Chemical Composition

Introduction to Chemical Composition

Chemical composition refers to the types and amounts of elements that make up a substance. Understanding chemical composition is essential for quantifying substances in chemical reactions and for converting between mass, moles, and number of particles.

Converting Between Grams and Moles of an Element

Understanding the Mole Concept

• Mole (mol): The SI unit for amount of substance, defined as containing exactly entities (Avogadro's number).

• Molar Mass: The mass of one mole of a substance, typically expressed in grams per mole (g/mol).

• The lighter the atom, the less mass in 1 mol of atoms; the lighter the atom, the more atoms there are in 1 g.

Example: Calculate the moles of carbon in 0.0265 g of pencil lead (pure carbon).

• Given:

• Calculation:

Converting Between Grams and Number of Atoms

Multi-Step Conversions

• To convert grams of an element to number of atoms, use the following steps:

• Grams of element → Moles of element → Number of atoms

Example: How many copper atoms are in a penny weighing 3.10 g (pure Cu)?

• Given: , atoms

• Calculation: atoms Cu

Counting Molecules by the Gram

Molar Mass of Compounds

• Molar Mass (of a compound): The mass of one mole of molecules or formula units of a compound.

• For compounds, add the molar masses of all atoms in the formula.

Example: Molar mass of :

• 2 mol H:

• 1 mol O:

• Total:

Converting Between Grams and Moles of a Compound

Application to Compounds

• To find moles from grams:

• To find grams from moles:

Example: Find the number of moles in a 22.5 g sample of dry ice ():

• Molar mass

Converting Between Grams of a Compound and Number of Molecules

Multi-Step Conversion

• Grams of compound → Moles of compound → Number of molecules

Example: Find the number of molecules in 22.5 g of dry ice:

• molecules

Converting Between Number of Molecules and Mass of a Compound

Reverse Multi-Step Conversion

• Number of molecules → Moles of compound → Grams of compound

Example: What is the mass of molecules?

• Molar mass

Chemical Formulas as Conversion Factors

Understanding Relationships in Chemical Formulas

• Chemical formulas indicate the ratio of elements in a compound.

• These ratios can be used as conversion factors between moles of compound and moles of constituent elements.

Example: In , 1 mol $H_2O$ contains 2 mol H and 1 mol O.

• To find moles of H in 50 mol :

Converting Between Moles of a Compound and Moles of a Constituent Element

Application to Compounds

• Use the subscripts in the chemical formula as conversion factors.

Example: Determine the number of moles of O in 3.7 mol of :

• contains 3 O atoms per formula unit.

Converting Between Grams of a Compound and Grams of a Constituent Element

Multi-Step Conversion

• Grams of compound → Moles of compound → Moles of element → Grams of element

Example: Determine the mass of carbon in 55.4 g of :

• Molar mass

• 10 mol C per 1 mol

Mass Percent Composition of Compounds

Definition and Calculation

• Mass percent composition is the percentage by mass of each element in a compound.

• Formula:

Example: A 0.358 g sample of chromium reacts with oxygen to form 0.523 g of metal oxide. What is the mass percent of chromium?

Mass Percent Composition as a Conversion Factor

Using Mass Percent in Calculations

• Mass percent can be used as a conversion factor between the mass of an element and the mass of the compound.

• For example, if a compound is 39% Na by mass, then 100 g of the compound contains 39 g Na.

Example: Find the mass of NaCl that contains 2.4 g of Na (39% Na):

Mass Percent Composition from a Chemical Formula

Calculation from Formula

• Mass percent can be determined from the chemical formula using molar masses.

• Formula:

Example: Find the mass percent of Cl in Freon-112 ():

• Molar mass Cl = 35.45 g/mol, 4 Cl atoms

• Molar mass

Conversion Factors from Chemical Formulas

Using Chemical Formulas for Quantitative Relationships

• Chemical formulas provide relationships between numbers of atoms, moles, and masses.

• These relationships are essential for determining the amounts of constituent atoms or molecules in a given sample.

Practice and Application

Worked Examples

• How many moles of are in 4.49 g?

• How many moles of P?

• How many moles of O?

Summary Table: Key Conversion Factors