Question

What is the mass per cent of oxygen in ethanol? (a) \(52.14 \%\) (b) \(13.13 \%\) (c) \(16 \%\) (d) \(34.73 \%\)

Short answer

The mass per cent of oxygen in ethanol is approximately 34.73%.

Step-by-step solution

Write the chemical formula for ethanol

The chemical formula for ethanol is C2H5OH. This indicates that each molecule of ethanol consists of 2 carbon (C) atoms, 6 hydrogen (H) atoms, and 1 oxygen (O) atom.

Calculate the molar mass of ethanol

The molar mass of ethanol can be calculated by summing the molar masses of all the atoms in a molecule. The atomic weights of C, H, and O are approximately 12.01 g/mol, 1.008 g/mol, and 16.00 g/mol, respectively. Thus, the molar mass of C2H5OH is: 2(12.01 g/mol) + 6(1.008 g/mol) + 16.00 g/mol = 24.02 g/mol + 6.048 g/mol + 16.00 g/mol = 46.068 g/mol.

Calculate the mass percent of oxygen in ethanol

The mass percent of an element in a compound is calculated using the formula: (Mass of the element in one mole of the compound / Molar mass of the compound) × 100%.For oxygen in ethanol: (16.00 g/mol / 46.068 g/mol) × 100% = (16.00 / 46.068) × 100% = 34.7303%.

Key concepts

Chemical Formula

Understanding the chemical formula of a substance is crucial as it reveals the types and numbers of atoms present in a molecule. For example, ethanol's chemical formula is C2H5OH, which tells us that one molecule of ethanol consists of 2 carbon atoms (C), 6 hydrogen atoms (H), and one oxygen atom (O). The subscripts (such as the '2' in C2) indicate the number of atoms of each element in one molecule of the compound. Knowing the chemical formula is the starting point for many calculations in chemistry, including determining the molar mass and finding the percentage composition of the compound.

When analyzing the chemical formula, one should also recognize the structural information it provides. In ethanol, the chemical formula implies that the oxygen atom is connected to a hydrogen atom, forming a hydroxyl group (-OH). This gives the molecule specific chemical properties, making it an alcohol.

Molar Mass

The molar mass is a fundamental concept in chemistry that refers to the mass of one mole of a substance. It is expressed in grams per mole (g/mol). To calculate the molar mass of a compound, such as ethanol, you add the atomic masses of all atoms present in the molecule based on its chemical formula. For instance, ethanol (C2H5OH) has a molar mass calculated by adding the mass of 2 moles of carbon (24.02 g/mol), the mass of 6 moles of hydrogen (6.048 g/mol), and the mass of 1 mole of oxygen (16.00 g/mol), leading to a total of 46.068 g/mol.

The concept of molar mass is crucial because it allows chemists to weigh out amounts of a substance that will have a known number of molecules or atoms. It can also link the microscale world of atoms and molecules to the macroscale world we experience every day, by relating a specific number of particles to a mass that can be measured on a balance.

Percentage Composition

Percentage composition refers to the percent by mass of each element in a compound. It allows chemists to understand the makeup of a substance in terms of its elemental constituents. To find the mass percent of oxygen in ethanol, you would take the mass of oxygen in one mole of ethanol (16.00 g/mol), divide by the total molar mass of ethanol (46.068 g/mol), and multiply by 100 to get a percentage.

This calculation reveals that oxygen makes up 34.7303% of ethanol by mass. This information is essential, as percentage composition informs the formulation and adjustment of chemical reactions, the nutritional content of food, the design of pharmaceuticals, and the verification of a substance's empirical formula. This percent composition is a necessary piece of data for both theoretical analysis and practical applications like quality control in product manufacturing.

Mole Concept

The mole concept is a bridge between the atomic, submicroscopic world and our everyday, macroscopic world. A mole represents Avogadro's number, which is approximately 6.022×10^23 of anything, usually atoms or molecules. The mole concept allows chemists to count particles like atoms, molecules, and ions by weighing them. Using the molar mass and the mole concept together, they can convert back and forth between the mass of a substance and the number of moles of the substance.

This is a fundamental concept for all quantitative work in chemistry because it lets you calculate the number of entities participating in a chemical reaction, which is critical for stoichiometric calculations. For instance, if you were informed that a container had 46.068 g of ethanol, you could deduce that it contains exactly one mole, or approximately 6.022×10^23 ethanol molecules, based on its molar mass.