Chemistry and Measurements: Study Notes for GOB College Students

Chapter 2: Chemistry and Measurements

Overview of Chemistry and Measurements

Chemistry relies on precise measurements to describe matter and its changes. Understanding units, significant figures, and conversion factors is essential for accurate scientific communication and calculations.

Units of Measurement

Importance of Units

• Units provide context to numerical values, indicating what property is being measured (e.g., mass, length, volume).

• Always include units with measurements to avoid ambiguity.

The Metric System and SI Units

• The metric system is used globally by scientists and health professionals for its logical structure based on powers of ten.

• The International System of Units (SI) is a modern form of the metric system and is the official system in most countries.

Volume

• Volume is the space occupied by a substance.

• Metric unit: liter (L); SI unit: cubic meter (m3).

• Common conversions: 1 L = 1.06 qt, 1 L = 1000 mL, 946 mL = 1 qt.

Length

• Length is measured in meters (m) in the metric system.

• 1 m = 1.09 yd, 1 m = 39.4 in., 2.54 cm = 1 in., 1 m = 100 cm.

Mass

• Mass is the measure of the quantity of matter in an object.

• Metric unit: gram (g); SI unit: kilogram (kg).

• Weight is mass under the influence of gravity: $\text{Weight} = \text{mass} \times \text{gravity}$.

• Common conversions: 1 kg = 1000 g, 1 kg = 2.20 lb, 454 g = 1 lb.

Temperature

• Metric unit: Celsius (°C); SI unit: Kelvin (K).

• Water freezes at 0°C (273 K), boils at 100°C (373 K).

• Kelvin does not use the degree symbol; absolute zero is 0 K.

Time

• Metric and SI unit: second (s).

• Conversions: 1 day = 24 hr, 1 hr = 60 min, 1 min = 60 s.

Measured Numbers and Significant Figures

Measured Numbers

• Obtained using measuring devices; include all certain digits plus one estimated digit.

• The last digit is always an estimate, reflecting the precision of the instrument.

Significant Figures (Sig Figs)

• Indicate the precision of a measured value.

• Rules:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Trailing zeros in a decimal number are significant.

  • Leading zeros are not significant.

  • Zeros used as placeholders without a decimal are not significant.

Exact Numbers

• Obtained by counting or defined equalities (e.g., 1 L = 1000 mL).

• Have an unlimited number of significant figures and do not affect the precision of calculations.

Significant Figures in Calculations

Rounding Rules

• When rounding, look at the first digit to be dropped:

• If it is 4 or less, drop it and all digits after.

• If it is 5 or greater, increase the last retained digit by 1 and drop all following digits.

• Maintain the magnitude of the number (replace dropped digits with zeros if necessary).

Multiplication and Division

• The final answer should have the same number of significant figures as the measurement with the fewest significant figures.

Addition and Subtraction

• The final answer should have the same number of decimal places as the measurement with the fewest decimal places.

Prefixes and Equalities

Metric System Prefixes

• Prefixes are used to indicate multiples or fractions of units (e.g., kilo-, centi-, milli-).

• Common prefixes: kilo- (103), centi- (10-2), milli- (10-3), micro- (10-6), nano- (10-9).

Equalities and Conversion Factors

• Equalities show the relationship between two units that measure the same quantity (e.g., 1 m = 100 cm).

• Can be written as fractions called conversion factors to convert between units.

• Defined equalities (within metric or US system) are exact; metric-US conversions may have limited significant figures unless defined (e.g., 1 in = 2.54 cm is exact).

The Cubic Centimeter (cm3 or cc)

• 1 cm3 = 1 mL; used interchangeably in medicine and science.

Problem Solving Using Unit Conversion

General Steps for Unit Conversion

1. Identify the given unit and the unit needed.

2. Write a plan for the conversion.

3. Identify the appropriate conversion factor(s).

4. Set up the equation and solve, ensuring correct significant figures.

Density and Specific Gravity

Density

Density is a physical property that relates the mass of a substance to its volume. It is unique for each substance and can be used to identify materials or predict whether an object will sink or float in water.

• Formula: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

• Common units: solids and liquids (g/mL), gases (g/L).

Calculating Density

• Measure mass and volume, then apply the formula above.

• Example: If a 0.258 g sample has a volume of 0.215 mL, $\text{Density} = \frac{0.258\ \text{g}}{0.215\ \text{mL}} = 1.20\ \text{g/mL}$ (rounded to 3 sig figs).

Using Density in Calculations

• To find mass: $\text{Mass} = \text{Density} \times \text{Volume}$

• To find volume: $\text{Volume} = \frac{\text{Mass}}{\text{Density}}$

Volume Displacement Method

• Used to determine the volume of irregular solids by measuring the change in liquid volume when the object is submerged.

• Formula: $\text{Volume} = \text{Final Volume} - \text{Initial Volume}$

Specific Gravity

• Specific gravity is the ratio of the density of a substance to the density of water at 4°C (1.00 g/mL).

• Formula: $\text{Specific Gravity} = \frac{\text{Density of Sample}}{\text{Density of Water}}$

• It is a unitless quantity and is commonly used in medical settings (e.g., urine analysis).

Applications in Medicine

• Specific gravity of urine is used to assess hydration and kidney function.

• Normal range for urine: 1.003 to 1.030.

• Abnormal values may indicate medical conditions such as diabetes, dehydration, or kidney disease.

Tables

Some Normal Laboratory Test Values

Substance in Blood	Normal Range
Albumin	3.5–5.4 g/dL
Ammonia	20–70 μg/dL (mcg/dL)
Calcium	8.5–10.5 mg/dL
Cholesterol	105–250 mg/dL
Iron (male)	80–160 μg/dL (mcg/dL)
Protein (total)	6.0–8.5 g/dL

Daily Values (DV) for Selected Nutrients

Nutrient	Amount Recommended
Calcium	1.0 g
Copper	2 mg
Iodine	150 μg (150 mcg)
Iron	18 mg
Magnesium	400 mg
Niacin	20 mg
Phosphorus	800 mg
Potassium	3.5 g
Selenium	70 μg (70 mcg)
Sodium	2.4 g
Zinc	15 mg

Additional info: This guide covers all foundational aspects of measurements in chemistry, including units, conversions, significant figures, and their application in laboratory and medical contexts. Mastery of these concepts is essential for success in general, organic, and biological chemistry courses.