Chapter 1: Chemistry and Measurements – Study Notes

Chemistry and Measurements

Introduction to Chemistry and Measurements

Chemistry is the study of matter, its properties, and the changes it undergoes. Accurate measurement is fundamental in chemistry, as it allows scientists to quantify observations and communicate results effectively. Measurements are essential in both laboratory and clinical settings, such as monitoring patient health and administering medications.

Units of Measurement

Metric and SI Units

Scientists use the metric system and the International System of Units (SI) as standardized methods for measuring physical quantities. These systems provide consistency in scientific communication worldwide.

• Length: meter (m)

• Volume: liter (L) in metric, cubic meter (m3) in SI

• Mass: gram (g) in metric, kilogram (kg) in SI

• Temperature: degree Celsius (°C) in metric, kelvin (K) in SI

• Time: second (s)

Measurement	Metric Unit	SI Unit
Volume	liter (L)	cubic meter (m3)
Length	meter (m)	meter (m)
Mass	gram (g)	kilogram (kg)
Temperature	degree Celsius (°C)	kelvin (K)
Time	second (s)	second (s)

Measured Numbers and Significant Figures

Measured vs. Exact Numbers

Measured numbers are obtained through measurement and contain some degree of uncertainty. Exact numbers are obtained by counting or by definition and have no uncertainty.

• Measured Number: Determined using instruments (e.g., length, mass, temperature).

• Exact Number: Obtained by counting (e.g., 3 apples) or defined relationships (e.g., 1 ft = 12 in.).

Reading Measurements and Estimation

When measuring, the last digit is always an estimate, reflecting the uncertainty in the measurement. For example, when using a ruler, the value between the smallest marks is estimated.

Significant Figures (SFs)

Significant figures reflect the precision of a measured number. They include all certain digits plus one estimated digit.

• All nonzero digits are significant.

• Zeros between nonzero digits are significant.

• Zeros at the end of a decimal number are significant.

• Leading zeros are not significant.

• Placeholder zeros in large numbers without a decimal are not significant.

Rule	Example	SFs
Nonzero digits	4.5 g	2
Zeros between digits	205 °C	3
Zeros at end of decimal	16.00 mL	4
Leading zeros	0.0004 s	1
Placeholder zeros	850 000 m	2

Scientific Notation and Significant Zeros

Scientific notation is used to clearly indicate significant zeros in large or small numbers. For example, $3.0 \times 10^2$ m has two significant figures.

Rounding Off and Calculations with Significant Figures

When performing calculations, the number of significant figures in the result depends on the operation:

• Multiplication/Division: The result has the same number of SFs as the measurement with the fewest SFs.

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

Prefixes and Equalities

Metric Prefixes

Prefixes are used to express multiples or fractions of units in the metric system. They make it easier to represent very large or very small quantities.

Prefix	Symbol	Value	Scientific Notation
kilo	k	1,000	$1 \text{ km} = 1 \times 10^3 \text{ m}$
centi	c	0.01	$1 \text{ cm} = 1 \times 10^{-2} \text{ m}$
milli	m	0.001	$1 \text{ mg} = 1 \times 10^{-3} \text{ g}$
micro	μ (mc in medicine)	0.000001	$1 \text{ μg} = 1 \times 10^{-6} \text{ g}$
nano	n	0.000000001	$1 \text{ nm} = 1 \times 10^{-9} \text{ m}$

Equalities and Conversion Factors

Equalities express the relationship between two units that describe the same quantity. Conversion factors are fractions derived from equalities and are used to convert between units.

• Example: $1 \text{ m} = 100 \text{ cm}$ gives conversion factors $\frac{100\ \text{cm}}{1\ \text{m}}$ and $\frac{1\ \text{m}}{100\ \text{cm}}$.

Temperature

Temperature Scales

Temperature is a measure of how hot or cold an object is. The three main temperature scales are Celsius (°C), Fahrenheit (°F), and Kelvin (K).

• Celsius: Freezing point of water is 0°C, boiling point is 100°C.

• Fahrenheit: Freezing point of water is 32°F, boiling point is 212°F.

• Kelvin: Absolute zero is 0 K (no negative values), same unit size as Celsius.

Temperature Conversions

• Celsius to Fahrenheit: $T_{\text{F}} = 1.8 \times T_{\text{C}} + 32$

• Fahrenheit to Celsius: $T_{\text{C}} = \frac{T_{\text{F}} - 32}{1.8}$

• Celsius to Kelvin: $T_{\text{K}} = T_{\text{C}} + 273.15$

Problem Solving Using Unit Conversion

Unit Conversion Method

To solve problems involving unit conversions, identify the given quantity and units, determine the needed units, and use appropriate conversion factors to reach the answer. Multiple conversion factors may be needed for complex problems.

Chemistry Link to Health: Toxicology

LD50 and Toxicity

The LD50 (lethal dose, 50%) is the amount of a substance that causes death in 50% of test animals, usually expressed in mg/kg of body mass. Lower LD50 values indicate higher toxicity.

Substance	LD50 (mg/kg)
Table sugar	29,700
Boric acid	5,140
Baking soda	4,220
Table salt	3,300
Ethanol	2,080
Aspirin	1,100
Codeine	800
Oxycodone	480
Caffeine	192
DDT	113
Cocaine (injected)	95
Dichlorvos	56
Ricin	30
Sodium cyanide	6
Parathion	3

Density

Definition and Calculation

Density is the ratio of mass to volume and is a characteristic property of substances. It is used to identify substances and determine whether objects will float or sink in a fluid.

• Formula: $\text{Density} = \frac{\text{mass}}{\text{volume}}$

• Units: g/mL (solids and liquids), g/L (gases)

Substance	Density (g/mL or g/L)
Cork	0.26
Ice	0.92
Water	1.00
Aluminum	2.70
Lead	11.3
Air (dry, g/L)	1.29

Volume Displacement Method

The volume of an irregular solid can be determined by the amount of water it displaces. The density is then calculated using the measured mass and displaced volume.

Concept Map: Chemistry and Measurements