# CHAPTER 3 Calculations with Chemical Formulas and Equations. atoms in a FORMULA UNIT

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1 CHAPTER 3 Calculations with Chemical Formulas and Equations MOLECULAR WEIGHT (M. W.) Sum of the Atomic Weights of all atoms in a MOLECULE of a substance. FORMULA WEIGHT (F. W.) Sum of the atomic Weights of all atoms in a FORMULA UNIT of a substance. Can be used for: MOLECULAR COMPOUNDS only. Example: Can be used for both: IONIC COMPOUNDS and MOLECULAR COMPOUNDS Example: MW (H 2 O) =? FW (Na 2 CO 3 ) =? One H 2 O molecule One Formula Unit of Na 2 CO 3 contains: contains: 2 H atoms: 2 Na + ions: 2 x 1.01 amu = 2.02 amu 2 x amu = amu 1 O atom: 1 C atom: 1 x amu = amu 1 x amu = amu 3 O atoms: 3 x amu = amu One H 2 O One Na 2 CO 3 molecule formula unit weighs amu weighs amu 1

2 THE MOLE CONCEPT THE MOLE: is the amount of substance that contains 6.02 x chemical units such as: atoms, ions molecules formula units electrons, etc x is abbreviated N A has been introduced in chemistry by the Italian scientist, Avogadro is referred to as Avogadro s Number Examples: 1 mole H atoms contains: x H atoms 1 mole H 2 molecules contains: x H 2 molecules: 1 mole H 2 O molecules contains: x H 2 O molecules: 2 x x H atoms = 1.20 x H atoms 2 x x H atoms = x H atoms 1 x x O atoms = x O atoms 1 mole NaCl contains: x formula units of NaCl: 1 x x Na + ions 1 x x Cl ions 1 mole Na 2 CO 3 contains: x formula units of Na 2 CO 3 : 2 x x Na + ions = x Na + ions 1 x x CO 3 2 ions = x CO 3 2 ions 1 x x C atoms 3 x x O atoms = x O atoms 2

3 MOLAR MASS (MM): is the mass of one mole of substance is equal to the Formula Weight expressed in grams 1 mole H 2 O 1 mole Na 2 CO 3 contains weighs contains weighs x g 6.02 x g molecules formula units Examples: 1. Calculate the mass in grams of one He atom. 1 mol He atoms 1 He atom x x = g 6.02 x He atoms 2. How many C atoms are there in 4.00 x 10 8 g of propane (C 3 H 8 )? 4.00 x 10 8 g C 3 H 8 x x x NOTE: Set up units so that they cancel out 3

4 3. How many moles of Na atoms are there in 50.0 g of Na? 4. What is the mass of 6.30 moles of lead (II) sulfate? 2+ 2 Formula of lead (II) sulfate: PbSO 4 PbSO 4?????? g PbSO 4? g PbSO 4 = 6.30 moles PbSO 4 x 1 mole PbSO 4 1 mole PbSO 4 = Molar Mass of PbSO 4 : 1 mole Pb = 1 x g = g 1 mole S = 1 x g = g 4 moles O = 4 x g = g g g PbSO 4? g PbSO 4 = 6.30 moles PbSO 4 x = 1.91 x 10 3 g PbSO 4 1 mole PbSO 4 4

5 5. Calculate the number of moles in 2.45 g of magnesium sulfate Formula of magnesium sulfate: MgSO 4 MgSO 4 1 mole MgSO 4? moles MgSO 4 = 2.45 g MgSO 4 x????? g MgSO 4 1 mole MgSO 4 = Molar Mass of MgSO 4 : 1 mole Mg = 1 x g = g 1 mole S = 1 x g = g 4 moles O= 4 x g = g g 1 mole MgSO 4? moles MgSO 4 = 2.45 g MgSO 4 x = moles MgSO g MgSO 4 5

6 PERCENT COMPOSITION The percent composition of a compound is the mass percent of each element in the compound. no. of X in formula x atomic mass of X Mass % X = x 100 molecular mass of compound The recommended daily allowance of table salt (sodium chloride) is mg NaCl. How many mg of Na are contained in 2,000. mg NaCl? First Step: Calculate the mass of 1 mole of compound 1 mole Na = 1 x 22.9 g = g 1 mole Cl = 1 x g = g 1 mole of NaCl = g Second Step: Calculate the % by mass of each element in the compound g % Na = x 100 = 39.34% g g % Cl = x 100 = 60.66% g Third Step: Check if the percentages add up to 100% % Na % Cl = % How many mg of Na are contained in 2,000. mg NaCl? mg Na? mg Na = 2,000. mg NaCl x = mg Na mg NaCl 6

7 Examples: 1. Calculate the percent composition of sodium phosphate (Na 3 PO 4 ). Calculate molar mass of sodium phosphate: 3 moles Na = 1 mole P = 4 moles O = 1 mole Na 3 PO 4 = % Na = ¾¾¾¾¾¾¾¾¾¾¾ x 100 = % P = ¾¾¾¾¾¾¾¾¾¾¾ x 100 = % O = ¾¾¾¾¾¾¾¾¾¾¾ x 100 = TOTAL % = % 2. Calculate the mass percentage of copper in the natural ore malachite. The formula of malachite is CuCO 3. Cu(OH) 2 First: Calculate the mass of 1 mole of CuCO 3. Cu(OH) 2 2 moles Cu = 2 x g = g Cu 1 mole C = 1 x12.01 g = g C 5 moles O = 5 x g = g O 2 moles H = 2 x 1.01 g = 2.02 g H 1 mole CuCO 3. Cu(OH) 2 = g g % Cu = x 100 = 57.49% g 7

8 3. Calculate the mass of copper in 1.37 grams of copper (II) oxide (CuO) 1 mole Cu = 1 x g = g 1 mole O = 1 x g = g 1 mole CuO = g g Cu? g Cu = 1.37 g CuO x = 1.09 g Cu g CuO 4. Seawater contains % (by mass) of bromine. How many grams of bromine are there in 2.50 L of seawater? The density of seawater is g/cm 3. 8

9 CHEMICAL FORMULAS Formulas give information about the type, the number, and arrangement of atoms in a substance Two important type of formulas will be discussed and compared: MOLECULAR FORMULAS EMPIRICAL FORMULAS (Simplest Formulas) can be written for molecular compounds only can be written for both molecular and ionic compounds indicate the number of atoms of each kind indicate the smallest whole number ratio in a molecule between the atoms or ions of a substance are multiples of empirical formulas (smallest whole number subscripts) Multiplier H 2 O 2 (hydrogen peroxide) HO 2 H 2 O H 2 O 1 C 2 H 2 (acetylene) CH 2 C 6 H 6 (benzene) CH 6 C 6 H 12 O 6 (glucose) CH 2 O 6 C 6 H 12 (cyclohexane) CH 2 6 C 6 H 4 Cl 2 C 3 H 2 Cl 2 Does not exist for an Ionic Compound NaCl Does not exist for an ionic compound SnO 2 9

10 Calculating the Empirical (Simplest Formula) Information Required: Percentage Composition by Mass, or Composition by weight Example: Tin (Sn) reacts with oxygen (O) to form an oxide of tin (Sn a O b ), which contains g Sn and g O. What is the Empirical Formula of the oxide? (that is: a =? b =?) Step 1: MASS MOLE 1 mole Sn? moles Sn = g Sn x = moles Sn atoms g Sn 1 mole O? moles O = g O x = moles O atoms g O Step 2: DIVIDE BY SMALL moles O Relative number of O atoms = = moles Sn moles Sn Relative number of Sn atoms = = moles Sn Empirical Formula: Sn O SnO 2 (includes experimental error) 10

11 Sample Problem # 1 Arsenic (As) reacts with oxygen (O) to form a compound that is 75.7 % As and 24.3 % O by mass. What is the empirical formula of this oxide? Step 1: PERCENT MASS 75.7 % As 24.3% O Assume 100 g compound 75.7 g As 24.3 g O Step 2: MASS MOLE 1 mole As? moles As = 75.7 g As x = 1.01 moles As atoms g As 1 mole O? moles O = 24.3 g O x = 1.52 moles O atoms g O Step 3: DIVIDE BY SMALL 1.52 moles O Relative number of O atoms = = moles As 1.01 moles As Relative number of As atoms = = moles As Empirical Formula: As 1.00 O 1.50??????????? (subscipts must be whole numbers) 11

12 Step 4: MULTIPLY TIL WHOLE: (As 1.00 O 1.50 ) x 2 = As 2 O 3 A Simple Rhyme for a Simple Formula (to be remembered) 1. Percent to Mass 2. Mass to Mole 3. Divide by small 4. Multiply til Whole 12

13 Sample Problem # 1 A sample of Freon contains g C, 2.50 g Cl, and 1.34 g F. What is the Empirical Formula of Freon? 1. Percent to Mass This step may be skipped since masses are already given: g C 2.50 g Cl 1.34 g F. 2. Mass to Mole? moles C =? moles Cl =? moles F = 3. Divide by small C = Cl= F= Empirical Formula: 13

14 Finding the Molecular Formula Information required: 1. Empirical Formula Ex: CH 2 2. Molecular Mass Ex: 56.0 amu What is the Molecular Formula of this compound? Empirical Formula Mass of CH 2 is: 1 C = 1 x 12.0 amu = 12.0 amu + 2 H = 2 x 2.0 amu = 2.0 amu 14.0 amu Molecular Formula: cannot be CH 2 (14.0 amu 56.0 amu) multiplier is not 1 Consider the possible multipliers and the resulting Molecular Mass: Multiplier (n): Molecular Formula: CH 2 C 2 H 4 C 3 H 6 C 4 H 8 Molecular Mass(amu): correct Molecular Formula (yields correct Molecular Mass) NOTE: Molecular Mass = Multiplier x Empirical Formula Mass MM = n x EFM 56.0 amu = 4 x 14.0 amu 56.0 amu Molecular Mass It follows: 4 = In General: n = 14.0 amu Empirical Formula Mass 14

15 Sample Problem 1 The empirical formula for benzene is CH. The molecular mass of benzene is 78 amu. What is the molecular formula of benzene? Empirical Formula Mass of CH is: 1 C = 1 x 12 amu = 12 amu + 1 H = 1 x 1 amu = 1 amu 13 amu Molecular Formula: (CH) n n =? Molecular Mass 78 amu n = = = 6 Empirical Formula Mass 13 amu Molecular Formula: (CH) 6 C 6 H 6 Sample Problem 2: A compound whose empirical formula is C 3 H 2 Cl has a Molecular Mass of amu. What is the Molecular Formula? Empirical Formula Mass of C 3 H 2 Cl is: 3 C = 3 x 12.0 amu = 36.0 amu + 2 H = 2 x 1.0 amu = 2.0 amu 1 Cl = 1 x 35.5 amu = 35.5 amu 73.5 amu Molecular Formula: (C 3 H 2 Cl) n n =? Molecular Mass 147 amu n = = = 2 Empirical Formula Mass 73.5 amu Molecular Formula: (C 3 H 2 Cl) 2 C 6 H 4 Cl 2 15

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