BackIntroduction to Chemistry: Measurement, Significant Figures, and Unit Conversions
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Measurement and Units in Chemistry
Metric System Units
In chemistry, measurements are made using the metric system, which is based on standardized units for length, mass, and other quantities.
Length: The metric unit for length is the meter (m).
Mass: The basic unit of mass in the metric system is the gram (g).
Smallest Particle: The smallest particle that represents an element is the atom.
Significant Figures
Counting Significant Figures
Significant figures indicate the precision of a measured or calculated quantity. The number of significant figures in a value is determined by the digits that are known with certainty plus one estimated digit.
Example: The value 6.022140857 x 1023 (Avogadro's constant) has 12 significant figures.
Example: The diameter of Saturn, 72,367.4 mi, has 6 significant figures.
Example: has 7 significant figures.
Example: m/s has 4 significant figures.
Example: g has 2 significant figures.
Metric Prefixes and Symbols
Common Metric Prefixes
Metric prefixes are used to express multiples or fractions of units. The following table summarizes some common prefixes:
PREFIX | SYMBOL | MULTIPLE |
|---|---|---|
tera | T | 1,000,000,000,000 |
giga | G | 1,000,000,000 |
centi | c | 0.01 |
milli | m | 0.001 |
Scientific Notation and Decimal Form
Converting Between Forms
Scientific notation is used to express very large or very small numbers. To convert to decimal form, move the decimal point according to the exponent.
Example: C = 0.0000000000000000001602 C
Example: m/s = 299,800,000 m/s
Example: Nm2C-2 = 0.000000009 Nm2C-2
Example: m = 25,740,000 m
Making Measurements and Reporting Significant Figures
Reading Instruments
When measuring with instruments, report values to the correct number of significant figures, including one estimated digit.
Example: A ruler measurement: 2.35 cm
Example: A graduated cylinder measurement: 36.5 mL
Mathematical Operations with Significant Figures
Addition and Subtraction
When adding or subtracting, the result should have the same number of decimal places as the measurement with the fewest decimal places.
Example: (rounded to 59.17 based on decimal places)
Example: (rounded to 0.4638 based on decimal places)
Percent Composition
Calculating Percent by Mass
Percent composition is the percentage by mass of each element in a compound or mixture.
Formula:
Example: For a sample of white gold: platinum
Density Calculations
Using Density to Find Mass or Volume
Density relates mass and volume and is a key concept in chemistry.
Formula:
Example: If the density of alcohol is g/mL, the mass in mL is g
Unit Conversions
Converting Between Units
Unit conversions are essential for solving chemistry problems. Use conversion factors to change from one unit to another.
Example: To convert gallons to liters:
Example: To find the number of buckets needed: buckets
Time and Speed Calculations
Calculating Travel Time
Use the relationship between distance, speed, and time to solve problems.
Formula:
Example:
Temperature Conversions
Converting Between Fahrenheit, Celsius, and Kelvin
Temperature can be expressed in different units. Use the following formulas for conversion:
Fahrenheit to Celsius:
Celsius to Kelvin:
Example:
Example:
