Chemical Quantities and Calculations

Molar Mass and Mole Calculations

Understanding molar mass and mole relationships is fundamental in chemistry for converting between mass, moles, and number of particles.

• Molar Mass: The mass of one mole of a substance, expressed in g/mol.

• Calculation:

• Example: If 1.85 × 10-3 moles of a hormone weigh 0.581 g, the molar mass is .

Atoms and Molecules: Counting and Mass Relationships

Calculating the number of atoms or molecules in a sample and determining the mass of specific elements within compounds is a key skill.

• Avogadro's Number: particles/mol.

• Example: To find the number of P atoms in 2.63 g of PBr5:

  • Calculate moles of PBr5:

  • Each mole of PBr5 contains 1 mole of P atoms.

  • Number of P atoms = moles × Avogadro's number.

• Mass of Element in Compound: Use the ratio of the element's molar mass to the compound's molar mass.

Chemical Reactions and Stoichiometry

Balancing Chemical Equations

Balancing equations ensures the law of conservation of mass is obeyed.

• Steps:

  1. Write the unbalanced equation.

  2. Balance atoms one at a time, starting with the most complex molecule.

  3. Check all atoms and adjust coefficients as needed.

• Example:

Limiting Reactant and Percent Yield

Identifying the limiting reactant and calculating percent yield are essential for reaction efficiency analysis.

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Percent Yield:

• Example: If 153 g of C2H5Cl is produced but the theoretical yield is 310.12 g, percent yield is .

Atoms, Elements, and Atomic Theory

Dalton's Atomic Theory

Dalton's atomic theory forms the basis for understanding chemical reactions and the nature of matter.

• Key Postulates:

  • Atoms are indivisible and indestructible.

  • Atoms of the same element are identical in mass and properties.

  • Atoms combine in simple whole-number ratios to form compounds.

  • Atoms of different elements have different masses and properties.

  • Atoms cannot be created or destroyed in chemical reactions.

• Inconsistent Statements: Atoms can be created; atoms of a compound are fixed and cannot rearrange; atoms of the same element can have different masses (isotopes exist, but Dalton did not account for them).

Isotopes and Mass Spectrometry

Isotopes are atoms of the same element with different numbers of neutrons. Mass spectrometry can differentiate isotopes based on their mass.

• Example: Phosphorus-32 and Sulfur-32 can be distinguished if the mass spectrometer has sufficient precision, e.g., g.

Compounds, Formulas, and Nomenclature

Empirical and Molecular Formulas

The empirical formula shows the simplest whole-number ratio of atoms in a compound, while the molecular formula shows the actual number of atoms.

• Empirical Formula Calculation:

  • Divide the number of moles of each element by the smallest number of moles.

  • Round to the nearest whole number to get the ratio.

• Example: For 11.34 mol C, 11.34 mol H, and 5.67 mol O, the empirical formula is C2H2O.

• Molecular Formula Calculation: , where .

Naming Compounds and Ions

Chemical nomenclature follows systematic rules for naming compounds and ions.

• Binary Ionic Compounds: Name the cation first, then the anion (e.g., MgI2 is magnesium iodide).

• Polyatomic Ions: Recognize common ions such as sulfate (SO42-), nitrate (NO3-), and tellurate (TeO32-).

• Hydrates: Compounds with water molecules attached, e.g., Ga(NO3)3 · 5 H2O.

Classification of Matter

Types of Elements and Compounds

Matter can be classified as elements, compounds, and mixtures, with further distinctions among monoatomic, diatomic, and polyatomic species.

• Monoatomic Element: Single atom species (e.g., Argon).

• Diatomic Element: Molecules with two atoms (e.g., O2, Cl2).

• Polyatomic Element: Molecules with more than two atoms of the same element.

• Molecular Compound: Composed of molecules (e.g., C12H22O6).

• Ionic Compound: Composed of ions (e.g., NH4Cl).

Periodic Properties and Chemical Bonding

Charge-to-Mass Ratio

The charge-to-mass ratio is important in characterizing particles such as ions and subatomic particles.

• Formula: , where is charge and is mass.

• Example: For an alpha particle (He) with mass 4.00151 amu, convert mass to kg and charge to C to calculate .

Acids, Bases, and Hydrates

Acid Nomenclature

Acids are named based on their anions. For example, HBr is hydrobromic acid, H2CrO4 is chromic acid, and H2PO3 is phosphorous acid.

• Common Mistakes: Ensure correct pairing of acid names and formulas.

Organic Chemistry Basics

Hydrocarbons: Alkanes, Alkenes, Alkynes

Hydrocarbons are classified based on the types of bonds between carbon atoms.

• Alkanes: Only single bonds (e.g., CnH2n+2).

• Alkenes: At least one double bond (e.g., CnH2n).

• Alkynes: At least one triple bond (e.g., CnH2n-2).

• Example: H3CCHCHCH2CH3 is an alkene.

Tables

Classification Table: Types of Molecules

Common Polyatomic Ions and Their Names

Additional info:

• Some problems involve advanced stoichiometry, empirical/molecular formula determination, and percent yield calculations, which are essential for General Chemistry exam preparation.

• Organic chemistry classification (alkane, alkene, alkyne) is included as a foundational topic.