CHEM 101A Review Problems Topics A-G

Problems: Be sure to show your work. Give clear explanations wherever requested.

1) A 1.461 g sample of table sugar is combusted in a constant volume (bomb) calorimeter that has a heat capacity of 1.560 kJ/C. The temperature of the calorimeter increases by 15.38C. a) Assuming that the table sugar is pure sucrose, C12H22O11(s), write the balanced

equation for the combustion reaction. Write all substances in their physical states at 25C.

b) Calculate E for the reaction as written in the balanced equation. c) Calculate H at 25C for the reaction as written in the balanced equation.

2) A sample of Br2 (g) takes 18.0 minutes to effuse through a membrane. How long would it

take for the same number of moles of Ar (g) to effuse through the same membrane?

3) Table salt, NaCl (s), and sugar, C12H22O11(s), are accidentally mixed. A 4.50-g sample is burned, and 3.30g of CO2(g) is produced. What is the mass percentage of the table salt in the mixture?

4) The following two transitions appear in the line spectrum of potassium:

[Ar]4p1 [Ar]4s1 = 769.9 nm [Ar]4p1 [Ar]3d1 = 1169 nm

Also, the first ionization energy of potassium is 419 kJ/mol. Given this information, calculate the energy required to ionize a potassium atom from its [Ar]3d1 state. (Hint: it may be helpful to construct an energy diagram showing the energies of the 3d, 4p, 4s, and ionized state.)

5) A 1.3876 gram sample of a compound with the formula XC4H10 is combusted, yielding 1.978 g of CO2 and 1.012 g of H2O, along with a white power. With this information, determine what element X is.

6) An unidentified transition metal M and an unidentified halogen X react to form a compound MX2. When heated, the compound decomposes by the reaction:

2 MX2(s) 2 MX(s) + X2(g) When 1.12 g of MX2 is heated, 0.720 g of MX is obtained, along with 62.38 mL of gas at 741.2 torr at 25C. What is the identity of the halogen X and of the metal M?

7) A student has two unlabeled bottles, both containing a while crystalline solid. She knows

that one bottle contains tartaric acid (H2C4H4O6) and the other bottle contains oxalic acid (H2C2O4), but she is not sure which is which. The student performs a titration using one of the compounds and determines the following: A 2.738 g sample of acid dissolved in water to make 20.00 mL of solution requires 43.24 mL of 0.844 M NaOH for complete neutralization. Which acid has she titrated? (Note that both of these acids are diprotic each has two acidic hydrogens per molecule.)

CHEM 101A Review Problems Topics A-G

8) A 5.000 gram sample of an unknown compound is combusted producing 9.553 g of CO2 and 5.864 g of H2O. The unknown compound contains carbon, hydrogen, and might contain oxygen. Determine the empirical formula of this compound.

9) Add electrons to the energy diagrams below to show the requested electron configurations. Use arrows to represent the electrons. (There may be more than one correct answer.) The ground state of oxygen An excited state of oxygen 3p ____ ____ _____ 3p ____ ____ _____ 3s ____ 3s ____ 2p ____ ____ _____ 2p ____ ____ _____ 2s ____ 2s ____ 1s ____ 1s ____

10) An element in period 2 has the following successive ionization energies (in kJ/mol). Identify the element.

IE1 = 801 IE2 = 2427 IE3 = 3659 IE4 = 25,022 IE5 = 32,822

11) An equilibrium mixture consists of 0.25 M NO2 (g) and 0.45 M N2O4 (g) in a 0.75 L

container. A chemist wishes to add some NO2 to this mixture in order to increase the equilibrium amount of NO2(g) to 0.50 moles. Assume the temperature and volume are constant. How many moles of NO2(g) must be added to the container?

12) You have two identical flasks, labeled A and B. Flask A contains helium gas and flask B

contains oxygen gas. The gases exert equal pressures. However, the temperature in flask A is higher than the temperature in flask B. Answer the following questions and explain your answers.

a) Which flask contains the larger number of molecules? b) In which flask is the average velocity higher? c) Which flask contains a larger fraction of molecules with kinetic energies less than 5000

J/mol?

CHEM 101A Review Problems Topics A-G

13) Containers A and B are joined by a valve. Container A has a volume of 647 mL. With the valve closed, container A holds nitrogen gas at a pressure of 0.574 atm and a container B holds argon gas at a pressure of 0.873 atm. Both gases are held at the same temperature. When the valve is opened, the gases mix and the total pressure is 0.621 atm. What is the volume of container B? (Assume that the temperature does not change and that no chemical reaction takes place).

14) A 1.39 g sample of an unknown binary compound reacts with excess oxygen gas to produce nitrogen gas (not collected) and 1.21 g of water vapor. The compound is known to have a molar mass of 62.09 g/mol. Determine the molecular formula of this compound. (Note: a binary compound is a compound that contains only two elements.)

15) A 0.652 g sample of a pure strontium halide reacts with excess sulfuric acid (H2SO4), and the solid strontium sulfate formed is separated, dried, and found to weigh 0.755 g. What is the formula of the original halide?

16) David and Torrey are designing a calorimetry experiment for a Chemistry101A class. They take an insulated coffee mug and place 66.0 grams of ice, with a temperature of -5.0C, in it. They then heat an aluminum block over a bunsen burner until it reaches a temperature of 188. C. After placing the hot block in the coffee mug, the system comes to an equilibrium temperature of 21.0 C. What was the mass of the aluminum block that David and Torrey used? You may assume that the coffee mug does not absorb any heat.

Specific heat of water: 4.18 J/gC Specific heat of ice: 2.03 J/gC Specific heat for aluminum: 0.890 J/gC Heat of fusion for water: 6.02 kJ/mol Heat of vaporization for water: 40.7 kJ/mol

17)

a) An unknown compound is subjected to combustion analysis. Combustion of a 0.4377g sample of this compound produces 0.9224 g of carbon dioxide and 0.3776 g of water. No other products are formed in the combustion. Determine the empirical formula of this compound.

b) The molar mass of this unknown compound is found to be between 250 and 350 g/mol. Determine the molecular formula of the compound.

18) When 2.585 g of C2H7N burns in excess oxygen, 11.299 g of products form. Use this information to determine whether the nitrogen-containing product is N2, NO, NO2, or N2O.

19) A mineral sample contains calcium carbonate, along with several other compounds. When this mineral is mixed with concentrated hydrochloric acid, the calcium carbonate reacts as follows: CaCO3(s) + 2 HCl(aq) CaCl2(aq) + CO2(g) + H2O(l) A 6.235 g sample of the mineral produces 1.788 g of carbon dioxide. Calculate the percentage of calcium carbonate in the mineral.

CHEM 101A Review Problems Topics A-G

20) Lithium metal reacts with N2 (g) according to the equation: 6 Li (s) + N2 (g) 2 Li3N (s)

A sample of Li (s) was placed in a sealed 1.00L flask containing N2 (g) at a pressure of 1.23 atm and a temperature of 20.0C. After complete reaction of lithium, the pressure inside the flask had decreased to 0.92 atm and the temperature remained unchanged. Calculate the mass of Li (s) originally present.

21) A solution contains only one of the following anions: CO32-, Cl-, or OH-. Based on the observations below, which ion must be present? Briefly explain your choice. You must use words in your explanation!

Observations:

1. When 1 M HNO3 is added to a sample of the solution, there is no visible change. 2. When 0.1 M Pb(NO3)2 is added to a fresh portion of the original solution, some solid

forms. 3. When excess 1 M HNO3 is added to a fresh portion of the solution and then 0.1 M

Pb(NO3)2 is added to the solution, no solid forms.

22) Estimate the change in enthalpy for the combustion of one mole or acetone. The structure of acetone is shown below, and its molecular formula is C3H6O. Average bond dissociation energies are given in the table below.

Bond BDE (kJ/mol) CO 360 C=O 736 OO 146 O=O 498 OH 464 HH 436 CH 414 CC 347 C=C 611

23) The Ka of hydrocyanic acid, HCN, is 6.2 x 10-10. Determine the pH of a 0.25 M solution

of hydrocyanic acid. Use the approximation method to solve the equation. Clearly show your work and indicate what approximation you made.

CHEM 101A Review Problems Topics A-G

24) Consider the following oxygenation reaction of hemoglobin (Hb) to answer the following questions:

Hb(aq) + O2(aq) HbO2(aq) Hemoglobin (Hb) is dark red in color, while oxygenated hemoglobin (HbO2) is bright red in color. a) Using Le Chateliers Principle, explain what color of blood you would expect to see

in an oxygen rich environment, such as the lungs. Explain your answer clearly. b) Using Le Chateliers Principle, which direction does the equilibrium shift in an

oxygen deificient environment? Explain your answer clearly. c) Carbon monoxide poisoning occurs because carbon monoxide, CO, also binds to Hb

and does so more strongly than O2 (the reaction with CO has a larger K value than the reaction with O2). If CO were introduced, what would happen to the equilibrium reaction given above? Use Le Chateliers Principle to explain.

25) A sealed container holds a mixture of ammonia, nitrogen, and hydrogen gas at equilibrium: 2 NH3(g) N2(g) + 3 H2(g)

When the temperature of the mixture decreased from 500C to 300C, equilibrium is reestablished at a new position. Using the bond dissociation energies from the table, determine if the new equilibrium partial pressure of NH3 will be higher or lower compared to that of the original equilibrium mixture. Explain your reasoning.

Bond BDE

(kJ/mol) NN 167 N=N 418 NN 942 NH 386 HH 432

26) A 5.00 L rigid flask contains an equilibrium mixture of 0.500 M NOCl(g), 0.020 M NO

and 0.010 M Cl2.

2 NOCl(g) 2 NO(g) + Cl2(g) A chemist injects an unknown amount of chlorine gas intro the mixture. After equilibrium is reestablished, she determines that the concentration of the NO(g) is now 0.015 M. How many moles of chlorine gas must have been injected?

27) When 0.100 mol of gaseous N2 and 0.100 mol of gaseous H2 are put into a 5.00 L container at 250 C, the following reaction occurs and reaches equilibrium.

N2(g) + 3 H2 (g) 2 NH3(g) The total pressure in the flask at equilibrium is 1.52 atm. Calculate KP and Kc for this reaction.

CHEM 101A Review Problems Topics A-G

28) A ground state hydrogen atom absorbs a photon with an energy of 1599.022 kJ/mol. It then emits a photon with an energy of 338.542 kJ/mol. What is the final state of the hydrogen atom? Show all your work and reasoning.

29) The bond energy of the Cl-Cl bond in chlorine gas is 239 kJ/mol and the bond length is 199 pm. a) If a Cl-Cl bond is broken, is energy released or absorbed? _____________________ b) Draw a bond energy diagram (a graph of potential energy versus the distance

between the nuclei) for the bond in Cl2. Be certain to include labels and units and to clearly indicate where the potential energy of the system equals zero.

30) For a 0.015 M solution of an unknown monoprotic acid, the pH is 3.24. Determine the pH

of a 0.10 M solution of the same acid.

31) An element from the 3rd period can combine with chlorine to form two different compounds XCl2 and XCl4. Both compounds are non-polar. Based on this information, what is the identity of element X? Explain your reasoning clearly. Include a clear sketch of the structure for each molecule.

32) The following gaseous compounds of nitrogen and oxygen are known to exist: N2O, NO, NO2, and N2O4. The reaction of metallic copper with dilute nitric acid produces only one of these gases, with the possible reactions being:

4 Cu(s) + 10 HNO3(aq) N2O(g) + 4 Cu(NO3)2(aq) + 5 H2O(l) 3 Cu(s) + 8 HNO3(aq) 2 NO(g) + 3 Cu(NO3)2(aq) + 4 H2O(l) Cu(s) + 4 HNO3(aq) 2 NO2(g) + Cu(NO3)2(aq) + 2 H2O(l) Cu(s) + 4 HNO3(aq) N2O4(g) + Cu(NO3)2(aq) + 2 H2O(l) The reaction of 0.492 g of copper with excess 1M HNO3 produces 130 mL of gaseous product at 18C and 721 torr. Identify the gaseous product of this reaction.

33) Gaseous ammonia and gaseous nitrogen chloride react to give ammonium chloride in a reversible reaction. An equilibrium mixture is found to contain 0.038 atm of ammonia, 0.0071 atm of hydrogen chloride, and 2.16 g of ammonium chloride in a 5.17 L container at 165C. Calculate the K and Kp for this reaction at 165C.

NH4Cl(s) NH3(g) + HCl(g)

34) Using the following Ka values, determine which solution has the highest pH. Briefly explain your choice. Ka values: HNO2 = 4.0 x 10-4 HClO = 3.5 x 10-8 HCN = 6.2 x 10-10 Choices: 0.010M HNO2, 0.010M HClO, 0.010M HCN

CHEM 101A Review Problems Topics A-G

35) Zinc metal reacts with hydrochloric acid according to the following net ionic equation:

Zn (s) + 2 H+ (aq) Zn2+ (aq) + H2 (g) The hydrogen gas is collected over water. The gas sample has a volume of 240.0 mL at 1.032 atm and 30.0C. What mass of Zn must have reacted? The vapor pressure of water is 0.042 atm at 30.0C.

36) An unknown metal M forms an oxide containing 25.9% oxygen by mass. Identify the metal if the formula of this oxide is M2O.

37) When a 0.105 g sample of propylene (molar mass: 42.08 g/mol) is burned in a bomb calorimeter, the temperature of the calorimeter increases from 25.00C to 27.14C. The equation and H at 25.00C are given below:

2 C3H6(g) + 9 O2(g) 6 CO2(g) + 6 H2O(l) H = -4249 kJ Use this information to determine the heat capacity of this bomb calorimeter. 38) For the reaction HCl (aq) + NaHCO3(aq) NaCl (aq) + CO2(g) + H2O(l), E = 13 kJ. Based on this information, which of the following statements MUST be true under all conditions? (PE = potential energy, KE = kinetic energy)

A) The PE of the reactants is greater than the PE of the products. B) The PE of the reactants equals the PE of the products. C) The PE of the reactants is less than the PE of the products. D) The KE of the reactants is greater than the KE of the products. E) The KE of the reactants equals the KE of the products. F) The KE of the reactants is less than the KE of the products.

39) Consider the two samples of gas described below: Sample A: 3.00 L of H2(g) at 0.0C, 2.00 atm Sample B: 3.00 L of Cl2(g) at 300.0C, 4.50 atm Which of these gas samples has the higher fraction of molecules with kinetic energies greater than 10,000 J/mol? Explain your answer clearly. 40) A boron compound reacts with water, converting all the boron in the compound to boric acid (H3BO3). The boric acid solution is titrated with NaOH solution. (NaOH reacts with boric acid in a 1:1 mole ratio.) A 0.2341 g sample of boron compound reacts with water and the resulting boric acid solution is titrated to an endpoint with 27.7 mL of NaOH solution. If the boron compound contains 27.4% by mass boron, what is the molarity of the NaOH solution?