Rucete ✏ AP Chemistry In a Nutshell
14. Experimental Chemistry
This chapter reviews the fundamental laboratory techniques and concepts essential to experimental chemistry, including measurement, data collection, precision, significant figures, physical property determination, and laboratory safety.
Data Gathering
• Observations can be quantitative (numerical) or qualitative (descriptive).
• All experimental details must be recorded in a bound, dated lab notebook with no erasures.
• Include description of the experiment, apparatus diagrams, sources of chemicals, observations, and conclusions.
Calculations
• Two main approaches: – Dimensional analysis for unit conversion – Use of equations with variables to solve for unknowns
• Units must be shown throughout to confirm cancellation and correct results.
Accuracy and Precision
• Accuracy: closeness to the true value; best assessed by comparison with independent methods.
• Precision: consistency of repeated measurements.
• Determinate (systematic) errors affect accuracy; indeterminate (random) errors affect precision.
Significant Figures
• Rules for significance: – Nonzero digits, embedded zeros, and trailing zeros after a decimal are significant. – Leading zeros are not significant.
• Use scientific notation to avoid ambiguity in large numbers.
• In multiplication/division: keep the fewest sig figs. In addition/subtraction: keep the fewest decimal places.
Uncertainty
• Absolute uncertainty: the ± range of the last digit (e.g., 45.47 ± 0.01 mL).
• Relative uncertainty = absolute uncertainty / measurement value.
• Use absolute uncertainty in addition/subtraction; use relative uncertainty in multiplication/division.
Rounding
• Round based on required sig figs or decimal places.
• If the next digit is <5, drop digits. If ≥5, round up.<5 digits.="" drop="" if="" p="" round="" up.="">
Graphs
• Plot independent variable on x-axis; dependent on y-axis.
• Draw smooth best-fit lines, not dot-to-dot. Use “method of least squares” for accuracy.
• Do not extend lines beyond data. Determine slope with (y2 – y1)/(x2 – x1).
Measurement Techniques
• Always read from eye level to avoid parallax error.
• Meniscus: read from the bottom of the curve.
• Weigh using calibrated balances and containers. Mass = sample + container – tare.
• Measure liquid volume with pipets or burets for precision; use TD and TC marks correctly.
• Temperature: immerse thermometer to marked line. Verify with ice (0°C) and boiling water (100°C).
Filtration and Separation
• Gravity filtration: separates solids from liquids using filter paper and funnel.
• Vacuum filtration: faster method using a Büchner funnel and vacuum flask.
• Decanting: pouring off liquid to leave behind solid sediment.
• Centrifugation: separates components by density using rapid spinning.
Chromatography
• Separates substances based on differences in solubility and adsorption.
• Mobile phase carries sample through stationary phase.
• Components travel at different rates (Rf = distance moved by component / distance moved by solvent front).
Distillation
• Separates liquids by differences in boiling points.
• Simple distillation for large differences; fractional distillation for close boiling points.
• Used to purify solvents or separate mixtures (e.g., ethanol and water).
Extraction
• Separates compounds based on differential solubility in immiscible solvents (like oil and water).
• Use separatory funnel to isolate layers.
• Acid–base extractions use pH changes to move substances between layers.
Titration Technique
• Standard solution added via buret to analyte until equivalence point is reached.
• Use indicator or pH meter to detect endpoint.
• Rinse buret with titrant before use, and deliver from the tip without air bubbles.
• Equivalence point: moles acid = moles base.
Colorimetry
• Measures solution concentration based on absorbance of specific wavelengths.
• Use blank for zeroing the spectrophotometer.
• Beer's Law: A = εlc (absorbance = molar absorptivity × path length × concentration).
Drying and Heating
• Use desiccators or drying ovens to remove moisture from solids.
• Crucibles: heat solids to high temperatures for mass analysis.
• Heat to constant mass: repeat heating and weighing until mass remains unchanged.
Laboratory Safety
• Always wear goggles and lab coat.
• Tie back hair, avoid loose clothing, wear closed-toe shoes.
• Know location of eyewash stations, showers, fire extinguishers.
• Dispose of chemicals properly—never down the drain unless instructed.
In a Nutshell
Experimental chemistry relies on careful measurement, precision, and safety. Understanding techniques like filtration, titration, chromatography, and spectrophotometry allows accurate data collection and analysis. Mastering lab practices ensures valid results and reinforces scientific investigation.