Measurement in Chemistry

Accuracy vs. Precision

In scientific measurement, it is important to distinguish between accuracy and precision:

• Accuracy: How close a measured value is to the true or accepted value.

• Precision: How reproducible or consistent repeated measurements are, regardless of their closeness to the true value.

Example: If you weigh a 10.00 g standard and obtain values of 9.98 g, 9.99 g, and 10.01 g, your measurements are both accurate (close to 10.00 g) and precise (very similar to each other).

Measurement and Uncertainty

All measurements in chemistry include some degree of uncertainty. This is because no instrument can measure with infinite precision. When recording a measurement:

• Record all certain digits plus one estimated digit.

• The last digit is uncertain and reflects the precision of the measurement.

Example: If a ruler measures to the nearest millimeter, and you estimate a length as 12.34 cm, the '4' is the estimated digit.

Significant Figures (Sig Figs)

Significant figures indicate the precision of a measured value. The rules for counting significant figures are:

• If there is a decimal point, count digits from left to right starting at the first nonzero digit.

• If there is no decimal point, count digits from right to left starting at the first nonzero digit.

• Exact counted values (e.g., 12 eggs) have an infinite number of significant figures.

Example: 0.00450 has three significant figures (4, 5, and the trailing 0).

Applying Significant Figures

• When performing calculations, the result should be reported with the correct number of significant figures based on the input values.

• For multiplication/division: The answer should have as many significant figures as the value with the fewest significant figures.

• For addition/subtraction: The answer should have as many decimal places as the value with the fewest decimal places.

Units and Dimensional Analysis

SI Units

The International System of Units (SI) is used in chemistry for consistency. The base SI units are:

• Mass: kilogram (kg)

• Length: meter (m)

• Time: second (s)

• Temperature: kelvin (K)

• Amount of substance: mole (mol)

• Electric current: ampere (A)

• Luminous intensity: candela (cd)

Common derived units include:

• Area:

• Volume:

Unit Conversion and Dimensional Analysis

Dimensional analysis is a method for converting between units using conversion factors so that unwanted units cancel out.

• Set up the calculation so that units cancel, leaving only the desired unit.

• Example: To convert 5.00 inches to centimeters using the conversion factor :

Areas of Chemistry and Scientific Approaches

Five Areas of Chemistry

• Analytical Chemistry: Study of the composition of substances.

• Biochemistry: Study of chemical processes in living organisms.

• Inorganic Chemistry: Study of inorganic compounds (non-carbon based).

• Organic Chemistry: Study of carbon-containing compounds.

• Physical Chemistry: Study of the physical properties and behavior of matter.

Pure vs. Applied Science

• Pure Science: Seeks to build knowledge for its own sake (e.g., discovering a new element).

• Applied Science: Uses scientific knowledge to solve practical problems (e.g., developing a new drug).

Properties and Classification of Matter

Intensive vs. Extensive Properties

Properties of matter can be classified as intensive or extensive:

Classification of Matter

• Homogeneous Mixtures: Uniform composition throughout (e.g., air, saltwater).

• Heterogeneous Mixtures: Non-uniform composition (e.g., oil and water, salad).

Separation Techniques for Mixtures

Several laboratory techniques are used to separate mixtures based on physical properties:

• Filtration: Separates solids from liquids using a porous barrier (e.g., sand from water).

• Evaporation: Removes a liquid from a solution to recover the dissolved solid (e.g., salt from saltwater).

• Distillation: Separates substances based on differences in boiling points (e.g., separating alcohol from water).

• Extraction/Separatory Funnel: Separates components based on solubility in different immiscible liquids (e.g., separating oil from water).

Example: To separate a mixture of sand and salt, add water to dissolve the salt, filter to remove the sand, then evaporate the water to recover the salt.