CALCULATIONS AND CHEMICAL EQUATIONS ATOMIC MASS: MASS OF AN ATOM

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1 CALCULATIONS AND CHEMICAL EQUATIONS ATOMIC MASS: MASS OF AN ATOM There is a unique relationship between molar mass and atomic weight: Oxygen's atomic weight is amu. 1 mole of oxygen is 6.02 x 1023 atoms of oxygen. Calculations and Chemical Equations Example: Hydrogen atomic weight = amu Carbon atomic weight = amu Atomic mass: Mass of an atom of an element, expressed in atomic mass units Atomic mass unit (amu): x 10-24g Carbon atoms are 12 times as massive as hydrogen atoms Practice: Ca atomic mass = amu Ne atomic mass = amu Atomic weight: Average mass of all isotopes of a given element; listed on the periodic table How many Neon atoms are required to give the same mass as one calcium atom? Avogadro s Number: The Mole 1 Dozen = 12 Oxygen s atomic weight is amu. 1 Gross = Mole = 6.02 x There is a unique relationship between molar mass and atomic weight: 1 mole of oxygen is 6.02 x 1023 atoms of oxygen amu = x 10-24g 1 mole of atoms = x 1023 atoms What is the molar mass (g/mole) of oxygen? 1 mole of molecules = x 1023 molecules amu/oxygen atom x x 10-24g/amu Molar Mass: the mass (in grams) of one mole a particular substance = x g/atom x 6.02 x 1023 atoms/mole =16.00 g/mole Molar mass (in grams) is always equal to the atomic weight of the atom! Molar mass (in grams) is always equal to the atomic weight of the atom! Mole (mol): The number of particles (atoms or molecules) in a sample of element or compound with a mass in grams equal to the atomic (or molecular) weight

2 Examples: Atomic weight of carbon is 12.01amu Molar mass of carbon is g/mol Example: Atomic weight of sodium = Atomic weight of helium is 4.00 amu g of sodium contains 1 mole Molar mass of helium is 4.00 g/mol (6.02 x 1023) of atoms 1 Calculations Using the Mole Practice: 1 mol S atoms = 6.02 x 1023 atoms 1 mole represents the mass of a sample that contains Avogadro s number of particles 6.02x 1023 atoms = 32 g S Example: Atomic wt. of Potassium (K)= 39 amu 1 mol S atoms = 32 g S 1 mol K atoms = 6.02 x 1023 atoms = 39 g K What is the mass in grams of 1 atom of Sulfur? Example: Atomic wt. of Sulfur (S) = 32 u 1 S atom = g 1 S atom x 32 g S/6.02 x 1023 atoms = g 1 mol S atoms = 6.02 x 1023 atoms = 32 g S 1 mol S atoms = 6.02 x 1023 atoms 1 S atom = 5.32 x10-23 g 6.02x 1023 atoms = 32 g S 1 mol S atoms = 32 g S Practice: Practice: 1 mol S atoms = 6.02 x

3 mol S atoms = 6.02 x 1023 atoms atoms 6.02x 1023 atoms = 32 g S 6.02x 1023 atoms = 32 g S 1 mol S atoms = 32 g S 1 mol S atoms = 32 g S How many moles of Sulfur in 98.6 grams? 98.6g = moles Sulfur 98.6g x 1 mole/32g = How many atoms in this sample of 98.6g of S? 98.6g = 98.6g x 6.02x1023 atoms/32g = moles sulfur = 3.07 moles sulfur Practice: atoms of S atoms of S = 1.85x1024 atoms S The mole concept applies to molecules, as well as to atoms 1 mol S atoms = 6.02 x 1023 atoms 6.02x 1023 atoms = 32 g S Chemical formulas indicate relative quantities of atoms within a compound 1 mol S atoms = 32 g S What is the mass of 1 atom of Sulfur? 1 atom = g 1 atom x 32g/6.02x1023 atoms = g Example: H2O has 2 H atoms for every 1 Oxygen atom C6H12O6 has how many atoms of each element? = 5.33 x10-23 g S

4 2 Formula Weight (F.W.): sum of all atomic weights of all atoms in a compound; expressed in amu Practice: Formula weight of H2O =? A mole of a molecule will have a mass in grams equal to its formula weight F.W. of H2O = atomic weight of H atoms + atomic weight of O atom Example: F.W. of CH4N2O (urea) =? Atomic weights: C = 12 amu Atomic weight of H =1amu, O =16 amu H = 1 amu N = 14 amu F.W. of H2O = O = 16 amu Molar mass of H2O = 18 g/mol 18 amu F.W. CH4N2O = (1) + 2 (14) + 16 = 60 amu Molar mass of CH4N2O is 60 g/mol More Practice: Chemical Equations Prozac, C17H18F3NO, is a widely used antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac? Chemical Reaction: Interaction between substances that results in one or more new substances being produced 17C (12.0) + 18H (1.0) + 3F (19.0) + 1N (14.0) + 1 O (16.0) = Example: hydrogen + oxygen Æ water 204 =

5 g/mole Reactants of a Reaction: Starting materials that undergo chemical change; written on the left side of the equation representing the reaction Products of a Reaction: Substances formed as a result of the reaction; written on the right side of the equation representing the reaction The arrow points towards the products formed by the reaction Individual products and reactants are separated by a plus sign Chemical Equation: A written statement using symbols and formulas to describe the changes that occur in a reaction Example: 2H2(g) + O2 (g) Æ 2H2O (l) Letter in parentheses indicates the state of the substance: gas (g), liquid (l), solid (s), dissolved in water (aq) If heat is required for the reaction to take place, the symbol Δ is written over the reaction arrow Types of Chemical Reactions Combination Reactions: Reaction in which 2 or more substances react to form a single substance Decomposition Reactions: Reaction in which a single substance reacts to form 2 or more simpler substances Single-Replacement Reactions: Reaction in which an element reacts with a compound and displaces another element from the compound Double-Replacement Reactions: Reaction in which 2 compounds react and exchange partners to form 2 new compounds 3 Precipitation Reactions: Reaction in which chemical change results in formation of one or more insoluble product Acid-Base Reactions: Reactions involving the transfer of a hydrogen ion, H+, from one reactant to another Redox Reactions: Reaction in which substances undergo oxidation and reduction; involve the transfer of electrons from one reactant to another Oxidation: loss of electrons Combination reaction: two or more elements or compounds combine to form one product. A + B AB Examples: H2 + Cl2 2HCl 2S + 3O2 2SO3 4Fe + 3O2

6 2Fe2O3 Reduction: gain of electrons Decomposition reaction: one substance is broken down (split) into two or more simpler substances. Single replacement reaction: one element takes the place of an element in a reacting compound A AB 2KClO3 AC Example: Zn(s) + HCl(aq) Examples: 2HgO + BC +B A + B ZnCl2(aq) + H2(g) 2Hg + O2 H2 2KCl + 3 O2 Zn HCl ZnCl2 Double replacement reaction: two elements swap places among the reactants (the positive ions in the reacting compounds) AB + CD AD + CB Combustion reaction: a reactant (often containing carbon) reacts with oxygen O2. Examples: C + O2 CO2 Examples: AgNO3 + NaCl AgCl + NaNO3 ZnS ZnCl2 + H2S + 2HCl

7 CH4 + 2O2 C3H8 + 5O2 CO2 + 2H2O 3CO2 + 4H2O 4 Oxidation and Reduction Reactions An oxidation-reduction reaction involves the transfer of electrons from one reactant to another. In oxidation, electrons are lost Zn2+ + 2e- (loss of electrons--leo) Zn In reduction, electrons are gained. Cu2+ + 2e- Cu (gain of electrons--ger) Balanced Equation: Equation in which the number of atoms of each element in the reactants is the same as the number of atoms of that element in the products Practice: Identify each of the following as an oxidation or a reduction reaction: A. Sn Sn4+ + 4e- Oxidation B. Fe3+ + 1e- Fe2+ Reduction C. Cl2 + 2e- 2Cl- Reduction Law of Conservation of Mass: Atoms are neither created nor destroyed in chemical reactions

8 Example: CaS + H2O Æ CaO + H2S Reactants Products Is this equation balanced? Example: NO(g) + O2(g) ÆNO2(g) Practice: SO2 + O2 Æ SO3 Is this equation balanced? Is this equation balanced? 2SO2 + O2 Æ 2SO3 Adjust the coefficient of the reactants and products to balance the equation 2NO + O2 Æ 2NO2 Reactants Product Practice: H2 + Cl2 Æ HCl Is this equation balanced? H2 + Cl2 Æ 2HCl x1023 CH x1023 O2 Æ6.02 x1023 CO x1023 H2O Chemical Equations and the Mole 1 mol CH4 + 2 mol O2Æ1 mol CO2 + 2 mol H2O Stoichiometry: Study of mass relationships in chemical reactions; ratios of different molecules 16.0g CH g O2 Æ 44.0g CO gH2O Example: CH4 + 2O2 Æ CO2 + 2H2O How many moles of O2 would be required to react with 1.72 mol CH4? 10 CH O2 Æ 10 CO H2O 6.02 x1023 CH x1023 O2 Æ6.02 x1023 CO x1023 H2O 1.72 mol CH4 = mol O2 1 mol CH4 + 2 mol O2Æ1 mol CO2 + 2 mol H2O 16.0g CH g O2 Æ 44.0g CO gH2O How many grams of H2O will be produced from 1.09 mol of CH4? 2-part problem: first find moles of H2Othen convert moles to grams Use this equation to obtain conversion factor for moles of CH4 to

9 moles of H2O: 1 mol CH4 + 2 mol O2Æ1 mol CO2 + 2 mol H2O 1.72 mol CH4 x 2 mol O2/1 mol CH4 = 3.44 mol O2 Theoretical and Percent Yield Theoretical yield: the maximum amount of product that would be formed from a particular reaction in an ideal world Actual yield: the amount of product formed from a particular reaction in the real world (usually less than the theoretical yield) Percent yield: ratio of actual yield to theoretical yield, times 100 % 1.09 mol CH4 = mol H2O % yield = actual yield/theoretical yield x 100% 1.09 mol CH4 x 2 mol H2O/1 mol CH4 = 2.18 mol H2O Example: theoretical yield = 39.2g water 2.18 mol H20 = grams H2O 2.18 mol H20 x 18.0g H2O/1 mol H2O = 39.2grams H2O actual yield = 35.5 g water % yield = 35.5g/39.2g x 100% = 90.6 % yield 6