High School Chemistry/Introduction to Methods of Chemistry

From Wikiversity
Jump to navigation Jump to search

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, feuls).
  2. Inorganic Chemistry: All substances inorganic (not containing carbon).
  3. Analytical Chemistry: Separate and identify matter (drug testing).
  4. Physical Chemistry: Behavior of chemicals (why does nilon stretch?/reactions).
  5. Biochemistry: Chemistry of living organisms (photosynthesis, metabolism, respiration).

Measurements and Data Collection[edit | edit source]

  • Can be quantitiative (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]
Attention
{{{1}}}

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]

Addition/Subtraction

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

  • 25.6 + 85.379 + 145.69 = 256.669
ROUNDED ANSWER: 256.700
Multiplication/Division

Round to the least number of sig figs TOTAL

  • 52.0 x 365 x 13 = 246,000
ROUNDED ANSWER: 250,000

More Practice[edit | edit source]

  • 37.2 + 18.0 + 380 = 435.2
ROUNDED ANSWER: 435.
  • 0.57 x 0.86 x 17.1 = 8.38242
ROUNDED ANSWER: 8.4
  • (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.