Measurements - Lesson 2 Summary - GEN-CHEM1 PDF

**LESSON 2: "MEASUREMENTS"** **Objectives:** - Differentiate between precision and accuracy; - Apply the rules for significant figures in calculations - Determine the density of liquids and solids **Measurement** is the assignment of numerical value to a physical property. **Unit** provi...

**LESSON 2: "MEASUREMENTS"** **Objectives:** - Differentiate between precision and accuracy; - Apply the rules for significant figures in calculations - Determine the density of liquids and solids **Measurement** is the assignment of numerical value to a physical property. **Unit** provide a standard reference point for measuring, allowing for consistent communication and comparison of quantities. **International System of Units** (SI) is the modern version of the metric system. **"Base Units"** **Quantity** **Unit** **Symbol** --------------------- ---------- ------------ Mass Kilogram kg Electric Current Ampere A Time Second s Temperature Kelvin K Amount of substance Mole mol Length Meter m Luminous intensity Candela cd **Metric System** uses decimal multiples, simplifying calculations. Based on powers of ten. Used globally in scientific contexts. "**Prefix and Notation"** The **prefix** is independent of the base unit it modifies. In conversion, it changes the unit of the measurement without changing its value. **Uncertainty in Measurement** - **Random Error** -- fluctuations in measurements due to uncontrollable factors - **Systematic Error** - consistent errors in measurements due to faulty tools or methods. - **Precision** -- consistency of repeated results. If you measure a quantity several times and the value agree closely with one another, then it is precise. - How **reproducible** a measurement is, regardless of accuracy. - **Accuracy** -- is how close the value to its **"true"** value. **Significant Figures** -- are digits in any measurement that are known with certainty, plus one uncertain digit. **Rules for Counting Significant Figures** 1. All nonzero digits are significant 2. Zeros between nonzero digits are significant 3. Zeros to the left of the first nonzero digit are not significant 4. If the number is less than 1, the only zero at the end of the number and the zeros between nonzero digits are significant. [Leading zero is not significant] 5. If the number is greater than 1, then all zeros written to the right of the decimal point are significant. [Trailing zeros after a decimal point are significant.] 6. For the numbers with trailing zero but without decimal point, the zeros may NOT significant. 7. Zeros used as place holders in a large number without a decimal point are not significant. **Scientific notation** -- the process of simplifying a very large number or a very small number. It is also a system in which an ordinary decimal number is expressed as a product of a number between 1 and 10 and a power of 10. - Exponent is positive if the decimal point is moved to the left. - Exponent is negative if the decimal point is moved to the right. **"Conversion of Units"** **Dimensional analysis** -- a process in which a conversion factor written in a form of ratio is used to change given in data to the unit desired. **Steps:** 1. Write the unknown quantity that is sought including the units. 2. Write all the known conversion factors needed. 3. Begin with what is known and multiply it by identified conversion factors, cancelling similar units to get the known unit. **Density Measurement** -- one common measurement used in the laboratory, involves getting the mass, volume, and temperature of an object. - **Mass** (quantity of matter in an object, g/kg) - **Volume** (amount of space occupied by a substance) - **Temperature** (how hot or cold an object is, celcius or Fahrenheit, K) - K = celcius + 273.15 - K= (F+ 459.67) 5/9 - **Density** (ratio of mass of an object to the volume it occupies.