Review Notes for Lab Exams

CHEM 1151 (Experiments 1 - 9)

General guidelines:

Study the pre-lab and post-lab questions from your lab manual for each experiment. Also review the pre-lab quizzes. Read through the lab experiments, thinking about safety and waste disposal and why different procedures were followed. Consider the accuracy of the measuring apparatus used and the significant figures reported for the experimental data (e.g., balance, burette, graduated cylinder, pipette, volumetric flask, barometer, thermometer used by you in the lab). Be able to perform the calculations you did for each experiment using the experimental data you collected in the lab. Know safety rules for working in the lab.

Experiment 1 - Fundamental Laboratory Techniques

• Interpolation (density)
• Accuracy and precision (definitions, range is a measure of precision)
• Calculate range
• Calculations involving density

Experiment 2 - Gravimetric Determination of Water of Hydration

• Purpose of heating to constant mass
• Why don't you heat until the crucibles are red hot when dehydrating the sample?
• Calculation of theoretical % water of hydration
• Calculation of experimental % water of hydration (calculation using experimental data)
• Calculation of % error

Experiment 3 - The Stoichiometry and Reactions of Some Copper Compounds

• Why did you dissolve Cu in HNO₃(aq) under the hood?
• What is digestion (of a precipitate)? What is its purpose?
• What is the purpose of Zn in this experiment?
• Why did you wash Cu with water at the end of the experiment?
• Why did you wash Cu with acetone at the end of the experiment?
• Safety precautions (e.g., why use hoods? treatment for acid spilled on yourself?)
• Complete and balance equations, net ionic equations, for reactions
• Stoichiometry problems (Calculate theoretical yield, calculate % yield)
• How did you dispose of the basic decantate obtained when CuO was precipitated? How did you dispose of the acidic solution decanted from Cu at the end of the experiment?

Experiment 4 - Solubility Rules and Net Ionic Equations

• Be able to apply the solubility rules to predict the solubility of an ionic salt in water.
• Apply solubility rules to predict what precipitate (if any) will be produced in a reaction.
• Write complete ionic equations.
• Write net ionic equations.
• Complete a chemical reaction (i.e. write the products given the reactants).

Experiment 5A - Standardization of NaOH by Titration with Sulfuric Acid

• What is neutralization? Define acid and base.
• What is the indicator used for this titration? What is the purpose of the indicator? What is the color of the indicator in acidic solution? in basic solution?
• Be able to write balanced equations for the reactions in this experiment
• Be able to solve dilution problems
• Be able to calculate mols of acid and mols of base from concentration and volume data.

Experiment 5B - Molar Mass Determination by Titration

• What is neutralization? Define acid and base.
• What is the indicator used for this titration? What is the purpose of the indicator? What is the color of the indicator in acidic solution? in basic solution?
• Be able to write balanced equations for the reactions in this experiment
• Be able to solve dilution problems
• Know the relationship of molarity, mols, volume, mass and molar mass and be able to solve stoichiometry problems involving solutions (i.e. solution stoichiometry)
• Calculate range (measure of precision)
• How is excess NaOH treated before disposal? How is excess acid treated before disposal?
• Perform calculations involving molarity and volume (reaction will be given)
• Calculate molar mass from the concentration and volume of titrant
• Calculate % error in the molar mass by comparing the experimental value to the substance closest to it

Experiment 6 - Thermochemistry: Heats of Solution

• Why is a Styrofoam cup (rather than just a beaker) used for the calorimeter?
• What is the purpose of measuring the temperature change when hot water is added to room temperature water in the calorimeter?
• Why is the final temperature determined by extrapolating the temperature vs. time plot to the time of mixing?
• Be able to calculate heat of reaction from heats of formation
• Be able to calculate heat of formation from heat of reaction and heats of formation for the other substances in the reaction
• Be able to solve Hess's law problems
• Be able to solve calorimeter problems (calculate heat from heat capacity and temperature change or heat capacity from heat and temperature change)
• Be able to do calculations in lab experiment (including reading graphs)

Experiment 7 - Thermochemistry: Enthalpy of Combustion

• Why is a Styrofoam cup used for the calorimeter?
• Why is the final temperature determined by extrapolating the temperature vs. time plot to the time of mixing?
• Be able to solve Hess's law problems
• Be able to plot data and read graphs
• Be able to calculate heat of combustion from calorimetry data
• Be able to calculate heat of formation from heat of reaction and heat of formations
• Add enthalpies of reaction for several reactions to determine the enthalpy of reaction for a different reaction

Experiment 8 - Molecular Geometry

• Lewis structures
• Predict structural pair and molecular geometries using VSEPR theory
• Be able to count electron pair interactions for a molecule: lone pair - lone pair (L-L), lone pair - bond pair (L-B), bond pair - bond pair (B-B) repulsions

• Be able to write and balance the equation for the reaction used to generate hydrogen
• Why must the inverted graduated cylinder be full of water?
• Why is it necessary to have the water levels equal in the graduated cylinder and water bath after the generation of hydrogen?
• What is the theoretical value of molar volume of a gas at STP?
• Interpolation (vapor pressure)
• Dalton's law of partial pressures (calculation of pressure of oxygen from barometric pressure and vapor pressure of water)
• Calculate % error
• Calculations from lab data
• Combined gas law problems

General guidelines

Study the pre-lab and post-lab questions from your lab manual for each experiment. Also review all pre-lab quizzes. Read through the lab experiments, thinking about safety and waste disposal and why different procedures were followed. Be able to explain terms and give reasons for the techniques used in the qualitative analysis scheme. Consider the accuracy of the measuring apparatus used and the significant figures reported for the experimental data (e.g., pipette). Be able to perform the calculations you did using the experimental data you collected in the lab. Know general safety rules for working in the lab.
 

For Experiments 16 - 18:

Know from memory:

• All confirmatory tests and net ionic equations for those tests.
• Precipitating reagents for Groups III, IV and V cations. (Be able to write balanced net ionic equations for the precipitation of each ion in each group.)
• Given reactants, recognize any equation involved in an analysis and be able to complete and balance the equation.

Be able to solve K_sp problems similar to those in prelabs and postlab questions for experiments.

General questions about qualitative analysis

• What is meant by "decantate"? "supematant"? "residue"? "precipitate"?
• What is the purpose of washing a precipitate?
• How do you check for completeness of precipitation? Why is this necessary? (What problems are encountered if this is not done?)
• What is the "white stuff" that sometimes settles on the equipment in your drawer during the week?
• Why is waste containing Ba²⁺, Cr³⁺ and Cr04^2- kept separate from other waste (containing Na⁺, K⁺, NH₄+, Ca²⁺, N0₃-, S0₄^2-, P0₄^3-, etc.)? (Know which ions must be maintained for disposal as "heavy metal waste.")

Experiment 10 - Molar Mass by Freezing Point Depression

• Determine the freezing point of a pure substance or a mixture from reading a graph
• Calculate the molar mass of an unknown solid given freezing point depression data
• What are appropriate units for K_f?
• Why was the sample heated to 10-15 degrees above the expected melting point?
• What effect does a solute have on the freezing point of the pure solvent?
• Contrast the temperature profile during a phase change for a pure substance with that for a mixture

Experiment 11 - Kinetics: Rates of Reactions and Activation Energies

• Be able to determine reaction order from appropriate plots of function of concentration vs. time
• Be able to plot and read graphs (know which functions to use for which order and for Arrhenius plot)
• Be able to do calculations in this experiment

Experiment 12 - Determining the Concentration of a Solution: Beer's Law

• What quantities are graphed in a Beer's Law plot?
• What is the purpose of making standard solutions of varying concentration?
• How is the colorimeter calibrated?
• Be able to calculate the concentration of an unknown solution using absorbance and concentration data

Experiment 13 - Chemical Equilibrium: Finding a Constant, K_c

• For what chemical reaction is the equilibrium constant being determined?
• Write the K_c expression for the reaction of interest.
• Calculate K_c from absorbance and concentration data

Experiment 14 - Titration of a Diprotic Acid: Identifying an Unknown

• How is the equivalence point determined from a plot of pH vs. volume of titrant?
• Be able to write acid ionization equations for each ionization in a polyprotic acid
• If an titration curve shows more than one equivalence point, which one should be used for calculations?
• Calculate the molar mass (molecular weight) of an unknown acid from titration data

Experiment 15 - Ionic Equilibrium - The Ionization Constant of an Unknown Acid

• Be able to calculate the concentration of weak acid when the solution is diluted with a solution containing the salt of the acid
• Be able to calculate Ka from pH and concentrations of a weak acid and the salt of that acid. How does a buffer resist change in pH? Be able to write equations to support your answer to this.

Experiment 16 - Qualitative Analysis of Group III and IV Cations

• In the analysis for Cr³⁺, why is it important to remove all the H₂0₂ before acidifying and testing for Cr?
• Which Group Ill ion does not precipitate in a NH₃/NH₄⁺ buffered basic solution?
• Why do you only report iron as Fe³⁺?
• In the analysis of Fe³⁺ and Cr³⁺, why is an excess of NaOH required?
• In the analysis for NH₄⁺, what is the indication for a positive test? What can cause a false positive test?
• Why do results appear different for the unknown than when doing the known?

Experiment 17 - Qualitative Analysis of Anions

• What is the difference between the confirmatory tests for N0₃- and N0₂-? Why must these tests be done in the hood?
• For Group II anions, why is it a good idea to run confirmatory tests on knowns along with your unknown?

Experiment 18 - Qualitative Analysis of a Salt Unknown

• Which analysis do you start with?
• How many cations and anions do you expect to identify?