Which of the following has the units of g/mol?
A. Molarity
B. Molar mass
C. Percent composition
D. Concentration
The Correct Answer and Explanation is:
The correct answer is:
B. Molar mass
Explanation:
Molar mass is defined as the mass of one mole of a substance, and its units are grams per mole (g/mol). In chemistry, molar mass is a fundamental property used to convert between the amount of substance (in moles) and the mass of the substance (in grams). By definition, one mole of any element or compound contains the same number of particles (atoms, molecules, or ions), known as Avogadro’s number, which is approximately (6.022 \times 10^{23}). Therefore, molar mass allows us to relate the microscopic properties of atoms and molecules to the macroscopic properties we observe and measure in the laboratory.
The units of molar mass, g/mol, provide a quantitative link between the atomic or molecular scale and the laboratory scale. For example, carbon has a molar mass of approximately 12 g/mol, meaning that one mole of carbon atoms has a mass of 12 grams. For compounds, the molar mass is calculated by adding the atomic masses (in g/mol) of all atoms in the molecule according to its chemical formula. For instance, the molar mass of water (H₂O) is calculated by adding the molar masses of two hydrogen atoms and one oxygen atom, resulting in approximately 18 g/mol.
Why the Other Options are Incorrect:
- Molarity (Option A) is a measure of concentration that describes the number of moles of solute per liter of solution. Its units are moles per liter (mol/L), not g/mol.
- Percent Composition (Option C) is the percentage by mass of each element in a compound. It is typically expressed as a percentage (%) and does not have units of g/mol.
- Concentration (Option D) is a general term referring to the amount of solute in a given quantity of solvent or solution. Its units vary (e.g., mol/L for molarity, g/L for mass concentration) and are not g/mol.
In summary, molar mass is the correct answer because it uniquely has the units of grams per mole (g/mol) and is a critical concept for calculating and understanding the relationships between mass, moles, and particles in chemical reactions.