Question

Para-cresol, a substance used as a disinfectant and in the manufacture of several herbicides, is a molecule that contains the elements carbon, hydrogen, and oxygen. Complete combustion of a \(0.345-\mathrm{g}\) sample of \(p\) -cresol produced \(0.983 \mathrm{~g}\) carbon dioxide and \(0.230 \mathrm{~g}\) water. Determine the empirical formula for \(p\) -cresol.

Short answer

The empirical formula for para-cresol is C7H8O.

Step-by-step solution

Find the moles of carbon and hydrogen in para-cresol

To find the moles of C and H in para-cresol, first, find the moles of CO2 and H2O that were formed during combustion. For CO2: Given mass = 0.983 g Molar mass of CO2 = 12.01 g/mol (for C) + 2 * 16.00 g/mol (for O) = 44.01 g/mol Moles of CO2 = \(0.983 \frac{\text{g}}{44.01\text{g/mol}}\) = 0.0223 mol CO2 For H2O: Given mass = 0.230 g Molar mass of H2O = 2 * 1.01 g/mol (for H) + 16.00 g/mol (for O) = 18.02 g/mol Moles of H2O = \(0.230 \frac{\text{g}}{18.02\text{g/mol}}\) = 0.0128 mol H2O Each mole of CO2 contains 1 mole of carbon, and each mole of H2O contains 2 moles of hydrogen. So, Moles of C = Moles of CO2 = 0.0223 mol Moles of H = 2 * Moles of H2O = 2 * 0.0128 mol = 0.0256 mol

Find the moles of oxygen in para-cresol

First, find the total mass of C and H in para-cresol: Mass of C = Moles of C * Molar mass of C = 0.0223 * 12.01 = 0.2679 g Mass of H = Moles of H * Molar mass of H = 0.0256 * 1.01 = 0.0258 g Now we can find the mass of oxygen in para-cresol by subtracting the total mass of C and H: Mass of O = 0.345 g (para-cresol) - (0.2679 g (C) + 0.0258 g (H)) = 0.0513 g Now, find the moles of O: Molar mass of O = 16.00 g/mol Moles of O = \(0.0513 \frac{\text{g}}{16.00\text{g/mol}}\) = 0.0032 mol

Calculate the molar ratio of C, H, and O

Divide each mole amount by the smallest mole amount to find the molar ratio: Molar ratio of C = 0.0223 mol / 0.0032 mol = 7 Molar ratio of H = 0.0256 mol / 0.0032 mol = 8 Molar ratio of O = 0.0032 mol / 0.0032 mol = 1

Determine the empirical formula

The empirical formula is the simplest whole-number ratio of moles. Thus, the empirical formula for para-cresol is C7H8O.

Key concepts

Chemical Combustion

Chemical combustion is a vital process in understanding the chemical behavior of compounds like para-cresol. In a combustion reaction, a substance reacts with oxygen to produce energy in the form of heat and light. This is typically seen when organic compounds burn, releasing carbon dioxide and water as by-products.
For organic molecules, combustion is often used to determine the amount of carbon and hydrogen present. When para-cresol is completely burned, it yields carbon dioxide and water. By measuring the mass of these products, chemists can back-calculate the amount of carbon and hydrogen originally present in the compound.
The information gathered from combustion analysis is crucial for calculating empirical formulas, which express the simplest ratio of elements within a molecule.

Molar Mass Calculation

Molar mass calculation is an essential step in finding the amount of each element in a compound. Molar mass is the mass of one mole of a substance, which correlates to its molecular weight expressed in grams.
In the case of combustion, we first determine the molar mass of carbon dioxide (CO2) and water (H2O) to calculate how much carbon and hydrogen are in the original compound.

• For CO2, the molar mass is calculated as follows: 12.01 g/mol (carbon) plus 16.00 g/mol (oxygen), times two for the two oxygen atoms, resulting in 44.01 g/mol.
• For water, the molar mass is 2 * 1.01 g/mol for hydrogen plus 16.00 g/mol for oxygen, equaling 18.02 g/mol.

Using these molar masses, one can transform the mass of the products back to moles, further aiding in the empirical formula determination.

Molecular Composition

The term molecular composition refers to the types and amounts of atoms that constitute a molecule. In the scenario with para-cresol, the atoms in focus are carbon, hydrogen, and oxygen. Through combustion, we determine the molecular composition by analyzing the resulting products.

• Moles of carbon are equal to moles of CO2, as each molecule of CO2 contains one carbon atom.
• Moles of hydrogen are determined based on water, with each molecule of H2O containing two hydrogen atoms.
• Finally, moles of oxygen within the molecule are inferred by calculating what remains after accounting for carbon and hydrogen, as determined by subtracting the masses of these elements from the original sample mass.

Understanding this concept is critical for deducing the empirical formula, a symbolic representation that showcases the simplest ratio of these atoms in the compound.

Organic Chemistry

Organic chemistry focuses on compounds primarily made of carbon, along with hydrogen, oxygen, nitrogen, and other elements. Para-cresol fits perfectly into this field as a compound containing carbon, hydrogen, and oxygen. Its study showcases how these elements interact and form complex structures.
Organic chemists often perform combustion analysis as a method to determine a compound's composition, as done with para-cresol. This kind of analysis provides crucial insight into chemical formulas and helps in identifying how substances like para-cresol can be utilized in real-world applications, such as disinfectants or herbicides.
Emphasizing empirical formulas, which are foundational concepts in organic chemistry, enables the simplification of complex molecules. This simplification is vital as it aids researchers in plotting chemical behavior and interactions in broader chemical systems.