Measurements in Chemistry

Introduction

Accurate measurement is fundamental to all scientific work, especially in chemistry. Standardized systems of units and clear rules for expressing measurements ensure that results are reliable and universally understood.

International Standard (SI System)

Overview of the SI System

• SI (Système International d'Unités) is the modern form of the metric system, established in France in 1790.

• It is based on standards for reference, originally tied to the Earth.

• All units are based on 10 or multiples of 10, making conversions straightforward.

• SI is the official measurement system for all but three countries (including the United States).

SI Base Units

Fundamental Quantities and Their Units

• Distance: meter (m)

• Time: second (s)

• Mass: kilogram (kg)

• Temperature: kelvin (K)

• Amount of substance: mole (mol)

• Electric current: ampere (A)

• Luminous intensity: candela (cd)

SI Derived Units

Common Derived Units in Chemistry

• Energy: joule (J)

• Force: newton (N)

• Pressure: pascal (Pa)

• Power: watt (W)

Unit Prefixes

Macro Prefixes (Larger Units)

Micro Prefixes (Smaller Units)

Temperature Scales

Celsius Scale

• Named after Swedish scientist Anders Celsius.

• Freezing point of water: 0 °C

• Boiling point of water: 100 °C

• The interval between these points is divided into 100 equal parts (degrees).

Kelvin Scale (SI Unit)

• Also known as the absolute temperature scale.

• Named after Lord Kelvin.

• Freezing point of water: 273 K

• Boiling point of water: 373 K

• Does not use the degree symbol (K, not °K).

• Zero on the Kelvin scale is called absolute zero.

• Conversion formulas:

Temperature Conversion Examples

• Example 1: Convert to K.

• Example 2: The boiling point of argon is . What is this in ?

Units of Volume

Definition and Common Units

• Volume is the space occupied by matter.

• Calculated as length × width × height.

• SI unit: cubic meter ()

• Common laboratory units: liter (L) and milliliter (mL)

• 1 L = 1 dm3 = 1000 mL

Measuring Density

Definition and Formula

• Density is the ratio of mass to volume.

• Formula:

• Common units: or

Density Example Problems

• Example 1: A metal with mass 614 g and volume 78 cm3 has density (likely iron).

• Example 2: A plastic ball (15.8 g, 19.7 cm3): . Since this is less than the density of gasoline, it would float.

• Example 3: Gold (density 19.3 g/cm3, volume 0.93 cm3):

• Example 4: Silicon (density 2.33 g/cm3, mass 62.9 g):

• Example 5: Substance (density 3.26 g/cm3, volume 0.350 cm3):

Significant Figures

Definition and Rules

• Significant figures (sig figs) are the digits in a measurement that are known with certainty plus one estimated digit.

• Rules:

  • All non-zero digits are significant.

  • Zeros between significant digits are significant.

  • Trailing zeros in the decimal portion are significant.

• When rounding, the answer cannot be more precise than the least accurate measurement.

Significant Figures in Calculations

• Addition/Subtraction: The result should have no more digits to the right of the decimal than the measurement with the least such digits.

• Multiplication/Division: The result should have no more significant figures than the measurement with the fewest significant figures.

Dimensional Analysis

Method for Unit Conversion

• Dimensional analysis is a systematic method for converting between units using conversion factors.

• Set up the problem so that units cancel appropriately, leaving the desired unit.

• Example: To convert 15 mg to g:

Scientific Notation

Purpose and Rules

• Scientific notation is used to express very large or very small numbers in a compact form.

• Format: , where and is an integer.

• Move the decimal point so that only one non-zero digit is to its left; the number of moves determines the exponent.

Scientific Notation Examples

• 5000 m = m

• 0.0006 g = g

• 14 km = km

• 12 mg = mg