Lab Exam

Systematic Error, A consistent bias that shifts all measurements in the same direction and affects accuracy but not precision. Examples include improperly calibrated instruments or consistently reading measurements incorrectly.

Random Error, Unpredictable variation between repeated measurements caused by limitations in instruments or human estimation. Random error affects precision and can be reduced by increasing sample size.

Gross Error, A major mistake during an experiment such as spilling a sample, using the wrong reagent, or recording data incorrectly. Gross errors often create obvious outliers.

Accuracy, How close an experimental result is to the true or accepted value. High accuracy means low systematic error.

Precision, How close repeated measurements are to one another regardless of whether they are correct. High precision means low random error.

Percent Error, A calculation comparing an experimental value to an accepted value to evaluate the accuracy of a result.

Percent RSD, Relative standard deviation expressed as a percentage. It is used to evaluate precision and compare variability between datasets.

68-95-99.7 Rule, In a normal distribution approximately 68% of data lie within one standard deviation of the mean, 95% within two standard deviations, and 99.7% within three standard deviations.

Population, The complete set of all possible measurements or observations that could exist for a system.

Sample, A subset of a population used to estimate properties of the entire population.

Error Propagation, The process by which uncertainty from individual measurements contributes to the uncertainty of a final calculated result.

Experiment 1 - Technique Used, Gravimetric analysis was used to verify the accuracy of laboratory glassware by measuring the mass of water delivered and converting it to volume using the density of water.

Experiment 1 - Gravimetric Analysis, An analytical method where mass measurements are used to determine an unknown quantity. In this experiment it was used to determine delivered volume.

Experiment 1 - Volume Verification Method, The mass of water delivered by a piece of glassware was measured and converted into volume using the known density of water at that temperature.

Experiment 1 - Imperfect Class Comparison, Different groups may have used water at slightly different temperatures, causing small differences in density and making class-wide comparisons less accurate.

Experiment 2 - Goal, Determine the concentration of aluminum by converting dissolved aluminum into an insoluble precipitate that could be isolated and weighed.

Experiment 2 - Purpose of pH Control, pH was controlled to ensure the aluminum precipitated efficiently while keeping excess ligand dissolved so it would not contaminate the precipitate.

Experiment 2 - Benefit of Working Near pKa, Near the pKa both protonated and deprotonated forms of the ligand exist, allowing control over solubility and precipitation behavior.

Experiment 2 - Filtration Method, Vacuum filtration was used to rapidly separate the solid precipitate from the liquid solution.

Experiment 2 - Incomplete Precipitation Effect, If some aluminum remains dissolved instead of precipitating, less mass is collected and the calculated aluminum concentration is artificially low.

Experiment 3 - KHP, Potassium hydrogen phthalate, a highly pure primary standard used to determine the exact concentration of NaOH.

Experiment 3 - Primary Standard, A compound with known purity and composition that can be accurately weighed to determine the concentration of another solution.

Experiment 3 - NaOH Standardization, The process of determining the exact molarity of NaOH by titrating a known amount of KHP.

Experiment 3 - Indicator Used, Phenolphthalein was used because its color change occurs near the expected equivalence point of the titration.

Experiment 3 - Reason Phenolphthalein Was Used, The pH range over which phenolphthalein changes color closely matches the equivalence point for the acids used in the experiment.

Experiment 3 - Endpoint, The point during a titration at which the indicator changes color, signaling the titration should stop.

Experiment 3 - Equivalence Point, The point at which chemically equivalent amounts of acid and base have reacted according to stoichiometry.

Experiment 3 - Equivalence Point Determination, The first derivative of the titration curve was used to mathematically locate the equivalence point.

Experiment 3 - Half-Equivalence Point, The point where half the acid has been neutralized and the pH is equal to the pKa of the acid.

Experiment 3 - Henderson-Hasselbalch Equation, An equation relating pH, pKa, and the ratio of conjugate base to acid, used to determine pKa from titration data.

Experiment 3 - CO₂ Contamination During KHP Titration, Dissolved CO₂ forms carbonic acid, introducing extra acid into the solution.

Experiment 3 - Effect of CO₂ on NaOH Molarity, Because extra acid must be neutralized, a larger volume of NaOH is used, making the calculated NaOH molarity artificially low.

Experiment 3 - Effect of CO₂ on Molecular Weight, Since NaOH molarity is calculated too low, the calculated molecular weight of the unknown acid becomes artificially high.

Experiment 3 - KHP Not Fully Dry, Water contributes to the measured mass even though it is not KHP, causing the amount of KHP to be overestimated.

Experiment 3 - Effect of Wet KHP on NaOH Molarity, Too many moles of KHP are assumed, making the calculated NaOH molarity artificially high.

Experiment 3 - Effect of Wet KHP on Molecular Weight, Because NaOH molarity is artificially high, the calculated molecular weight of the unknown acid becomes artificially low.

Experiment 3 - pH Meter Reading Too High, Every pH value is shifted upward, causing the calculated pKa to be higher than the true value.

Experiment 3 - pH Meter Reading Too Low, Every pH value is shifted downward, causing the calculated pKa to be lower than the true value.

Experiment 3 - Removing KHP After Weighing, Less KHP is present than assumed, causing NaOH molarity to be underestimated and molecular weight to be overestimated.

Experiment 4 - ABBA Mnemonic, The ferrioxalate synthesis follows an Acid-Base-Base-Acid sequence that helps remember the order of reagent additions.

Experiment 4 - Purpose of Boiling Water Bath, A boiling water bath provides a constant temperature of 100°C, allowing controlled heating without overheating the reaction.

Experiment 4 - Fe(II) Oxalate Color, The first iron oxalate precipitate formed during the synthesis is yellow.

Experiment 4 - Ferrioxalate Intermediate Color After Potassium Oxalate, Addition of potassium oxalate produces an orange ferrioxalate solution.

Experiment 4 - Ferric Hydroxide Color, Oxidation and basification form brown ferric hydroxide.

Experiment 4 - Ferrioxalate Product Color, Addition of oxalic acid converts the brown ferric hydroxide into the characteristic green ferrioxalate complex.

Experiment 4 - Fe-Bipyridine Complex Color, When iron reacts with bipyridine during analysis, a red complex forms that can be measured by UV-Vis spectroscopy.

Experiment 4 - Ferrioxalate Color Sequence, White → Yellow → Orange → Brown → Green → Clear → Red represents the visual progression of compounds formed throughout the synthesis and analysis.

Experiment 4 - Too Little Ethanol During Crystallization, Not enough ethanol reduces product precipitation, causing more ferrioxalate to remain dissolved and lowering yield.

Experiment 4 - Reason Too Little Ethanol Lowers Yield, Product that remains dissolved passes through the filter and is not collected.

Experiment 4 - Too Much Ethanol During Crystallization, Excess ethanol can cause impurities such as oxalate salts to precipitate along with the desired product.

Experiment 4 - Reason Too Much Ethanol Lowers Mass Percent Iron, Extra impurity mass contains little or no iron, increasing total mass while iron content remains unchanged.

Experiment 4 - Boiling Too Long During Recrystallization, Ethanol evaporates from solution, reducing crystallization efficiency and lowering yield.

Experiment 4 - Calcium Interference with Bipyridine, Calcium can compete for ligands or interfere with complex formation, reducing the amount of red iron-bipyridine complex formed.

Experiment 4 - Incomplete Crystal Transfer, Some ferrioxalate is lost before analysis, resulting in a lower measured iron concentration.

Experiment 4 - Calcium Oxalate Remaining in Solution, Suspended calcium oxalate particles scatter light during UV-Vis analysis.

Experiment 4 - Reason Calcium Oxalate Raises Mass Percent Iron, Scattered light lowers measured transmittance, which increases absorbance and makes iron concentration appear artificially high.

Experiment 4 - Mass Percent Iron Determination, A calibration curve was used to convert absorbance into iron concentration and ultimately calculate the percentage of iron in the compound.

Experiment 4 - Empirical Formula Determination, Iron content, charge balance, and remaining mass were used to determine the empirical formula of ferrioxalate.

Experiment 5 - UV-Vis Measures Directly, The instrument directly measures the amount of light transmitted through a sample at each wavelength.

Experiment 5 - Value Usually Reported, Absorbance is reported because it is linearly related to concentration according to Beer’s Law.

Experiment 5 - Reason Absorbance Is Preferred, Absorbance produces a straight-line relationship with concentration, making quantitative analysis easier.

Experiment 5 - Beer-Lambert Law, A = εbc where absorbance is proportional to concentration, path length, and molar absorptivity.

Experiment 5 - A in Beer-Lambert Law, Absorbance, which measures how much light is absorbed by the sample.

Experiment 5 - ε in Beer-Lambert Law, Molar absorptivity, a constant describing how strongly a substance absorbs light at a particular wavelength.

Experiment 5 - b in Beer-Lambert Law, Path length of the cuvette, usually 1 cm.

Experiment 5 - c in Beer-Lambert Law, Concentration of the absorbing species.

Experiment 5 - Lambda Max (λmax), The wavelength at which a substance exhibits its maximum absorbance and therefore provides the strongest analytical signal.

Experiment 5 - Reason for Using λmax, Measurements taken at λmax provide the highest sensitivity and greatest accuracy for concentration determination.

Experiment 5 - Molar Absorptivity (ε), A measure of how effectively a substance absorbs light at a particular wavelength.

Experiment 5 - Characteristics of Good Calibration Curve, The relationship should be linear, have an intercept near zero, and include the concentration range of unknown samples.

Experiment 5 - Interpolation, Determining unknown values using points that fall within the range of known standards and is generally more reliable.

Experiment 5 - Extrapolation, Estimating values outside the calibration range, which introduces greater uncertainty and is less reliable.

Experiment 5 - Preferred Method for Calibration Curves, Interpolation because it uses data within the validated range of the calibration standards.

Experiment 5 - Reliable Absorbance Range, Approximately 0.1–1.0 because Beer’s Law is most accurate and detector performance is best in this range.

Experiment 5 - Reason Absorbances Above 1 Are Unreliable, Very little light reaches the detector, causing larger measurement errors and deviations from Beer’s Law.

Experiment 5 - Professor's Preferred First Dilution, A 10:1 dilution because it usually places concentrations near the useful absorbance range.

Experiment 5 - Goal, Use UV-Vis spectra and mathematical decomposition to identify dyes present in an unknown mixture.

Experiment 5 - Values Required in Report, λmax, εmax, and the concentrations of the two dyes that make up the unknown sample.

Experiment 5 - Spectral Decomposition, A mathematical process that separates a mixture spectrum into contributions from individual known dye spectra.

Experiment 5 - Least-Squares Fitting, A mathematical method that minimizes total error between experimental and predicted spectra to identify dye concentrations.

General Lab Skill - Reading a Buret, Read the bottom of the meniscus and estimate one additional digit beyond the smallest graduation.

General Lab Skill - Purpose of pH Meter Calibration, Ensures pH measurements are accurate and minimizes systematic error.

General Lab Skill - Most Common Filtration Technique Used, Vacuum filtration because it quickly separates solids from liquids and helps dry the solid.

General Lab Skill - Ideal Calibration Curve Intercept, Close to zero because zero concentration should ideally produce zero absorbance.

General Lab Skill - Data Point Outside Calibration Range, Requires extrapolation and produces less reliable concentration estimates.

General Lab Skill - Water Bath Advantage Over Hot Plate, Water cannot exceed its boiling point under normal conditions, providing consistent and controlled heating.

General Lab Safety, Always wear appropriate PPE and follow safe laboratory practices because chemistry experiments involve chemicals, glassware, and heat sources that can cause injury.

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