Chemical Composition and Calculations in Compounds

Chemical Composition

Compounds and Molecules

Compounds are substances formed when two or more elements chemically combine in fixed ratios. Molecules are the smallest units of compounds that retain their chemical properties.

• Compound: A substance made of two or more different elements bonded together.

• Molecule: A group of atoms bonded together, representing the smallest unit of a chemical compound.

• Example: Water (H2O) is a compound made of hydrogen and oxygen atoms.

Molar Mass

The molar mass of a compound is the sum of the atomic masses of all atoms in its formula, expressed in grams per mole (g/mol).

• Definition: The mass of one mole of a substance.

• Calculation: Add the atomic masses of each element multiplied by the number of times it appears in the formula.

• Formula:

$\text{Molar Mass} = \sum (\text{number of atoms of element} \times \text{atomic mass of element})$

• Example: For CO2 (carbon dioxide):

$\text{Molar Mass of CO}_2 = (1 \times 12.01) + (2 \times 16.00) = 44.01\ \text{g/mol}$

Calculating Mass from Moles

To find the mass of a given number of moles of a compound, multiply the number of moles by the molar mass.

• Formula:

$\text{Mass} = \text{Number of moles} \times \text{Molar mass}$

• Example: Calculate the mass of 1.7 mol of water (H2O):

$\text{Molar Mass of H}_2\text{O} = (2 \times 1.01) + (1 \times 16.00) = 18.02\ \text{g/mol}$

$\text{Mass} = 1.7\ \text{mol} \times 18.02\ \text{g/mol} = 30.634\ \text{g}$

Percent Composition

Percent composition refers to the percentage by mass of each element in a compound.

• Formula:

$\text{Percent by mass of element} = \frac{\text{mass of element in 1 mole}}{\text{molar mass of compound}} \times 100\%$

• Example: For CO2:

Carbon: $\frac{12.01}{44.01} \times 100\% = 27.3\%$ Oxygen: $\frac{32.00}{44.01} \times 100\% = 72.7\%$

Sample Table: Molar Masses of Common Compounds

Compound	Formula	Molar Mass (g/mol)
Water	H2O	18.02
Carbon Dioxide	CO2	44.01
Ammonia	NH3	17.03
Sodium Chloride	NaCl	58.44
Glucose	C6H12O6	180.16

Additional info:

• Some terms and calculations were inferred from context due to unclear handwriting and fragmented notes.

• Examples and table entries were expanded for clarity and completeness.