In the multiple-choice section, this topic appears in about 7 out of 75 questions. In the free-response section, this topic appears every year.
There will be some questions on the test that are specifically about lab technique, but most “lab” questions will be about specific chemistry topics, placed in a lab setting to make them seem more intimidating. You just need to remember some basic rules and combine them with science and common sense. The free-response questions may require you to remember some specifics about some basic experiments and laboratory techniques. The best way to prepare for this is to review your lab notebooks.
These are the 22 experiments that the College Board recommends that AP Chemistry classes perform during the school year.
Here are some basic safety rules that might turn up in test questions.
Here are some rules for ensuring the accuracy of experimental results.
Filtration—In filtration, solids are separated from liquids when the mixture is passed through a filter. Typically, porous paper is used as the filter. To find the amount of solid that is filtered out of a mixture, the filter paper containing the solid is allowed to dry and then weighed. The initial weight of the clean, dry filter is then subtracted from the weight of the dried filter paper and solid.
Distillation—In distillation, the differences in the boiling points of liquids are used to separate them. The temperature of the mixture is raised to a temperature that is greater than the boiling point of the more volatile substance and lower than the boiling point of the less volatile substance. The more volatile substance will vaporize, leaving the less volatile substance.
Chromatography—In chromatography, substances are separated by the differences in the degree to which they are adsorbed onto a surface. The substances are passed over the adsorbing surface, and the ones that stick to the surface with greater attraction will move slower than the substances that are less attracted to the surface. This difference in speeds separates the substances. The name chromatography came about because the process is used to separate pigments.
Titration is one of the most important laboratory procedures. In titration, an acid-base neutralization reaction is used to find the concentration of an unknown acid or base. It takes exactly one mole of hydroxide ions (base) to neutralize one mole of hydrogen ions (acid), so the concentration of an unknown acid solution can be found by calculating how much of a known basic solution is required to neutralize a sample of given volume. The most important formula in titration experiments is derived from the definition of molarity.
Molarity = Moles/Liters
Moles = (Molarity)(Liters)
The moment when exactly enough base has been added to the sample to neutralize the acid present is called the equivalence point. In the lab, an indicator is used to tell when the equivalence point has been reached. An indicator is a substance that is one color in acid solution and a different color in basic solution. Two popular indicators are phenolphthalein, which is clear in acidic solution and pink in basic solution, and litmus, which is pink in acidic solution and blue in basic solution.
Spectrophotometer—A spectrophotometer measures slight variations in color. It can be used to measure the concentration of ions that produce colored solutions.
The concentration of the ions will be directly proportional to the absorbance of the solution measured by the spectrophotometer. The relationship between absorbance and concentration is given by Beer’s law.
One of the most useful ways of identifying unknown ions in solution is precipitation. You can use the solubility rules given in the solubility chapter to see how the addition of certain ions to solution will cause the specific precipitation of other ions. For instance, the fact that BaSO4 is insoluble can be used to identify either Ba2+ or SO
in solution; if the solution contains Ba2+ ions, then the addition of will cause a precipitation reaction. The inverse is true for a solution that contains ions.
In the same way, the insolubility of AgCl can be used to identify either Ag+ or Cl– in solution.
You can tell whether a solution contains ions by checking to see if the solution conducts electricity. Ionic solutes conduct electricity in solution; nonionic solutes do not.
Some ions burn with distinctly colored flames. Flame tests can be used to identify Li+ (red), Na+ (yellow), and K+ (purple), as well as the other alkali metals. The alkaline earths, including Ba2+ (green), Sr2+ (red), and Ca2+ (red) also burn with colored flames.
, CO2 gas is produced.
), a strong oxidizing agent, turns a solution purple.
), also a strong oxidizing agent, turns a solution orange. Chromate (CrO42–) turns a solution yellow.The pictures below show some standard chemistry lab equipment.
Questions 1–4
(A) Oxidation-reduction
(B) Neutralization
(C) Fusion
(D) Combination
(E) Decomposition
Which of the reaction types listed above best describes each of these processes?
1. CO2(g) + CaO(s) → CaCO3(s)
2. 2 Fe3+(aq) + 2 I–(aq) → 2 Fe2+(aq) + I2(aq)
3. CH3COOH(aq) + NaOH(aq) →
CH3COONa(aq) + H2O(l)
4. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
Questions 5–8
(A) Na+
(B) Cu2+
(C) Ag+
(D) Al3+
(E) NH4+
5. This ion turns an aqueous solution deep blue.
6. This ion forms a white precipitate when added to a solution containing chloride ions.
7. This ion produces a yellow flame when burned.
8. This ion produces a strong odor when added to a basic solution.
9. Which of the following indicators would be most useful in identifying the equivalence point of a titration for a solution that has a hydrogen ion concentration of 7 × 10–4 M at the equivalence point?
(A) Methyl violet (pH range for color change is 0.1–2.0)
(B) Methyl yellow (pH range for color change is 1.2–2.3)
(C) Methyl orange (pH range for color change is 2.9–4.0)
(D) Methyl red (pH range for color change is 4.3–6.2)
(E) Bromthymol blue (pH range for color change is 6.1–7.6)
10. The volume of a liquid is to be measured. Which of the following cylindrical flasks would take the most accurate measurement?
(A) A flask with 1 ml gradations and a diameter of 1 cm
(B) A flask with 1 ml gradations and a diameter of 3 cm
(C) A flask with 5 ml gradations and a diameter of 1 cm
(D) A flask with 5 ml gradations and a diameter of 3 cm
(E) A flask with 10 ml gradations and a diameter of 1 cm
11. Which of the following is (are) considered to be proper laboratory procedure?
I. Reading the height of a fluid in a buret from a point level with the fluid’s meniscus.
II. Placing a sample to be weighed directly on the pan of the balance.
III. Stirring a solution constantly during a titration.
(A) I only
(B) III only
(C) I and III only
(D) II and III only
(E) I, II, and III
12. A 0.1-molar NaOH solution is to be released from a buret in a titration experiment to measure the hydrogen ion concentration of an unknown acid. Which of the following laboratory procedures would cause an error in the measure of the concentration of the acid?
I. The buret was rinsed with the NaOH solution immediately before the
titration.
II. The buret was rinsed with distilled water immediately before the titration.
III. The buret was rinsed with the unknown acid immediately before the titration.
(A) I only
(B) III only
(C) I and II only
(D) I and III only
(E) II and III only
13. Which of the following is NOT proper procedure for transferring a solution with a pipette?
(A) Rinsing the pipette with the solution to be transferred.
(B) Using your mouth to draw the solution into the pipette.
(C) Covering the top of the pipette with your index finger to keep the solution from escaping.
(D) Draining the solution into a waste beaker until the meniscus drops to the calibration mark.
(E) Touching the pipette to the side of the destination beaker at the end of the transfer.
14. An object that was weighed on a balance was later found to be slightly heavier than the weight that was recorded by the balance. Which of the following could have caused the discrepancy?
I. There was some foreign matter on the weighing paper along with the object.
II. The object was hot when it was
weighed.
III. The experimenter neglected to account for the weight of the weighing paper.
(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III
15. An experimenter wishes to use test paper to find the pH of a solution. Which of the following is part of the proper procedure for this process?
(A) Dipping the test paper in the solution while stirring.
(B) Dipping the test paper in the solution without stirring.
(C) Pouring some of the solution onto the dry test paper.
(D) Dipping the test paper in distilled water and slowly adding the solution to the water while stirring.
(E) Dipping the test paper in distilled water and slowly adding the solution to the water without stirring.
1. A titration experiment was conducted to determine the pH of a known volume of a strong monoprotic acid solution. A hydroxide solution of known concentration was poured into a buret and then titrated into the acid solution. The volume of hydroxide solution titrated into the acid solution was measured at the equivalence point and used to calculate the concentration of the acid solution. Which of the following situations would cause an error in the calculated value of the pH of the acid solution? Explain each answer.
(a) The buret was rinsed with the hydroxide solution before the solution was poured into the buret.
(b) The experimenter did not notice that a few drops of hydroxide solution spattered outside the acid solution container during the titration.
(c) The buret was rinsed with distilled water before the hydroxide solution was added.
(d) The experimenter read the hydroxide solution level from the top of the fluid instead of the bottom of the meniscus both before the titration and at the equivalence point.
(e) Some hydroxide solution was spilled while the experimenter was pouring it into the buret.
2. An experiment was conducted to determine the molecular weight of a pure salt sample. The mass of the salt sample was known. The salt was dissolved in a container of water of known mass and the freezing point of the solution was measured. The molecular weight was calculated by the freezing point depression method. How would the calculated value of the molecular weight be affected by each of the following?
(a) The experimenter failed to take the dissociation of the salt into account.
(b) The experimenter mistook molarity for molality, and used liters of solution instead of kilograms of solvent in the calculation to find the number of moles of solute.
(c) The container used for the experiment was not rinsed and contained dust particles.
(d) The experimenter misread the thermometer and recorded a freezing point that was higher than the true value.
(e) The experimenter did not notice that some solid salt did not completely dissolve.
3. Use your knowledge of chemical principles to answer or explain each of the following:
(a) When helium gas is to be collected in a jar by the displacement of air, the opening of the jar must be directed downward. When carbon dioxide gas is to be collected in a jar by the displacement of air, the opening of the jar must be directed upward.
(b) Will the molar quantity calculated for a gas collected over water be too large or too small if the experimenter fails to take into account the vapor pressure of water?
(c) Why is it easier to separate oxygen gas from hydrogen gas by the method of successive effusion than it is to separate oxygen gas from nitrogen gas by the same method?
(d) Give an explanation for why an attempt to separate two liquids by distillation may fail.
1. D In a combination (or composition, or synthesis) reaction, two substances combine to form a more complex substance.
A combination reaction is the opposite of a decomposition reaction, so if this reaction occurred in reverse, it would be a decomposition reaction.
2. A In an oxidation-reduction reaction, electrons are transferred between the reactants, causing the oxidation state of the element that is oxidized to increase and the oxidation state of the element that is reduced to decrease. In this reaction, Fe3+ is reduced to Fe2+ and I– is oxidized to I0.
3. B In a neutralization reaction, an acid (in this case, CH3COOH) and a base (NaOH) react to form water and a salt (CH3COONa).
4. A This reaction, the combustion of an organic compound, is also an oxidation-reduction reaction. As we said above, in a redox reaction, electrons are transferred between the reactants, causing the oxidation state of the element that is oxidized to increase and the oxidation state of the element that is reduced to decrease. In this reaction, H– is oxidized to H+ and O0 is reduced to O2–.
5. B Copper, along with most of the transition metals, forms colored solutions with water. This is true because the d electrons of the transition metals are constantly changing energy levels and emitting radiation in the visible spectrum.
6. C Silver is the only ion listed that forms an insoluble chloride.
7. A Sodium, along with the other Group IA elements, produces a colored flame in the flame test.
8. E Ammonium ion reacts with hydroxide ion to form ammonia, which has a strong, distinct odor. This reaction is shown below.
NH4+ + OH– → NH3 + H2O
9. C If [H+] is 7 × 10–4 M, then the pH must be between 3 and 4. Only methyl orange changes color between 3 and 4.
10. A Volume = (height)(cross-sectional area)
The smaller the gradations, the more accurately the height can be measured. The smaller the area, the farther apart the 1 ml gradations will be and the more accurately the height of the fluid can be measured.
11. C Choices (I) and (III) are proper experimental procedures.
(II) is not; a sample should always be weighed in a glass or porcelain container to prevent a reaction with the balance pan.
12. E Rinsing the buret with the NaOH solution (I) is proper procedure and will not change the concentration of the NaOH solution and will not cause an error. Rinsing the buret with distilled water (II) will dilute the NaOH solution, lowering the concentration and causing an error, and rinsing the buret with the unknown acid (III) will cause some of the NaOH solution to be neutralized in the buret, lowering its concentration and causing an error.
13. B You should never use your mouth to draw solution into a pipette. Instead, you should use a rubber suction bulb.
All of the other choices are part of the proper procedure.
14. B When a hot object is weighed (II), convection currents around the object can reduce the apparent mass measured by the balance.
Choices (I) and (III) would both cause the weight measured by the balance to be greater than the actual weight of the object. We’re looking for the opposite effect.
15. C (A) and (B) are wrong because there is a danger of contaminating the solution by adding the paper.
(D) and (E) are wrong because adding the solution to distilled water completely changes the solution and defeats the purpose of testing it.
So pouring the solution onto the dry test paper (C) is the proper procedure.
1. A note for the answers:
In this experiment, the volume of OH– added is directly measured.
Moles = (molarity)(volume) is used to find the moles of OH– added.
The moles of OH– added to reach the equivalence point is equal to the number of moles of H+ originally present in the acid solution.
Molarity = 
is used to find [H+] of the acid solution.
pH = –log[H+] is used to calculate the pH.
(a) This is proper experimental procedure and will have no adverse effect on the calculated value of the acid solution.
(b) This will make the measured volume of the hydroxide solution larger than the actual amount added, the calculated value for the moles of OH– and H+ will be too large, the calculated value of [H+] will be too large, and the calculated pH will be too small.
(c) This will dilute the hydroxide solution, which means that too large a volume of hydroxide solution will be added, the calculated value for the moles of OH– and H+ will be too large, the calculated [H+] will be too large, and the calculated pH will be too small.
(d) The levels were read consistently although they were read from the wrong spot. The two errors should cancel, and the calculated pH should be correct.
(e) This will not affect the concentration of the hydroxide solution or the measurement of the volume poured into the acid solution, so the calculated pH should not be affected.
2. A note for the answers:
In this experiment, the freezing point of the solution is measured, and from the freezing point the freezing-point depression, ∆T, is calculated.
The equation m = 
is used to calculate the molality of the solution.
Moles = (molality)(kg of solvent) is used to calculate the number of moles of salt.
MW = 
is used to calculate the molecular weight of the salt.
(a) The experimenter makes x = 1, instead of 2 or 3. So the calculated value of m will be too large, the calculated value of moles of salt will be too large, and the calculated MW will be too small.
(b) Moles = (molality)(kg of solvent) Moles = (molarity)(liters of solution)
Because the solvent is water, the distinction between kilograms and liters is not important (for water, 1kg = 1 L), but the distinction between solvent and solution may make a difference.
Liters of solution will be a little larger than kilograms of solvent, so the calculated value for moles of salt will be too large, and the calculated MW will be too small.
(c) Extra particles in the solution will cause the measured freezing point to be too low. ∆T will be too large, m will be too large, and the calculated MW will be too small.
(d) If the freezing point is too high, the calculated ∆T will be too small, m will be too small, and the calculated MW will be too large.
(e) If some salt does not dissolve, then the grams of salt used in the calculation will be larger than the amount actually in the solution, and the calculated MW will be too large.
3. (a) Helium (MW = 4 g/mol) is less dense than air, so it will rise to the top of the jar, displacing air downward.
Carbon dioxide (MW = 44 g/mol) is denser than air, so it will sink to the bottom of the jar, displacing air upward.
(b) If the vapor pressure from water is ignored, the pressure of the gas used in the calculation will be too large.
n = 
If P is too large, then n, the calculated molar quantity of gas, will be too large.
(c) Separation of gases by successive effusion depends on Graham’s law.
The greater the difference in molecular weights, the greater the difference in average molecular speeds, and the greater the difference in rates of effusion.
Oxygen gas (32 g/mol) and nitrogen gas (28 g/mol) have similar molecular weights.
Oxygen gas (32 g/mol) and hydrogen gas (2 g/mol) have very different molecular weights.
(d) The most likely reason for the failure of separation by distillation would be that the boiling points of the two liquids are too close together.