formula.
9. Q: How is combustion analysis used to determine empirical and molecular formulas? A: In combustion analysis, a compound is burned in excess oxygen to produce carbon dioxide and water. By measuring the masses of CO2 and H2O produced, the amount of carbon and hydrogen in the original compound can be determined, which can be used to find the empirical formula. If the molar mass of the compound is known, the molecular formula can also be determined.
10. Q: How can you determine the molecular formula of a compound from its empirical formula and its molar mass? A: If you know the molar mass of the compound, you can divide it by the molar mass of the empirical formula to find the number of empirical formula units in each molecule of the compound. This number is then multiplied by the subscript of each element in the empirical formula to find the molecular formula.
11. Q: What does a subscript in a molecular or empirical formula represent? A: A subscript in a molecular or empirical formula represents the number of atoms of that element in the compound.
12. Q: What is a hydrate, and how is its formula typically represented? A: A hydrate is a compound that includes water molecules within its crystalline structure. The formula of a hydrate is represented by the formula of the compound followed by a dot and the number of water molecules per formula unit. For example, the formula of copper(II) sulfate pentahydrate is CuSO4·5H2O.
13. Q: How can empirical formulas be used in stoichiometry? A: Empirical formulas can be used in stoichiometry to calculate the amounts of reactants needed or products formed in a chemical reaction. They provide the mole ratio of the elements involved in the reaction.
14. Q: Why can’t the molecular formula always be determined from the empirical formula alone? A: The empirical formula only provides the simplest whole-number ratio of atoms in the compound, but it does not provide information about the actual number of atoms of each element in a molecule of the compound. Without additional information, such as the molar mass of the compound, the molecular formula cannot be determined.
15. Q: How is the empirical formula related to the formula mass? A: The empirical formula mass is the sum of the atomic masses (in atomic mass units) of all atoms in the empirical formula. It provides the mass of one mole of the empirical formula units.
16. Q: How can the empirical formula be determined from experimental data? A: Experimental data, such as mass or percentage composition of the elements in a compound, can be used to calculate the ratio of moles of each element. This ratio can then be simplified to whole numbers to give the empirical formula.
17. Q: What information does the empirical formula provide about a compound? A: The empirical formula provides information about the ratio of different elements in a compound. This ratio is represented as the simplest whole number ratio of atoms of each element.
18. Q: How can a molecular formula be converted into an empirical formula? A: A molecular formula can be converted into an empirical formula by dividing the subscript of each element in the molecular formula by the greatest common factor.
19. Q: How is the percent composition of an element in a compound calculated? A: The percent composition of an element in a compound can be calculated by dividing the total mass of the element in the formula by the formula mass of the compound, and then multiplying by 100%.
20. Q: Why do empirical and molecular formulas not provide information about the structure of a molecule? A: Empirical and molecular formulas provide information about the ratio and the number of atoms of each element in a molecule, respectively. However, they do not provide information about how these atoms are arranged or bonded in the molecule. That information is provided by the structural formula of the molecule.