Lecture Notes: Units and Measurements in Chemistry

Introduction to Units

• Units such as lumens, foot, and kilograms are arbitrary.
• Example: Feeling 9 volts by closing a circuit with the tongue.
• The International Prototype Kilogram (IPK) defines the kilogram.
  • A kilogram is the mass equivalent to the IPK.
  • Units like seconds, weeks, volts, and newtons are human-made.

Base and Derived Units

• The International System of Units has seven base units.
  • All other units are derived.
• Examples of derived units:
  • Speed: Length/Time
  • Acceleration: Speed/Time (m/s²)
  • Force: Mass x Acceleration (F=ma)
  • Work: Force x Distance
  • Power: Work/Time
• Infinite possible derived units, but not all are named.

Naming Units After Scientists

• Units like watts and hertz are lowercase.
• Named after scientists: Watt, Hertz.
• Having a unit named after you is prestigious.

Measurement: Seconds

• One standard unit is based on a real value: The second.
  • Defined based on Earth’s rotation.
  • Earth’s rotation is slowing, but seconds remain constant for consistency.
• Leap seconds are added to adjust for Earth's slowing rotation.

Importance of Units in Science

• Crucial in chemistry and science to avoid errors.
• Mars Climate Orbiter crashed due to unit mismanagement.

Converting Units

• Example: Converting 60 miles/hour to lightyears/second.
  • Process involves canceling units.
  • Result: 9.3 * 10^-12 lightyears/second.*

Exact vs Measured Numbers

• Exact Numbers: Fully defined, e.g., seconds in a minute.
• Measured Numbers: Have inherent uncertainty, e.g., car speed.
  • Measured to a certain precision.
• Importance of precision in calculations.

Significant Figures

• Preserve precision in calculations.
  • Addition/Subtraction: Focus on decimal places.
  • Multiplication/Division: Based on least precise measurement.
• Scientific notation simplifies determining significant figures.

Practical Applications

• Unit conversion is widely applicable.
• Using correct significant figures prevents misinformation.
• Being precise in measurements is essential in scientific calculations.

Conclusion

• Understanding units and significant figures is crucial in chemistry and beyond.
• Significant figures help maintain precision and honesty in reporting data.
• Engaging with units can improve scientific literacy and accuracy in experiments.