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MASS-VOLUME STOICHIOMETRY •Many chemical reactions involve gases. •For example, barbecues, furnaces and car engines. •To solve the mass-volume problems involving gases use the 4 step process for stoichiometric calculations. •Gas formulas may be used in these calculations. n = V or PV = nRT Vm Sample Problem 1 (M-V): Finding volume of gas not at STP or SLC. In a gas barbeque, propane was burnt in oxygen to form carbon dioxide and water vapour. 22.0 g of CO2 is collected and weighed. Find the volume of propane at 200oC and 101 325 Pa. Solution: Step 1: Write a balanced equation and identify known and unknown. C3H8 (g) + 5O2 (g) -->3CO2 (g) + 4H20 (g) unknown known V = ??? m = 22.0 g Step 2: Calculate n (known). n(CO2) = m / M = 22.0 / (12 + 2 x 16) = 22.0 / 44 = 0.5 mol Step 3: From equation find ratio of n(unknown) to n(known). n(C3H8) = 1/3 x n(CO2) = 1/3 x 0.5 = 0.167 mol Step 4: Answer Question – Find Volume of propane. P = 101 325 Pa = 101.3 kPa T = 200oC = 473 K PV = nRT V (C3H8) = nRT / P = 0.167 x 8.31 x 473 / 101.3 = 6.5 L VOLUME-VOLUME STOICHIOMETRY •Chemical reactions where all reactants and products are gases. •If the gases are at the same pressure and temperature, their molar ratios are equal to their volume ratios. •For example: N2 (g) + 3H2 (g) -->2NH3 (g) molar ratio: 1 3 2 volume ratio: 10 mL 30 mL 20 mL Sample Problem: Finding volume of gas when conditions of pressure and temperature are constant. Hydrogen sulfide may be oxidised to sulfur dioxide and water. What volume of oxygen is needed to react with 10 mL of hydrogen sulfide? Solution: Step 1: Write a balanced equation and identify known and unknown. 2H2S (g) + 3O2 (g) --> 2H2O (g) + 2S02 (g) known unknown V = 10 mL V = ??? Step 2: From equation find ratio of V(unknown) to V(known). Volume ratio is same as molar ratio because of constant pressure and temperature. V (O2) = 3/2 x V (H2S) = 3/2 x 10 = 15 mL |
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Sample Problem 2 (M-V):
Finding volume of gas at STP or SLC. What volume of hydrogen gas at STP is formed when 0.30 grams of magnesium metal is reacted completely with dilute sulfuric acid? Solution: Step 1: Write a balanced equation and identify known and unknown. Mg (s) + H2SO4 (aq) --> MgSO4 (aq) + H2 (g) known unknown m = 0.30 g V = ??? Step 2: Calculate n (known). n(Mg) = m / M = 0.30 / 24.3 = 0.012 mol Step 3: From equation find ratio of n(unknown) to n(known). n(H2) = 1/1 x n(Mg) = 1 x 0.012 = 0.012 mol Step 4: Answer Question – Find Volume of hydrogen. VSTP = 22.4 L n = V / Vm V (H2) = n x Vm = 0.012 x 22.7 = 0.2724 L = 0.27 L Comparing fuel efficiency •The coefficients of the reactants in a thermochemical equation indicate the amounts (moles) of each substance that reacts to give the specified enthalpy change. •Thus to you need to work the given equation and given mass together in a stoichiometric fashion to determine: 1.How much mole of the fuel you actually have. 2.The actual energy released for this number of mole. Example: Calculate the energy released , in MJ, when 20.0 kg of methane undergoes complete combustion. CH4(g) + 2O2 (g) --> CO2(g) + 2H2O (l) ΔH= -890KJ/mol Solution: n=m/M, n= 20000/16 = 1250 mol. Using the equation- 1 mole of methane releases 890KJ of energy. For 1250 moles, Energy = 1250/1 x890 = 1112500KJ (using ratios) Energy = 1113 MJ = 1.11 x 103 MJ •Thermochemical equations can be used to compare fuels. •These equations can be used to calculate the energy per mole the fuel releases. •The equations also allow for the calculation of carbon dioxide or carbon monoxide released as a product into the environment. |