3.8
Formula Mass and Mole Concepts of Compounds
Formula Mass of Covalent Compounds
Chemical formulas represent the elemental makeup of substances. For covalent compounds, the formula represents the…
The smallest representative unit of a compound is the molecule (or the formula unit), depicted by a chemical formula.
The atomic masses of all atoms in the chemical formula, if added, give the molecular mass (or formula mass) of the compound, measured in atomic-mass-unit.
For example, to find the molecular mass of sucrose or table sugar, the atomic mass of each element is multiplied with the number of atoms of the corresponding element. The sum-total gives the molecular mass of sucrose.
The mass of all molecules (or formula units) present in one mole of a chemical compound is called its molar mass, measured in g/mol.
The molar mass of a compound is numerically equal to its molecular mass (or formula mass). For example, sucrose, with molecular mass of 342.3 amu, will have a molar mass of 342.3 g/mol.
Knowing the actual number of molecules or particles present in a chemical substance is fundamental to understanding its composition, interactions, and combinations.
However, in a given sample of a compound, counting the actual number of molecules by ordinary means is impractical. By employing the relationship between molar mass, mole, and Avogadro’s number, the total molecules present can be calculated.
According to the concept, the molar mass of a compound is equivalent to one mole of the compound, constituting 6.022 × 1023 particles. Therefore, 342.3 grams of sucrose contain 6.022 × 1023 sucrose molecules.
However, if a sucrose sample weighs 150 grams for instance, then the number of molecules can be determined by calculation using a cross-multiplication method. The result shows that 150 grams of sucrose constitute 2.63 × 1023 sucrose molecules.
The percentage of each element in a compound can be expressed using a mass-percent value. To compute, the mass of the element in one mole of the compound is divided by the total mass of one mole of the compound and converted to a percentage.
In practice, to calculate the mass percent of hydrogen in sucrose, the total mass of hydrogen atoms and the total mass of sucrose molecules is included in the formula. The computed value shows that sucrose contains 6.4 mass-percent hydrogen.
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Q1: What is the difference between molecular mass and molar mass?
Molecular mass is the sum of atomic masses of all atoms in a chemical formula, measured in atomic mass units (amu). Molar mass is the mass of one mole of a compound, measured in grams per mole (g/mol). The numerical values are equal; for example, sucrose has a molecular mass of 342.3 amu and a molar mass of 342.3 g/mol.
Q2: How do you calculate the number of molecules in a sample using molar mass and Avogadro's number?
Use the relationship that one mole of any compound contains 6.022 × 10²³ particles. First, divide the sample mass by the molar mass to find moles, then multiply by Avogadro's number. For example, 150 grams of sucrose (molar mass 342.3 g/mol) equals 2.63 × 10²³ molecules.
Q3: Why is formula mass calculated the same way for both ionic and covalent compounds?
Both ionic and covalent formula masses are calculated by summing the average atomic masses of all atoms in the chemical formula. However, for ionic compounds, the formula does not represent a discrete molecule but rather the ratio of cations and anions. For covalent compounds like chloroform (CHCl₃), the formula represents a single molecule.
Q4: What does mass percent composition tell you about a compound?
Mass percent composition shows the relative abundance of each element in a compound. It is calculated by dividing the total mass of an element in one mole of the compound by the total molar mass, then converting to a percentage. For sucrose, hydrogen comprises 6.4 mass percent of the compound.
Q5: How does the electron mass affect formula mass calculations for ionic compounds?
The electron mass is negligibly small compared to atomic mass, so it is ignored in formula mass calculations. Although sodium cations are slightly lighter than sodium atoms and chloride anions are slightly heavier than chlorine atoms, these differences offset each other. Therefore, average atomic masses of neutral atoms are used for ionic compounds.
Q6: What is the relationship between chemical formulas and the smallest representative unit of a compound?
A chemical formula represents the smallest representative unit of a compound, which is either a molecule (for covalent compounds) or a formula unit (for ionic compounds). The formula shows the types and numbers of atoms or ions present. Adding the atomic masses of all atoms in the formula gives the molecular or formula mass.
Q7: Why is counting individual molecules impractical, and how does the mole concept solve this problem?
Counting individual molecules by ordinary means is impractical because compounds contain enormous numbers of particles. The mole concept solves this by establishing that one mole of any compound contains exactly 6.022 × 10²³ particles. This allows chemists to relate measurable mass quantities to the number of molecules or particles present in a sample.