AP CHEMISTRY FINAL EXAM REVIEW

AP CHEMISTRY FINAL EXAM REVIEW

• A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration.Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration.

The original solution used to make the solutions for the standard curve was prepared by dissolving 2.60g of CoCl2 (molar mass 130.g/mol) in enough water to make 100.mL of

solution. What is the molar concentration of the solution?: A) 0.200M

Correct. Molarity =moles of solute/volume of solution=(2.60gCoCl/2100.mL)×(1mol- CoCl2/130.gCoCl2)×(1000mL/1L)=0.200M

• The diagram above shows the distribution of speeds for a sample of N2(g) at 25°C. Which of the following graphs shows the distribution of speeds for a sample of O2(g) at 25°C (dashed line) ?: A) https://assets.learnosity.com/organisations/537/VH912643.g05.png

Correct. Two gases at the same temperature have molecules with the same average kinetic energy. Since KE=1/2*mA*v2A=1/2*mB*v2B for two gases, AA and BB, the gas with the higher molar mass will have molecules with a lower average speed.This graph shows the O2(g)O2(g) molecules with a lower average speed than the

N2(g)N2(g) molecules.

• The electron cloud of HF is smaller than that of F2 , however, HF has a much higher boiling point than F2 has. Which of the following explains how the dispersion-force model of intermolecular attraction does not account for the unusually high boiling point of HF?: D) Liquid F2F2 has weak dispersion force attractions between its molecules, whereas liquid HFHF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules.

Correct. The hydrogen bonding interactions in HFHF are much stronger than the weak dispersion forces between F2F2 molecules.

• How many moles of Na+ ions are in 100.mL of 0.100MNa3PO4(aq) ?: -

C)0.0300molCorrect. Na3PO4Na3PO4 forms three Na+Na+ ions per formula unit when it dissociates in water. 100mL100mL of 0.100MNa3PO40.100MNa3PO4 contains 100mL×0.100mol/1000mL=0.0100mol100mL×0.100mol/1000mL=0.0100mol of Na3PO4Na3PO4, which dissociates into 0.0300mol0.0300mol of Na+Na+ ions.

• The survival of aquatic organisms depends on the small amount of O2 that dissolves in H2O. The diagrams above represent possible models to explain this phenomenon. Which 1 / 2

diagram provides the better particle representation for the solubility of O2 in H2O, and why?:

• Diagram 2, because the polar H2OH2O molecules can induce temporary dipoles on the

electron clouds of O2O2 molecules.

Correct. The relatively small solubility of O2O2 in water is due to dipole-induced dipole forces between the polar H2OH2O molecules and the nonpolar O2O2 mole- cules. The permanent dipole moment of the H2OH2O molecules induces temporary dipole moments on the electron clouds of the nonpolar O2O2 molecules.

• Diagram 1 above shows equimolar samples of two gases inside a container fitted with a removable barrier placed so that each gas occupies the same volume. The barrier is carefully removed as the temperature is held constant. Diagram 2 above shows the gases soon after the barrier is removed. Which statement describes the changes to the initial pressure of each gas and the final partial pressure of each gas in the mixture and also indicates the final total pressure?: C) The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases.Correct. For each gas, the partial pressure in the mixture is half its initial pressure because the volume occupied has doubled under constant nn and TT. Since P±1V×constantP±1V×constant, Pf=12×PiPf=12×Pi for each gas. According to Dalton's law, the final total pressure (PTf)(PTf) is given by PTf=Pf,gas1+Pf,gas2=12(Pi,gas1+Pi,gas2)PTf=Pf,gas1+Pf,gas2=12(Pi,gas1+Pi,gas2

• 2F2(g)+2NaOH(aq)’OF2(g)+2NaF(aq)+H2O(l)

A 2mol sample of F2(g) reacts with excess NaOH(aq) according to the equation above. If the reaction is repeated with excess NaOH(aq) but with 1 mol of F2(g), which of the following is

correct?: B) The amount of OF2(g)OF2(g) produced is halved.

Correct. The NaOH(aq)NaOH(aq) is in excess. If the amount of reactants is halved, the amount of each product is halved. The coefficients in the equation do not need to be taken into account in this case.

• Two different ionic compounds each contain only copper and chlorine. Both compounds are powders, one white and one brown. An elemental analysis is performed on each powder. Which of the following questions about the compounds is most likely to be answered by the results of the analysis?: B) What is the formula unit of each compound?

Correct. From the elemental analysis, the moles of copper and chlorine can be calculated, then the mole ratio of copper to chlorine can be calculated. This ratio is the formula unit of the ionic compound.

• The mass spectrum of the element Sb is most likely represented by which of the following?: B)

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