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Chemistry SOL Review by Anne Mooring (Jamestown High School, Williamsburg VA, 2006)

Chemistry SOL Review by Anne Mooring (Jamestown High School, Williamsburg VA, 2006). Part 4: Molar Relationships The mole and molar calculations Stoichiometry Gas Laws (Boyle, Charles, Combined, Ideal, Dalton, Graham) Solution Concentrations Chemical Equilibrium Acid/Base Theory

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Chemistry SOL Review by Anne Mooring (Jamestown High School, Williamsburg VA, 2006)

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  1. Chemistry SOL Reviewby Anne Mooring (Jamestown High School, Williamsburg VA, 2006) • Part 4: Molar Relationships • The mole and molar calculations • Stoichiometry • Gas Laws (Boyle, Charles, Combined, Ideal, Dalton, Graham) • Solution Concentrations • Chemical Equilibrium • Acid/Base Theory • Use the SOL periodic table. Click here for link You will need a calculator and periodic table to complete this section. This section represents 8/50 of the SOL questions

  2. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations One mole = 6.02 x 1023 representative particles One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure One mole = the atomic mass listed on the periodic table. For example: one mole of Helium contains 6.02 x 1023 atoms of Helium and it has a mass of 4.00260 grams. At 0°C and one atmosphere of pressure, it would occupy 22.4 Liters. • Sample problem: How many liters would 2.0 moles of Neon occupy? • Answer: • 2.0 moles Ne x 22.4 Liters Ne = 44.8 Liters Ne • 1.0 moles Ne • Sample problem: How many moles are in 15.2 grams of Lithium? • Answer: • 15.2 g Li x 1 mole Li = 2.19 mole Li • 6.941 g Li

  3. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations One mole = 6.02 x 1023 representative particles One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure One mole = the atomic mass listed on the periodic table. • Sample problem: How many liters would 14 grams of Helium occupy? • Answer: • 14 g He x 1 mole He x 22.4 L He = 78 Liters He • 4.0026 g He 1 mole He

  4. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations One mole = 6.02 x 1023 representative particles One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure One mole = the atomic mass listed on the periodic table. You try one: What is the mass of 9.0 Liters of Argon gas at 0°C and one atmosphere of pressure?

  5. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations One mole = 6.02 x 1023 representative particles One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure One mole = the atomic mass listed on the periodic table. You try one: What is the mass of 9.0 Liters of Argon gas at 0°C and one atmosphere of pressure? 9.0 L Ar x 1 mol Ar x 39.948 g Ar = 16 g Ar 22.4 L Ar 1 mole Ar

  6. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations The molar mass = the sum of all the atomic masses. Example Ca(NO3)2 = 40.08 + 2(14.01) + 6(16.00) = 164.10 grams You try one: What is the gram formula mass (molar mass) of Mg3(PO4)2?

  7. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations The molar mass = the sum of all the atomic masses. Example Ca(NO3)2 = 40.08 + 2(14.01) + 6(16.00) = 164.10 grams You try one: What is the gram formula mass (molar mass) of Mg3(PO4)2? 3(24.305) + 2(30.97376) + 8(15.9994) = 262.86 grams

  8. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations The molar mass = the sum of all the atomic masses. Example Ca(NO3)2 = 40.08 + 2(14.01) + 6(16.00) = 164.10 grams You try one: What is the gram formula mass (molar mass) of Mg3(PO4)2? 3(24.305) + 2(30.97376) + 8(15.9994) = 262.86 grams What is the percent Magnesium in Mg3(PO4)2? Answer: 3(24.305) x 100 = 27.7% 262.86 You try one: What is the percent Lithium in Li2SiO3? molar mass = 2(6.941) + 28.0855 + 3(15.9994) = 89.9657 g % Li = 2(6.941) x 100 = 15.4% 89.9657

  9. Chemistry SOL Review— Molar Relationships A Brief Return to Empirical Formulas Empirical Formulas are the reduced form of Molecular formulas. For example: The empirical formula for C5H10 is CH2. A favorite SOL type question: What is the empirical formula of a compound that contains 30% Nitrogen and 70% Oxygen? This is really a percent composition problem. Figure out which compound contains 30% nitrogen. a) N2O b) NO2 c) N2O5 d) NO

  10. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations • At Standard Temperature and Pressure (STP) 1 mole of gas = 22.4 L • You can use this to calculate the density of a gas in g/Liter at STP. • Example: What is the density of CO2 gas at STP? • The molar mass of CO2 = 12.0111 + 2(15.9994) = 44.0099 g • Density = mass/volume = 44.0099 g/22.4 L = 1.96 g/L

  11. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations • At Standard Temperature and Pressure (STP) 1 mole of gas = 22.4 L • You can use this to calculate the density of a gas in g/Liter at STP. • Example: What is the density of CO2 gas at STP? • The molar mass of CO2 = 12.0111 + 2(15.9994) = 44.0099 g • Density = mass/volume = 44.0099 g/22.4 L = 1.96 g/L • You try one: • What is the density of Cl2 gas at STP?

  12. Chemistry SOL Review— Molar Relationships The Mole and Mole Calculations • At Standard Temperature and Pressure (STP) 1 mole of gas = 22.4 L • You can use this to calculate the density of a gas in g/Liter at STP. • Example: What is the density of CO2 gas at STP? • The molar mass of CO2 = 12.0111 + 2(15.9994) = 44.0099 g • Density = mass/volume = 44.0099 g/22.4 L = 1.96 g/L • You try one: • What is the density of Cl2 gas at STP? • Answer: molar mass = 2(35.453) = 70.906 g • 70.906 g/22.4 L = 3.165 g/L

  13. Chemistry SOL Review— Molar Relationships Mole ratio Stoichiometry For reaction calculations, the molar ratio is used. Example: How many moles of nitrogen will react with 9 moles of hydrogen to produce ammonia according to this equation? 2N2(g) +3 H2(g) → 2NH3(g) Given: 9 moles H2, Find moles N2

  14. Chemistry SOL Review— Molar Relationships Stoichiometry For reaction calculations, the molar ratio is used. Example 2: How many grams of nitrogen are needed to react with 2.0 grams of hydrogen using this equation? 2N2(g) +3 H2(g) → 3NH3(g) Given: 2.0 grams H2, Find grams N2

  15. Chemistry SOL Review— Molar Relationships Gas Laws • General Properties of GasesThere is a lot of “free” space in a gas.Gases can be expanded infinitely.Gases fill containers uniformly and completely.Gases diffuse and mix rapidly. Always use degrees Kelvin °C + 273 = K The Gas Laws The Combined Gas Law Boyle’s Law Inverse relationship Charles Law

  16. Chemistry SOL Review— Molar Relationships Gas Laws Some ProblemsA balloon contains 8.0 liters of gas at 100 K. What is the balloon’s volume at 200K? Answer: = 16 Liters A balloon contains 10. Liters at 3 atmospheres and 275 K. What is the volume of the balloon at 0.50 atmospheres and 200K? Answer: = 45 Liters

  17. Chemistry SOL Review— Molar Relationships Gas Laws • The Ideal Gas Law • Memorize: PV = nRT • P= pressure in kPa • V= liters • N= moles • T= temperature in Kelvin • R = universal gas law constant = 8.31 • The SOL test uses

  18. Chemistry SOL Review— Molar Relationships Gas Laws The Ideal Gas Law Memorize: PV = nRT Example 1:A 15 liter tank contains 2.0 moles of nitrogen gas at 27 °C. What is the pressure of nitrogen inside the tank? Answer: P=?, V=15 L, n=2.0, T=300K (remember to convert) P(15)=2.0(8.31)(300) so P = 332.4 kPa You try: How many moles of Hydrogen gas are in a 20. L tank pressurized to 1000. kPa at 300K? Answer: P=1000., V=20. L, n=? T=300K (1000.)(20) = n(8.31)(300) so n = 8.0 moles Hydrogen

  19. Chemistry SOL Review— Molar Relationships Gas Laws Dalton’s Law of Partial Pressures Memorize: Ptotal = P1 + P2 + P3 + … • Example A tank containing nitrogen, hydrogen and ammonia gas has a total pressure of 12 atmospheres. The partial pressure of the hydrogen is 6 atmospheres, the partial pressure of the ammonia is 4 atmospheres. What is the partial pressure of the nitrogen? • Answer: • Ptotal = 12 atm, PN2=?, PH2=6, PNH3=4 • 12 = PN2 + 6 + 4 so PN2 = 2

  20. Chemistry SOL Review— Molar Relationships Solution Concentrations • Calculating molarity: • Memorize this equation: Molarity = moles/liters or M = mol/L • Memorize conversion factor: 1000 mL = 1 L • Some example of using this equation: • Example 1: the molarity of 2.0 moles of HCl in a 0.50 L solution of water is: • molarity = 2.0 mole HCl/0.50 L = 4.0 Molar or 4 M • Example 2: The molarity of 0.40 moles of HCl in a 300. mL L solution of water is: • molarity = 0.40 moles HCl/0.300. L = = 1.3 M

  21. Chemistry SOL Review— Molar Relationships Solution Concentrations • Example 3: • The molarity of 72.9 g of HCl in 5.0 liters of aqueous solution is: • Answer: first calculate the moles of HCl Then calculate molarity of solution: 2.00 mol HCl/5.0 L = 0.40 M HCl

  22. Chemistry SOL Review— Molar Relationships Solution Concentrations • You try one: • What is the molarity of 1.2 grams LiF in a 50. mL aqeous solution? • Answer: first calculate the moles of LiF Then calculate molarity of solution (remember convert mL to Liters): 0.046 mol LiF/0.050 L = 0.95 M LiF

  23. Chemistry SOL Review— Molar Relationships Solution Concentrations • Diluting concentrated solutions • Memorize: M1V1 = M2V2 • M1 and V1 are the beginning molarities and volumes • M2 and V2 are the ending molarities and volumes • V1 and V2 can be in Liters or mLs, but must be the same units for both • Example: • What is the molarity of a 10. mL sample of 2.0 M aqueous HCl diluted to 40. mL • Answer: • (2.0)(10.) = (M2)(40.) so M2 = 0.5 Molar HCl

  24. Chemistry SOL Review— Molar Relationships Solution Concentrations • Diluting concentrated solutions • Memorize: M1V1 = M2V2 • M1 and V1 are the beginning molarities and volumes • M2 and V2 are the ending molarities and volumes • V1 and V2 can be in Liters or mLs, but must be the same units for both • You try one: • How many milliliters of 6.0 Molar HCl are required to prepare 240 mL of 2.0 Molar HCl? • Answer: • (6.0)(V1) = (2.0)(240) so V1 = 80. mL HCl

  25. Chemistry SOL Review--Molar Relationships Chemical Equilibrium Exothermic and Endothermic Reactions Catalysts lower the Activation energy barrier, making reactions faster.

  26. Chemistry SOL Review--Molar Relationships Chemical Equilibrium Reversible Reactions Some reactions are REVERSIBLE, which means that they can go backwards (from product to reactant) Example: The reaction between nitrogen and hydrogen, where a “” indicates a reversible reaction N2(g) + 3H2(g)  2 NH3(g) + heat The forward reaction takes place at the same rate as the reverse reaction. The equilibrium position of products and reactants depends on the conditions of the reaction. If we change the reaction conditions, the equilibrium changes.

  27. Chemistry SOL Review--Molar Relationships Chemical Equilibrium Reversible Reactions Le Chatelier’s Principle: If a system at equilibrium is stressed, the equilibrium will shift in a direction that relieves that stress. Equilibrium will shift AWAY from what is added. Here, N2 is added. N2 More “product” made N2(g) + 3H2(g)  2 NH3(g) + heat

  28. Chemistry SOL Review--Molar Relationships Chemical Equilibrium Reversible Reactions Le Chatelier’s Principle: If a system at equilibrium is stressed, the equilibrium will shift in a direction that relieves that stress. Equilibrium will shift AWAY from what is added. Here, NH3 is added. More “reactants” made NH3 N2(g) + 3H2(g)  2 NH3(g) + heat

  29. Chemistry SOL Review--Molar Relationships Chemical Equilibrium Reversible Reactions Le Chatelier’s Principle: If a system at equilibrium is stressed, the equilibrium will shift in a direction that relieves that stress. Equilibrium will shift TOWARDS what is removed. Here H2 is removed. H2 N2(g) + 3H2(g)  2 NH3(g) + heat More “reactants” made

  30. Chemistry SOL Review--Molar Relationships Chemical Equilibrium Reversible Reactions Le Chatelier’s Principle: If a system at equilibrium is stressed, the equilibrium will shift in a direction that relieves that stress. Equilibrium will shift TOWARDS what is removed. Here heat is removed. heat N2(g) + 3H2(g)  2 NH3(g) + heat More “product” made

  31. Chemistry SOL Review--Molar Relationships Chemical Equilibrium • Methods to Speed up Reactions: • Use a catalyst • Reduce the particle size • Increase the heat • Increase reactant concentration

  32. Chemistry SOL Review--Molar Relationships Acid/Base Theory Acids and Bases Generic formula for acids = HX (HCl, HNO3, H2SO4) Generic formula for bases = MOH where M is any metal (NaOH, KOH, Ca(OH)2 Ammonia, NH3, is also a base. Acid solutions have a pH less than 7 Basic solutions have a pH more than 7 Arrhenius acids: Taste _______ turn litmus paper red. Arrhenius bases Taste _______ Feel __________ Turn litmus paper blue. SAFETY NOTES Always add acid to water when diluting If you spill acid or base on yourself, rinse with lots of water. sour bitter slippery

  33. Chemistry SOL Review--Molar Relationships Acid/Base Theory • What is pH? • pH indicates the hydrogen ion molarity [H+] in a solution • pH = -log[H+] • pOH indicates the hydroxide ion molarity [OH-] in a solution. • pOH = -log[OH-] • Example: A 1.0 x 10-3 molar solution of HCl would have a pH of ___ • Example: A 1.0 x 10-4 molar solution of KOH would have a pOH of ___ • Memorize: pH + pOH = 14. • Example: A solution with a pH of 8 will have a pOH of: ____. 3 4 6

  34. Chemistry SOL Review--Molar Relationships References http://www.markrosengarten.com/ for New York Regent’s exam powerpoint.

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