GOB Chemistry Study Guide: Fundamental Concepts and Problem Solving

Measurement and Significant Figures

Understanding Significant Figures

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

• Rules for Counting Significant Figures:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Leading zeros are not significant.

  • Trailing zeros are significant only if there is a decimal point.

• Example: The number 4.192 has four significant figures.

Calculations: When adding or subtracting, the result should have the same number of decimal places as the measurement with the fewest decimal places. When multiplying or dividing, the result should have the same number of significant figures as the measurement with the fewest significant figures.

Example Calculation:

• Solve: $4.192 + 9.13 + 0.471$

• Answer: $4.192 + 9.13 + 0.471 = 13.793$ (rounded to two decimal places: $13.79$)

Units, Conversions, and Dimensional Analysis

Metric System and Unit Conversions

The metric system is used for scientific measurements. Common units include meters (m), liters (L), and grams (g). Dimensional analysis is a method to convert between units using conversion factors.

• Example: To convert 49.9 mL to liters, use the conversion $1~\text{L} = 1000~\text{mL}$.

• Calculation: $49.9~\text{mL} \times \frac{1~\text{L}}{1000~\text{mL}} = 0.0499~\text{L}$

Writing Conversion Factors: Conversion factors are ratios that express how many of one unit are equal to another unit.

• Example: $1~\text{hour} = 60~\text{minutes}$, so $\frac{1~\text{hour}}{60~\text{minutes}}$ or $\frac{60~\text{minutes}}{1~\text{hour}}$

Physical and Chemical Changes

Definitions and Examples

Understanding the difference between physical and chemical changes is fundamental in chemistry.

• Physical Change: A change that does not alter the chemical composition of a substance (e.g., melting, freezing, boiling).

• Chemical Change: A change that results in the formation of new substances with different properties (e.g., burning, rusting).

• Example: Boiling water is a physical change; burning wood is a chemical change.

Classification of Matter

Elements, Compounds, and Mixtures

Matter can be classified based on its composition.

• Element: A pure substance made of only one kind of atom (e.g., O2).

• Compound: A substance made of two or more elements chemically combined (e.g., H2O).

• Mixture: A combination of two or more substances that are not chemically combined.

• Homogeneous Mixture: Uniform composition throughout (e.g., saltwater).

• Heterogeneous Mixture: Non-uniform composition (e.g., salad).

Atomic Structure

Subatomic Particles

Atoms are composed of protons, neutrons, and electrons.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Electron: Negatively charged particle outside the nucleus.

Atomic Number (Z): Number of protons in the nucleus.

Mass Number (A): Total number of protons and neutrons.

• Example: An atom with atomic number 4 and mass number 12 has 4 protons, 8 neutrons, and 4 electrons.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Elements and the Periodic Table

Element Symbols and Names

Each element is represented by a unique chemical symbol.

• Zn: Zinc

• K: Potassium

• S: Sulfur

Periodic Table Organization: Elements are arranged by increasing atomic number. Periods are rows; groups/families are columns.

• Example: The element in period 2, family 4, atomic number 47 is Silver (Ag).

• Additional info: Family 4 is typically Group 4, but atomic number 47 is Silver (Ag), which is in Group 11. There may be a mismatch in the original question.

Density and Measurement

Density Calculations

Density is a physical property defined as mass per unit volume.

• Formula:

$\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

• Example: An object with mass 200.0 g and volume 10 mL has density $\frac{200.0~\text{g}}{10~\text{mL}} = 20.0~\text{g/mL}$

Temperature Scales

Celsius and Kelvin

Temperature can be measured in Celsius (°C) or Kelvin (K).

• Conversion Formula:

$K = ^\circ C + 273.15$

• Example: 25°C = 298.15 K

Scientific Notation

Expressing Numbers

Scientific notation is used to express very large or very small numbers in the form $a \times 10^n$.

• Example: 0.00049 = $4.9 \times 10^{-4}$

Summary Table: Classification of Matter

Type	Description	Example
Element	Pure substance, one type of atom	O2, Fe
Compound	Pure substance, two or more elements chemically combined	H2O, NaCl
Homogeneous Mixture	Uniform composition	Saltwater, air
Heterogeneous Mixture	Non-uniform composition	Salad, sand and water

Additional info:

• Some questions in the original file reference specific exam preparation problems, so examples and explanations have been generalized for study purposes.

• Where original questions were ambiguous or incomplete, standard academic context has been provided to ensure completeness.