Chapter 2 - Scientific Method
The importance of measurement
Accuracy and precision [Practice
]
Significant digits in measurements [Practice
]
Significant digits in calculations
The metric system [Practice
] [Reference]
Units: length [Practice
]
Units: volume [Practice
]
Units: mass [Practice
]
Measuring density [Practice
] [Reference]
Specific gravity
Measuring temperatures [Practice
]
Measuring heat
Specific heat capacity [Practice
] [Reference]
Chapter 2 2-1 Measurement- Quantitative measurements
produce a definite, specific value. - Qualitatvie measurements are usually nonnumerical descriptions.
- Measurements remove doubt from the minds of and settle disagreements among several observers.
2-2 Accuracy vs. Precision [Practice]- Accuracy is the state of correctness (right or wrong). - Precision
is how specific your answer (either right or wrong) is. - You can be accurate and not precise: (4.2 * 2 = 8) - You can be precise and not accurate (2 + 2 = 9.1352845834589)
- You can be inaccurate and imprecise (2 + 2 = 9) - You can be both accurate and precise (4.1212 + 8.1212 = 12.2424) 2-3 Significant Digit [
Practice]s- Examples: - 4 (1 significant digit)
- 40000 (1 significant digit) - 4.0000 x 10^4 (5 significant digits) - 4.0000 (5 significant digits) - 0.00004 (1 significant digit)
- 0.40000 (5 significant digits) - 4001.0 (5 significant digits)
2-4 Significant Digits in Calculations- Addition and subtraction: answer can have no more digits to the right of the decimal place than the term
with the least number of right-of-the-decimal-point numbers had. Examples: - 4 + 4 = 8 (each term has 0 numbers to right of decimal, so answer has 0 as well).
- 4.1 + 4 = 8 (term with lowest number of right-of-the-decimal-point numbers dictates answer length) - 4.1111111111 + 4 = 8 (same as previous)
- 4.25 + 4.25 = 8.50 (terms both have 2 right-of-decimal-point numbers, so answer does as well)
- 4.5 + 4 = 9 (2nd term has 0 right-of-decimal-point numbers, so answer does as well. 8.5 [rounded] -> 9)
- Multiplication and division: answer can have no more significant digits than the term with the least number of them. Examples:
- 2 x 2 = 4 (both terms have 1 sig. digit, so answer does as well) - 9 x 9 = 80 (each term has 1 sig. digit, so answer does as well [rounding])
- 4 x 4 = 20 (same as above, with rounding up) - 400 x 400 = 200000 (each term still only has 1 signficant digit)
- 4 x 4.12355366453 = 20 (first term only has one sig. digit) 2-5 The Metric System [
Practice] [Reference]
- All units based on 10 or multiples of 10. - SI = systeme international d'unites. Revised metric system. - Simple; used by all scientists. - SI:
- Length: meter (m) - Mass: kilogram (kg) - Time: second (s) - Electric current: ampere (A) - Thermodynamic temperature: kelvin (K)
- Amount of substance: mole (mol) - Luminous intensity: candela (cd) - [derived] Volume: cubic meters (m^3)
- [derived] Density: grams per cubic centimeter (g/(cm^3)) - Pressure: pascal (pa) - Energy: joule (J) 2-6 Units of Length [
Practice] [Reference]- meter. - see Chempire's Online Metric Prefixes Chart for prefixes. 2-7 Units of Volume [Practice] [Reference]-- 1 millileter (mL) = 1 cm^3.
2-8 Units of Mass [Practice] [Reference
]-- Mass is how much matter is in something. - "Weight" ¹
"Mass." Weight is the gravitational pull on something. (You weigh less on the moon than you do on earth, but your mass stays the same.) 2-9 Measuring Density [Practice] [Reference
]- Density is the ratio of mass to volume.
- A styrofoam peanut has the same volume of a real peanut, yet it has less mass. Therefore it is less dense than a real peanut. There's less stuff in the same amount of space.
- D = m/v 2-10 Specific Gravity- ratio of densities
- Specific gravity = (density of substance)/(density of water) 2-11 Measuring Temperature [
Practice]- Temperature is degree of hotness or coolness. It is the average amount of energy in the substance. - Scales:
- Fahrenheit: not used in chemistry. - Celsius: metric scale. 0°C = freezing of water, 100°C = boiling of water, -273°C = absolute zero (coldest anything can get).
- Kelvin: SI scale. 273 K = freezing of water, 373 K = boiling of water, 0 K = absolute zero (coldest anything can get). 2-12 Measuring Heat- Heat is the total amount of energy in a substance. - Units: - Joule: SI unit.
- The calorie (cal): amount of energy needed to raise 1 g of pure water 1°C (or 1 K) - The Calorie (C): 1 Calorie = 1000 calories = 1 kilocalorie. 2-13 Specific Heat Capacity [Practice] [Reference
]
- The specfic heat (SpH) of a substance is the amount of heat (energy) needed to raise 1 g of the substance 1° (1 K) in temperature.
- SpH = (heat [J or cal])/(mass [g] x change in temperature [degrees celsius])
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