Chemical Calculations

Mole-Mole Calculations The coefficients from the balanced equation are used to write conversion factors called mole rations. With these mole rations, you can relate moles of reactants to moles of product.

Calculating Moles of a Product How many moles of ammonia are produced when 0.60 mol of nitrogen reacts with hydrogen? Known: 0.60 mol N2 Unknown: moles of NH3

Calculating Moles of a Product Start with what is given and work from there. 0.60 mol N2 x 2 mol NH3/1 mol N2 = 1.2 mol NH3

Calculating Moles of a Product - Practice 8. 4Al + 3O2 2Al2O3 A. How many moles of aluminum are needed to form 3.7 mol Al2O3?

Calculating Moles of a Product - Practice 9. according to the equation in problem 8: A. how many moles of oxygen are required to react completely with 14.8 mol Al? B. How many moles of Al2O3 are formed when 0.78 mol O2 reacts with aluminum?

Mass-Mass Calculations The amount of a substance is usually determined by measuring its mass in grams. From the mass of a reactant or product, the mass of any other reactant or product in a given chemical equation can be calculated.

Mass-Mass Calculations If a given sample is measured in grams, the mass can be converted to moles by using the molar mass. Then the mole ration from the balanced equation can be used to calculate the number of moles of the unknown.

Mass-Mass Calculations If it is the mass of the unknown that needs to be determined, the number of moles of the unknown can be multiplied by the molar mass.

Steps in Solving a Mass-to-Mass Problem 1. Change the mass G to moles of G(mass G  mol G) by using the molar mass of G. Mass G x (1 mol G/molar mass G) = mol G 2. Change the moles of G to moles of W (mol G  mol W) by using the mole ratio from the balanced equation. Mol G x (b mol W/a mol G) = mol W

Steps in Solving a Mass-to-Mass Problem 3. Change the moles of W to grams of W (mol W  mass W) by using the molar mass of W. Mol W x (molar mass W/1 mol W) = mass W

Steps in Solving a Mass-to-Mass Problem Calculate the number of grams of NH3 produced by the reaction of 5.4g of hydrogen with and excess of nitrogen: N2(g) + 3H2(g)  2NH3(g) Step 1: change grams of hydrogen into moles of hydrogen 5.4g H2 x (1 mol of H2/2g of H2) = 2.7 mol H2

Steps in Solving a Mass-to-Mass Problem Step 2: Change moles of H to moles of NH3 2.7 mol H2 x (2 mol NH3/3 mol H2) = 1.8 mol NH3 Step 3: Change moles of NH3 to grams of NH3 1.8 mol NH3 x (17g/1 mol NH3) = 31g NH3

Practice 10. How many grams of acetylene (C2H2) are produced by adding water to 5g CaC2? CaC2(s) + 2H2O(l)  C2H2(g) + Ca(OH)2(aq)

Practice 11. Using the same equation, determine how many moles of CaC2 are needed to react completely with 49g H2O.

Other Stoichiometric Calculations In any stoichiometry problems, the given quantity is first converted to moles. Then the mole ratio from the balanced equation is used to calculate the number of moles of the wanted substance. Finally, the moles are converted to any other unit of measurement related to the unit mole, such as volume, mass, and representative particles.

Calculating Molecules of a Product Practice How many molecules of oxygen are produced when a sample of 29.2g of water is decomposed by electrolysis? 2H2O(l)  2H2(g) + O2(g) Step 1: Grams of water to moles of Water 29.2g x (1 mol H2O/18g) = 1.622 mol

Calculating Molecules of a Product Practice Step 2: Moles of H2O to Moles of O2: 1.622 mol H2O x (1 mol O2/2 Mol H2O) = .81 mol O2 Step 3: Moles of O2 to grams of O2: .81 mol O2 x (6.02 x 1023 molecules O2/1 mol O2) = 4.88 x 1023 molecules O2

Practice 12. How many molecules of oxygen are produced by the decomposition of 6.54g of potassium chlorate (KClO3)? 2KClO3(s)  2KCl(s) + 3O2(g)

Practice 13. How many grams of nitrogen dioxide must react with water to produce 5.00 x 1022 molecules of nitrogen monoxide? 3NO2(g) + H2O(l)  2HNO3(aq) + NO(g)

Volume-Volume Practice How many liters of nitrogen dioxide are produced when 34 L of oxygen reacts with an excess of nitrogen monoxide? 2NO(g) + O2(g)  2NO2(g) Step 1: Turn L of O2 into moles of O2: 34L x (1 mol O2/22.4 L) = 1.52 mol O2

Volume-Volume Practice Step 2: Turn moles of O2 into Moles of NO2: 1.52 mol O2 x (2 mol NO2/1 mol O2) = 3.04 mol NO2 Step 3: turn mol of NO2 into L of NO2: 3.04 mol NO2 x (22.4L NO2/1 mol NO2) = 68 L NO2

Volume-Volume Practice 14. How many liters of oxygen are required to burn 3.86L of carbon monoxide? 2CO(g) + O2(g)  2CO2(g)

Volume-Volume Practice 15. How many liters of phosphine are formed when 0.42 L of hydrogen reacts with phosphorus? P4(s) + 6H2(g)  4PH3(g)