High School Chemistry/Introduction to Methods of Chemistry

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

• 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

• 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

Combination of two regular units:

• Area (length times 2): m²
• Volume (length times 3): m³
• Density: ${\tfrac {mass}{volume}}$
• Speed: meters per second (m/s)

Scientific Notation

• Many measurements in science involve very small or very large numbers.
• Scientific notation is an easy way to express either.
• Format: Coefficient x 10^exponent
• 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

• 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

• 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

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

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

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

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

• 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

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

Calculating Average Atomic Mass

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

• ABD = Abundance

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

High School Chemistry

Atomic Structure, The Periodic Table, and Electrons or Light →