AP Final Exam-Practice Problems 1a) A 35.0 mg sample of a compound containing only C, H, and N undergoes completion producing 0.7504 L of CO2 and 41.1 mg of H2O. i) Determine the mass of CO2 produced. ii) Determine the mass of carbon present in the original sample. iii) Determine the mass of hydrogen present in the original sample. iv) Determine the mass of nitrogen present in the original sample. v) Determine the empirical formula of the original compound. b) When the sample is dissolved into 100 mL of water, solution freezes at - 0.014o. What is the molecular weight of the unknown compound? c) What is the molecular formula of the original compound? d) Write the balanced equation for the combustion reaction that took place in (a). e) What was the oxidizing agent in the reaction? f) Draw the Lewis dot structures for both water and carbon dioxide. i) identify the molecular shape, polarity, bond angle, and hybridization for each molecule. ii) Which molecule contains bonds of shorter length? Explain. iii) Which molecule has the higher vapor pressure? Explain. 2. First Ionization Second Ionization Third Ionization Energy (kJ mol-1) Energy (kJ mol-1) Energy (kJ mol-1) Element 1 Element 2 Element 3 Element 4 1251 496 738 1000 2300 4560 1450 2250 3820 6910 7730 3360 The table above shows the first three ionization energies for atoms of four elements from the third period of the periodic table. The elements are numbered randomly. Use the information in the table to answer the following questions. a) b) c) d) e) f) Which element is most metallic in character? Explain your reasoning. Identify element 3. Explain your reasoning. Write the complete electron configuration for an atom of element 3. What is the expected oxidation state for the most common ion of element 2? What is the chemical symbol for element 2? A neutral atom of which of the four elements has the smallest radius? 3. Answer the following problems about gases. (a) The average atomic mass of naturally occurring neon is 20.18 amu. There are two common isotopes of naturally occurring neon as indicated in the table below. Isotope Ne-20 Ne-22 Mass (amu) 19.99 21.99 (i) Using the information above, calculate the percent abundance of each isotope. (ii) Calculate the number of Ne-22 atoms in a 12.55 g sample of naturally occurring neon. (b) A major line in the emission spectrum of neon corresponds to a frequency of 4.34 x 1014 s-1. Calculate the wavelength, in nanometers, of light that corresponds to this line. (c) In the upper atmosphere, ozone molecules decompose as they absorb ultraviolet (UV) radiation, as shown by the equation below. Ozone serves to block harmful ultraviolet radiation that comes from the Sun. O3(g) UV O2(g) + O(g) A molecule of O3(g) absorbs a photon with a frequency of 1.00 x 1015 s-1. (i) How much energy, in joules, does the O3(g) molecule absorb per photon? (ii) The minimum energy needed to break an oxygen-oxygen bond in ozone is 387 kJ mol-1. Does a photon with a frequency of 1.00 x 1015 s-1 have enough energy to break this bond? Support your answer with a calculation. 4. A student performs an experiment to determine the molar mass of an unknown gas. A small amount of the pure gas is released from a pressurized container and collected in a graduated tube over water at room temperature, as shown in the diagram above. The collection tube containing the gas is allowed to stand for several minutes, and its depth is adjusted until the water levels inside and outside the tube are the same. Assume that: the gas is not appreciably soluble in water the gas collected in the graduated tube and the water are in thermal equilibrium a barometer, a thermometer, an analytical balance, and a table of the equilibrium vapor pressure of water at various temperatures are also available. (a) Write the equation(s) needed to calculate the molar mass of the gas. (b) List the measurements that must be made in order to calculate the molar mass of the gas. (c) Explain the purpose of equalizing the water levels inside and outside the gas collection tube. (d) The student determines the molar mass of the gas to be 64 g mol-1. Write the expression (set-up) for calculating the percent error in the experimental value, assuming that the unknown gas is butane (molar mass 58 g mol-1). Calculations are not required.