GOB Chemistry: Foundational Concepts, Calculations, and Applications

Scientific Method and Measurement

Steps of the Scientific Method

The scientific method is a systematic approach used in scientific study to investigate observations, solve problems, and test hypotheses. The steps are:

• Observation: Gathering information through the senses or instruments.

• Question: Formulating a question based on the observation.

• Hypothesis: Proposing a tentative explanation or prediction that can be tested.

• Experiment: Designing and conducting experiments to test the hypothesis.

• Data Collection and Analysis: Recording and analyzing the results of the experiment.

• Conclusion: Drawing conclusions based on the data; determining whether the hypothesis is supported or refuted.

• Communication: Sharing results with the scientific community.

Measured vs. Exact Numbers and Significant Figures

In chemistry, it is important to distinguish between measured numbers (obtained by measurement, subject to uncertainty) and exact numbers (defined values or counted quantities, with no uncertainty). Significant figures (SFs) indicate the precision of a measured value.

• Measured Numbers: Obtained using instruments; have a limited number of significant figures.

• Exact Numbers: Defined or counted values; have an infinite number of significant figures.

Examples:

• 0.170 L (measured; 3 SFs)

• 6.3 × 10−6 (measured; 2 SFs)

• 4 knives (exact; infinite SFs)

• 1 m = 100 cm (exact; infinite SFs)

Significant Figures in Calculations

Rules for Significant Figures

• In multiplication/division, the result should have as many SFs as the value with the fewest SFs.

• In addition/subtraction, the result should have as many decimal places as the value with the fewest decimal places.

Example Calculations:

• a. $45.25 \times 0.01088$ (2 SFs in result)

• b. $2.63/24$ (2 SFs in result)

• c. $(4.0 \times 8.00)/(16)$ (2 SFs in result)

Applications of Chemistry in Health and Medicine

Dosage Calculations

Calculating the number of tablets or the amount of a substance required for a specific dose is a common application in health sciences.

• Example: If a tablet contains 75 mg of a drug and the prescribed dose is 0.150 g, convert grams to milligrams and divide by the amount per tablet to find the number of tablets needed.

Density Calculations

Density is defined as mass per unit volume:

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

• Units: g/mL or g/cm3

• Example: If 0.38 g of LDL is in 0.362 mL, density = $\frac{0.38}{0.362}$ g/mL

Unit Conversions in Medicine

• To convert volume of blood to mass: $\text{Mass} = \text{Volume} \times \text{Density}$

• Example: 3.0 L of blood, density = 1.06 g/mL. Convert L to mL, then multiply by density.

Classification of Matter

Pure Substances vs. Mixtures

• Pure Substance: Has a fixed composition; can be an element or a compound.

• Mixture: Contains two or more substances physically combined; can be homogeneous (uniform) or heterogeneous (non-uniform).

Examples:

• Element: Oxygen (O2)

• Compound: Water (H2O)

• Homogeneous mixture: Salt water

• Heterogeneous mixture: Salad

Energy and Temperature

Energy Units and Conversions

• Joule (J): SI unit of energy

• Calorie (cal): 1 cal = 4.184 J

• Example: To convert 360 J to calories: $\text{cal} = \frac{360}{4.184}$

Temperature Conversions

• Celsius to Kelvin: $K = ^\circ C + 273.15$

• Celsius to Fahrenheit: $^\circ F = (^\circ C \times \frac{9}{5}) + 32$

• Example: 113°F to °C: $^\circ C = (^\circ F - 32) \times \frac{5}{9}$

Heat Calculations

Heat lost or gained can be calculated using:

$q = m \times c \times \Delta T$

• q = heat (J or cal)

• m = mass (g)

• c = specific heat (J/g·°C or cal/g·°C)

• ΔT = change in temperature (°C)

Chemical and Physical Changes

Classification

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

• Chemical Change: Change that produces new substances (e.g., rusting, burning).

Examples:

• Gold is hammered to form gold leaf: Physical

• Gasoline burns in air: Chemical

• Garlic is chopped: Physical

Elements, Atoms, and the Periodic Table

Atomic Structure

• Protons: Positively charged particles in the nucleus; define the element.

• Neutrons: Neutral particles in the nucleus; number = mass number − atomic number.

• Electrons: Negatively charged particles in orbitals around the nucleus; in a neutral atom, number equals protons.

Example: For an atom with mass number 68 (zinc):

• Protons: 30 (atomic number of Zn)

• Neutrons: 68 − 30 = 38

• Electrons: 30 (if neutral)

Isotopes

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

• Notation: $^{A}_{Z}\text{X}$, where A = mass number, Z = atomic number, X = element symbol.

• Example: Vanadium-50: $^{50}_{23}\text{V}$; Vanadium-51: $^{51}_{23}\text{V}$

Electron Arrangement and Lewis Symbols

• Electron Arrangement: Distribution of electrons in shells around the nucleus (e.g., O: 2,6; Cl: 2,8,7).

• Lewis Symbol: Element symbol surrounded by dots representing valence electrons.

Periodic Table Classification

• Groups: Vertical columns; elements in the same group have similar properties.

• Periods: Horizontal rows.

• Metals, Nonmetals, Metalloids: Metals are typically shiny, conductive, malleable; nonmetals are brittle, non-conductive; metalloids have intermediate properties.

Ionization Energy

• Ionization Energy: The energy required to remove an electron from an atom in the gas phase.

• Increases across a period (left to right), decreases down a group (top to bottom).

• Example: Cl has higher ionization energy than Mg; F has higher than N or C.

Sample Table: Classification of Substances

Substance	Type	Homogeneous or Heterogeneous
Element (e.g., O2)	Pure Substance	Homogeneous
Compound (e.g., H2O)	Pure Substance	Homogeneous
Salt water	Mixture	Homogeneous
Salad	Mixture	Heterogeneous

Additional info:

• Some questions require students to perform calculations or conversions, such as unit conversions, density, and temperature.

• Other questions focus on conceptual understanding, such as classifying changes or drawing Lewis structures.