Measurement and Unit Conversions

SI Units and Metric Prefixes

Understanding the International System of Units (SI) and metric prefixes is essential for accurate scientific measurement and conversion.

• SI Base Units: The standard units for mass (kilogram, kg), length (meter, m), volume (liter, L), and temperature (Celsius, °C or Kelvin, K).

• Metric Prefixes: Used to express multiples or fractions of base units. For example, milli- (m, 10-3), kilo- (k, 103), micro- (μ, 10-6), nano- (n, 10-9).

Example: To convert 384 liters to milliliters, multiply by 1,000 (since 1 L = 1,000 mL):

Significant Figures

Significant figures reflect the precision of a measurement. When performing calculations, the result should have the same number of significant figures as the least precise measurement.

• Rules: 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.

Temperature Conversions

• Celsius to Kelvin:

• Celsius to Fahrenheit:

• Fahrenheit to Celsius:

Density and Calculations

Density Formula

Density is the mass of a substance per unit volume.

• Formula:

• Units: Commonly expressed in g/mL or g/cm3.

Example: If a 25.0 mL sample has a mass of 17.2 g:

Specific Heat and Calorimetry

Specific Heat Capacity

Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Formula:

• Where: = heat (cal), = specific heat (cal/g°C), = mass (g), = change in temperature (°C)

Example: How many calories are needed to heat 156 g of aluminum from 21.9°C to 128°C? (SH = 0.220 cal/g°C)

Atomic Structure and Electron Configuration

Subatomic Particles

• Protons (p+): Positively charged, found in the nucleus, determine atomic number.

• Neutrons (n0): Neutral, found in the nucleus, contribute to mass number.

• Electrons (e-): Negatively charged, found in orbitals around the nucleus.

Electron Configuration and Orbital Diagrams

Electrons fill orbitals in a specific order (Aufbau principle), following the Pauli exclusion principle and Hund's rule.

• Order of Filling: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p, etc.

• Example: Sodium (Na, atomic number 11): 1s2 2s2 2p6 3s1

Isotopes

• Definition: Atoms of the same element with different numbers of neutrons (different mass numbers).

• Example: and are isotopes of chlorine.

Ions and Ionic Compounds

Formation of Ions

• Cations: Positively charged ions formed by loss of electrons (e.g., Na+).

• Anions: Negatively charged ions formed by gain of electrons (e.g., Cl-).

Table: Ion Formation

Lewis Dot Structures for Ions

• Show valence electrons as dots around the element symbol.

• Arrows indicate electron transfer in ionic bond formation.

• Example: Na + Cl → Na+ + Cl- (with electron transfer shown by arrows).

Formulas of Ionic Compounds

• Combine cations and anions in ratios that yield a neutral compound.

• Example: Magnesium oxide: Mg2+ and O2- combine to form MgO.

Covalent Compounds and Molecular Geometry

Lewis Dot Structures

• Represent valence electrons as dots around atomic symbols.

• Shared pairs (bonds) are shown as lines or pairs of dots.

VSEPR Theory and Molecular Shape

• VSEPR: Valence Shell Electron Pair Repulsion theory predicts molecular shapes based on repulsion between electron groups.

• Common Shapes: Linear, bent, trigonal planar, tetrahedral, trigonal pyramidal.

Table: Molecular Geometry and Polarity

Classification of Matter and Changes

States and Properties of Matter

• States: Solid, liquid, gas.

• Physical Change: Change in state or appearance without changing composition (e.g., melting ice).

• Chemical Change: Produces new substances (e.g., burning fuel).

Mixtures and Pure Substances

• Homogeneous Mixture: Uniform composition (e.g., salt water).

• Heterogeneous Mixture: Non-uniform composition (e.g., sand in water).

Additional Info

• Some questions require application of concepts to real-world scenarios, such as medication dosing and density calculations.

• Electron configuration and periodic table trends are foundational for understanding chemical reactivity and bonding.