Chemistry EOC Review Part 4: Molar Relationships

Chemistry EOC Review 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 You will need a calculator and periodic table to complete this section. This section represents 8/50 of the EOC questions

Chemistry EOC 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 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

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 L He = 78 Liters He g He mole He

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?

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 g Ar = 16 g Ar 22.4 L Ar mole Ar

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

The Mole and Mole Calculations The molar mass = the sum of all the atomic masses. Example Ca(NO3)2 = (14.01) + 6(16.00) = grams You try one: What is the gram formula mass (molar mass) of Mg3(PO4)2? 3(24.305) + 2( ) + 8( ) = grams

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

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 EOC type question: What is the empirical formula of a compound that contains 30% Nitrogen and 70% Oxygen? a) N2O b) NO2 c) N2O5 d) NO This is really a percent composition problem. Figure out which compound contains 30% nitrogen.

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 = ( ) = g Density = mass/volume = g/22.4 L = 1.96 g/L

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 = ( ) = g Density = mass/volume = g/22.4 L = 1.96 g/L You try one: What is the density of Cl2 gas at STP?

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 = ( ) = g Density = mass/volume = 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) = g g/22.4 L = g/L

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) → 3NH3(g) Given: 9 moles H2, Find moles N2 9 mol H2 x 2 mol N2 = 6 mol N2 3 mol H2 Mole ratio

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 2.0 g H2 x 1 mol H2 2 mol N2 g N2 = g N2 2.016g H2 3 mol H2 1 mol N2

Gas Laws General Properties of Gases There 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 = K The Gas Laws The Combined Gas Law P1V1T2 = P2V2T1 Boyle’s Law Inverse relationship P1V1 = P2V2 Charles Law V1T2 = V2T1

Gas Laws Some Problems A balloon contains 8.0 liters of gas at 100 K. What is the balloon’s volume at 200K? P1V1T2 = P2V2T1 P1V1 = P2V2 V1T2 = V2T1 8 = V2 100 200 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? (3.0)(10) = (0.50)V2 275 200 Answer: = 45 Liters

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 EOC test uses kPa x L Moles x K

Gas Laws The Ideal Gas Law Memorize: PV = nRT R = 8.31 kPa x L Moles x K 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 = kPa You try: How many moles of Hydrogen gas are in a 20. L tank pressurized to kPa at 300K? Answer: P=1000., V=20. L, n=? T=300K (1000.)(20) = n(8.31)(300) so n = 8.0 moles Hydrogen

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 = PN so PN2 = 2

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/ L = = 1.3 M

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 72.9 g HCl x 1 mol HCl = 2.00 mol HCl 36.46 g HCl Then calculate molarity of solution: 2.00 mol HCl/5.0 L = 0.40 M HCl

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 1.2 g LiF x 1 mol LiF = mol LiF 25.94 g LiF Then calculate molarity of solution (remember convert mL to Liters): 0.046 mol LiF/0.050 L = 0.95 M LiF

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

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

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

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.

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

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

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

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

Chemical Equilibrium Methods to Speed up Reactions: Use a catalyst Reduce the particle size Increase the heat Increase reactant concentration

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 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

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

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