Measurement & Significant Figures

Understanding Significant Figures

Significant figures are the digits in a measurement that are known with certainty plus one digit that is estimated. They reflect the precision of a measured quantity.

• Determining Significant Figures: Count all digits except leading zeros and trailing zeros without a decimal point.

• Rounding Calculations: Round the final answer to the correct number of significant figures based on the calculation type.

• Calculation Rules:

  • Addition/Subtraction: The result should have the same number of decimal places as the measurement with the fewest decimal places.

  • Multiplication/Division: The result should have the same number of significant figures as the measurement with the fewest significant figures.

• Scientific Notation: Used to express very large or small numbers; only significant digits are shown.

• SI Base Units: Recognize units such as meter (m), kilogram (kg), second (s), mole (mol), ampere (A), kelvin (K), and candela (cd).

Example: In the number 0.00450, there are three significant figures (4, 5, and the trailing 0).

Dimensional Analysis

Unit Conversion and Density

Dimensional analysis is a method for converting between units using conversion factors. It is essential for solving chemistry problems involving measurements.

• Setting Up Conversions: Arrange conversion factors so units cancel, leaving the desired unit.

• Density as a Conversion Factor: Density () relates mass () and volume (): Use density to convert between mass and volume.

• Common Conversions:

  • English ↔ Metric (e.g., inches to centimeters)

  • Feet/inches → millimeters

  • Celsius ↔ Kelvin:

Example: To convert 5.0 inches to centimeters:

Atomic Structure

Subatomic Particles and Isotopes

Atoms are composed of protons, neutrons, and electrons. Their arrangement determines the properties of elements and ions.

• Protons: Positively charged particles in the nucleus; number defines the atomic number ().

• Neutrons: Neutral particles in the nucleus; number affects mass number ().

• Electrons: Negatively charged particles orbiting the nucleus; number determines charge.

• Mass Number ():

• Atomic Number ():

• Charge:

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Cations: Positively charged ions (loss of electrons).

• Anions: Negatively charged ions (gain of electrons).

Calculating Atomic Mass

• Weighted Average: Atomic mass is calculated using the percent abundance and mass of each isotope.

• Converting Percent Abundance: Divide percent by 100 to get decimal.

• Molar Mass: Mass of one mole of a substance (g/mol).

• Avogadro’s Number: particles per mole.

• Conversions:

  • Grams ↔ moles:

  • Moles ↔ grams:

  • Moles ↔ atoms:

• Compounds with Parentheses: For example, Ca(OH)2 has a molar mass calculated by summing the masses of Ca, O, and H (with O and H multiplied by 2).

Example: Calculate the molar mass of Ca(OH)2:

Chemical Nomenclature

Naming and Writing Formulas

Chemical nomenclature is the system for naming compounds and writing their formulas. It distinguishes between ionic and molecular compounds.

• Ionic Compounds: Composed of metals and nonmetals; names include cation (metal) and anion (nonmetal or polyatomic ion).

• Metals with Invariant Charges: Metals that always have the same charge (e.g., Na+, Mg2+).

• Transition Metals: May have variable charges; use Roman numerals in names (e.g., FeCl2 is iron(II) chloride).

• Polyatomic Ions: Groups of atoms with a charge; memorize names and formulas. Common Polyatomic Ions:

  • Ammonium: NH4+

  • Hydroxide: OH-

  • Nitrate: NO3-

  • Nitrite: NO2-

  • Carbonate: CO32-

  • Hydrogen carbonate (bicarbonate): HCO3-

  • Sulfate: SO42-

  • Sulfite: SO32-

  • Phosphate: PO43-

  • Cyanide: CN-

  • Acetate: C2H3O2-

• Molecular Compounds: Composed of nonmetals; use prefixes to indicate the number of atoms (mono-, di-, tri-, tetra-, penta-, etc.).

Example: CO2 is named carbon dioxide; N2O4 is dinitrogen tetroxide.

Memorization

Essential Elements, Polyatomic Ions, and Acids

Memorization is crucial for quick recall during exams. Focus on the following:

• Essential Elements: Know the names and symbols of common elements (e.g., H, He, Li, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca).

• Polyatomic Ion Table: Memorize the names, formulas, and charges of common polyatomic ions listed above.

• Acids: For this exam, focus on binary acids (e.g., HCl, HBr, HF). Binary acids consist of hydrogen and one other nonmetal.

Example: HCl is hydrochloric acid; HF is hydrofluoric acid.

Practice Calculations

Types of Calculations to Master

Be able to solve problems involving the following calculations:

• Significant figures

• Dimensional analysis

• Celsius ↔ Kelvin conversions

• Density calculations

• Weighted average atomic mass

• Molar mass

• Grams ↔ moles

• Moles ↔ grams

• Moles ↔ atoms (using Avogadro's Number)

Example: Convert 10.0 g of NaCl to moles:

Topics Not Covered on Exam 1

Excluded Topics

The following topics will not be tested on Exam 1 but may be covered in future assessments:

• Oxyacids

• Percent composition

• Empirical formulas

• Balancing chemical equations

Additional info: These topics are foundational for later chapters and should be reviewed for future exams.