High School Chemistry/Introduction to Methods of Chemistry

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Chemistry is the study of the composition of matter, which is anything with mass and volume. They're five major branches of chemistry:

  1. Organic Chemistry: All substances containing the element: carbon (all living things, fuels).
  2. Inorganic Chemistry: All substances inorganic (not containing carbon {But Carbon compounds such as carbides (e.g., silicon carbide [SiC2]), some carbonates (e.g., calcium carbonate [CaCO3]), some cyanides (e.g., sodium cyanide [NaCN]), graphite, carbon dioxide, and carbon monoxide are classified as inorganic}).
  3. Analytical Chemistry: Separate and identify matter (drug testing).
  4. Physical Chemistry: Behavior of chemicals (why does nylon stretch?/reactions).
  5. Biochemistry: Chemistry of living organisms (photosynthesis, metabolism, respiration).

Measurements and Data Collection[edit | edit source]

  • Can be quantitative (numerical) or qualitative (subjective).

[qualitative deals with odor, color, and texture]

  • Must be...:
    • Accurate: How close your measurements are close to the known value.
    • Precise: Measurements are simply close to each other through repeated trials.
    • Easy to communicate

Metric System and International System of Measurement[edit | edit source]

  • Allows for scientists to easily communicate data and results.
  • Based on standard units (SI units)
    • Length (meters (m))
    • Mass (kilogram (kg))
    • Temperature (Kelvin (K)) [K = Celsuis + 273]
    • Time (seconds (s))
    • Amount of a substance (moles (moL))

Derived Units[edit | edit source]

Combination of two regular units:

  • Area (length times 2): m2
  • Volume (length times 3): m3
  • Density:
  • Speed: meters per second (m/s)

Scientific Notation[edit | edit source]

  • Many measurements in science involve very small or very large numbers.
  • Scientific notation is an easy way to express either.
  • Format: Coefficient x 10exponent
  • Coefficient is a number between 1 and 9. If the exponent is positive, its a big number, while if it negative, its a small number.

The SI (Metric) System Continued[edit | edit source]

  • Another way scientists express very large/small numbers.
  • The metric system uses universal units for ease of communication and prefixes to make huge/tiny numbers more manageable
Tera (T) 1,000,000,000,000 [1 x 1012]
Giga (G) 1,000,000,000 [1 x 109]
Mega (M) 1,000,000 [1 x 106] ← x's bigger than
Kilo (K) 1000 [1 x 103]
Hecto (h) 100 [1 x 102]
Deka (da) 10 [1 x 101]

BASE UNIT (grams, liters, meters, seconds, moles) ↑ Bigger ↓ Smaller

Deci (d) 10 [1 x 10-1]
Centi (c) 100 [1 x 10-2]
Milli (m) 1000 [1 x 10-3]
Micro (µ) 1,000,000 [1 x 10-6] ← x's smaller than
Nano (n) 1,000,000,000 [1 x 10-9]
Pico (p) 1,000,000,000,000 [1 x 10-12]

Uncertainty in Measurement[edit | edit source]

  • There's ALWAYS some error in taking measurements because instruments were made by people and are used by people.
  • This is one reason for the need for repeated trials in science.
  • Even so, in EVERY measurement there's always at least 1 uncertain digit (always the last one).
  • So, you always measure to the place you know for sure, plus one more (in other words, one place past the scale of the instrument).

Signifcant Figures/Digits[edit | edit source]

It would be tough if we had to report uncertainty every time, so we use significant figures (sig figs). The number of sig figs in a measurement, such as 2.531, is equal to the number of digits that are known with some degree of confidence. When you take a measurement, you'll use the same technique as above and omit the +/-. The number of sig figs in your measurement depends on the scale of the instrument.

As we improve the sensitivity of the equipment used to make a measurement, what do you think happens to the number of sig figs? Increases.

Counting Significant Figures[edit | edit source]

  1. Always count nonzero digits:
    • 21
    • 8.926
  2. Never count leading zeros:
    • 034
    • 0.091
  3. Always count zeros which fall somewhere between 2 nonzero digits:
    • 20.8
    • 00.1040090
  4. Count trailing zeros if and only if the number contains a decimal point:
    • 210
    • 210000000
    • 210.0
    • 25000.
  5. For numbers expressed in scientific notation, ignore the exponent:
    • -4.2010 x 1028

Calculating and Rounding using Significant Figures[edit | edit source]

Usually, experiments/measurements are repeated to ensure precision. To report results, we usually take an average of data. So, how do you know where to round? We'll see:

NOTE: Your calculation can be no more specific than the LEAST specifics of your original measurements/numbers.

Rounding Rules to Memorize[edit | edit source]


Round to the least number of sig figs after the decimal point

  • 25.6 + 85.379 + 145.69 = 256.669

Round to the least number of sig figs TOTAL

  • 52.0 x 365 x 13 = 246,000

More Practice[edit | edit source]

  • 37.2 + 18.0 + 380 = 435.2
  • 0.57 x 0.86 x 17.1 = 8.38242
  • (8.13 x 1014) / (3.8 x 102) = 2.139473684 x 1012
ROUNDED ANSWER: 2.1 x 1012

Percent Error[edit | edit source]

Expirmented Value - Accepted Value
___________________________________ • 100 = [ANSWER]
Accepted Value

Calculating Average Atomic Mass[edit | edit source]

(amu1 • abd1) + (amu2 • abd2) = average atomic mass [of the element]

  • ABD = Abundance

For the percentages, move the decimal two places to the left.