Question

The percentage of oxygen in a compound is \(4 \%\). Its minimum molecular mass will be (a) 100 (b) 400 (c) 200 (d) 32

Short answer

The minimum molecular mass of the compound will be 100 g/mol, so the correct answer is (a) 100.

Step-by-step solution

Understand the concept of percentage composition

The percentage composition expresses the proportion of each element in a compound in terms of percent by mass. Since the oxygen's percentage is given as 4%, it means that in every 100 g of the compound, 4 g is oxygen.

Calculate the mass of oxygen corresponding to its percentage

To find the mass of oxygen corresponding to the given percentage, consider the mass of the compound to be 100 g. Therefore, the mass of oxygen is 4% of 100 g, which is calculated as \( \frac{4}{100} \times 100 g = 4 g \).

Find the molar mass of oxygen

The molar mass of oxygen (O) is approximately 16 g/mol. Therefore, one mole of oxygen atoms weighs 16 g.

Calculate the minimum molar mass of the compound

Since 4 g of oxygen is present in the compound, the number of moles of oxygen is given by \( \frac{4 g}{16 g/mol} = 0.25 \text{ mol} \). To find the minimum molecular mass, assume the compound has only one atom of oxygen. Therefore, the molecular mass will be \(0.25 \text{ mol} \times 16 g/mol = 4 g \). Since oxygen is 4% of the compound, this value corresponds to the 4%, and thus the minimum molecular mass of the compound is \( \frac{100}{4} \times 4 g = 100 g/mol \).

Key concepts

Molar Mass Calculation

Molar mass is a fundamental concept in chemistry, representing the mass of one mole of a given substance and is expressed in grams per mole (g/mol). To calculate the molar mass of a compound, you need to sum the molar masses of the individual elements that make up the compound, taking into account their respective numbers in the chemical formula.

For example, to find the molar mass of water (H₂O), you would first need the molar masses of hydrogen and oxygen from the periodic table, which are about 1 g/mol and 16 g/mol respectively. Then, you multiply the molar mass of hydrogen by its count in the compound (2), add to this the molar mass of oxygen (1), and sum the parts:

• Hydrogen: 2 * 1 g/mol = 2 g/mol
• Oxygen: 16 g/mol
• Total molar mass = 2 g/mol + 16 g/mol = 18 g/mol

The ability to calculate molar mass is essential for converting between grams and moles, which is a typical requirement in stoichiometry problems.

Molecular Mass Determination

Molecular mass refers to the mass of a single molecule, measured in atomic mass units (amu). Determination of molecular mass is vital in chemistry for understanding the composition of substances. It involves calculating the sum of the atomic masses of all atoms present in the molecule.

Using the periodic table, each element's atomic mass is noted, and by using the chemical formula of the compound, the total molecular mass can be established. For instance, the molecular mass of carbon dioxide (CO₂) is calculated by adding the atomic mass of one atom of carbon (approximately 12 amu) to the atomic mass of two oxygen atoms (2 * 16 amu).

• Carbon: 12 amu
• Oxygen: 2 * 16 amu = 32 amu
• Total molecular mass = 12 amu + 32 amu = 44 amu

It's important to differentiate between molar mass and molecular mass; while they numerically equal for a single molecule, molar mass is used for a mole of molecules.

Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships that govern the reactants and products in chemical reactions. It relies heavily on the concept of molar mass for converting between masses and amounts (in moles) of substances.

Stoichiometric calculations often involve balancing chemical equations, understanding mole ratios, and applying the mole concept to determine the relative quantities of reactants needed or products formed. The stoichiometric coefficient, which indicates the number of moles of a substance involved in the reaction, plays a crucial role in these calculations. For instance, in the reaction 2H₂ + O₂ → 2H₂O, the mole ratio of hydrogen to oxygen is 2:1, meaning 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water. Grasping stoichiometry is crucial for anyone looking to solve chemistry problems effectively, as it touches several aspects of chemical reactions.

Chemistry Problem Solving

Chemistry problem solving is a skill that requires understanding key concepts, identifying what is being asked, and systematically applying appropriate methods to arrive at a solution. It often starts with an analysis of the given data, followed by the identification of the relevant formulas or principles.

In solving problems, a logical approach from general to specific helps simplify complex problems. Common steps include:

• Reading the problem thoroughly and noting given quantities and required outcomes.
• Diagramming the problem, if applicable, to visualize the scenario.
• Choosing the right equation or stoichiometric relationship that correlates the data to the unknown quantity.
• Carefully substituting known values into the equation and solving mathematically.

Additionally, understanding the concepts of molar mass, molecular mass, and stoichiometry is essential since they often serve as starting points for solving a wide range of chemistry problems.