Balance the following equation: KClO 3 + C 12 H 22 O 11 KCl + CO 2 + H 2 O

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1 Balance the following equation: KClO 3 + C 12 H 22 O 11 KCl + CO 2 + H 2 O Ans: 8 KClO 3 + C 12 H 22 O 11 8 KCl + 12 CO H 2 O 3.2 Chemical Symbols at Different levels Chemical symbols represent both a microscopic and macroscopic level. Microscopic level- chemical formulas represent the behavior of (a few) atoms and molecules. Macroscopic level- Chemical formulas and chemical equation now represent the large- scale ( gram scale) behavior of atoms and molecules that give rise to observable properties. 3.3 Avogadro s Number and the Mole To deal with macroscopic behavior of chemicals requires that the number (or coefficient) ratio from the balanced equation be converted to a mass ratio. Mass ratio is determined using molecular and formula masses. Molecular mass = sum of the atomic masses of all atoms in the molecule Formula mass = Sum of the atomic masses of all atoms in the formula unit of any compound, molecular or ionic. For NaCl: atomic mass of Na = amu atomic mass of Cl = amu formula mass of NaCl = amu 1

2 For C 2 H 5 Cl: atomic mass of 2 C = 2 x amu atomic mass of 5 H = 5 x 1.01 amu atomic mass of Cl molecular mass of C 2 H 5 Cl = amu = 5.05 amu = amu = amu Mass Number Relationship Samples of different substances contain the same number of molecules (or formula units) whenever their mass ratio is the same as their molecular mass ratio. ex., the mass ratio of one formula unit of NaCl to one molecule of C 2 H 5 Cl is to One mole ( mol) = x particles. This number is called Avogadro s Number (NA) i.e., 1 mol of HNO 3 = x units of HNO 3 Also one mole of any substance is the amount whose mass- its molar mass- is equal to the molecular or formula mass of the substance in grams. 1 mol of C 2 H 5 Cl = x molecules = molar mass of C 2 H 5 Cl = g 1 mol of NaCl = x formula units = molar mass of NaCl = g 2

3 The molar mass is in units of g/ mol i.e., Molar mass of NaCl = 58.5 g/ mol Now, whenever you see a balanced equation, the coefficients tell how many moles of each substance are needed. From the # of moles you can also calculate the masses required. 3 H 2 + N 2 2 NH 3 Mole relationship 3 mol of H 2 reacts with 1 mol of N 2 to produce 2 mol of NH 3 Mass relationship (3 mol x 2.02 g/mol) = 6.06 g of H 2 reacts with (1 mol x g/mol) = g of N 2 to produce (2mol x g/mol) = g of NH 3 PROBLEM: What is the formula mass (rounded to 2 decimal places) of ammonium dihydrogen phosphate? What is the molar mass? Ans: formula mass= amu molar mass= g/mol PROBLEM: For the balanced equation C 5 H O 2 5 CO H 2 O How many moles of oxygen will react with 0.50 mol of C 5 H 12? Ans: 4.0 mol of O 2 3

4 Stoichiometry : Chemical Arithmetic Stoichiometry- refers to the conversion between moles and grams of reactants and products in a chemical equation. to convert mol of A mass of A mol of A x (molar mass conversion factor) = mass of A to convert mass of A mol of A mass of A x (molar mass conversion factor) = mol of A Useful molar mass conversion factors are: 1 mol of A and molar mass of A molar mass of A 1 mol of A Example: How many moles of PCl 3 are present in 8.0 g PCl 3? Calculate molar mass of PCl 3 = g/mol + (3 x g/mol) = g/mol This molar mass represents conversion factors: g PCl 3 or 1 mol PCl 3 1 mol PCl g PCl 3 mol of PCl 3 = 8.0 g PCl 3 x 1 mol PCl g PCl 3 = or mol PCl 3 PROBLEM: How many g are in 8.33 x 10-3 C 12 H 22 O 11? Ans: 2.85 g mol of 4

5 Calculating the grams of reactants needed or the grams of product produced knowing the grams or moles of another reactant or product 1. Begin with balanced equation 2. With the known grams of reactant, convert to moles 3. Using coefficients in balanced equation, convert between moles of substances 4. Knowing mole, convert to grams The figure on the right shows how to calculate the mass of reactant B needed to react with a given mass of reactant A EXAMPLE: NaOCl, household bleach, can be prepared by the reaction of sodium hydroxide with chlorine following the balanced equation: 2 NaOH (aq) + Cl 2 (g) NaOCl (aq) + NaCl (aq) + H 2 O (l) How many grams of NaOH are needed to react with 25.0 g of Cl 2? SOLUTION: The general strategy is outlined in Figure 3.2 below: 5

6 Calculate molar mass of Cl 2 = g/mol USE THESE AS Calculate molar mass of NaOH = g/mol CONVERSION FACTORS 1. Convert mass of Cl 2 to mol Cl g of Cl 2 x 1 mol of Cl 2 = mol Cl g Cl 2 2. Convert moles of Cl 2 (given) to moles of NaOH (required) mol Cl 2 x 2 mol NaOH = mol NaOH required 1 mol Cl 2 3. Convert mol of NaOH (required) to mass of NaOH (required) mol NaOH x g NaOH = or 28.2 g NaOH required 1 mol NaOH Alternate method: Combine all 3 steps into one large equation grams of NaOH = 25.0 g Cl 2 x 1mol Cl 2 x 2 mol NaOH x g NaOH g Cl 2 1 mol Cl 2 1 mol NaOH PROBLEM. How many grams of HCl can be theoretically produced if we reacted 8.00 g of phosphorous trichloride (PCl 3 ) with excess water? The balanced equation is: PCl H 2 O H 3 PO HCl Ans: 6.37 g of HCl 6

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