Chapter 7. Chapter 7. Chemical Formulas Express Composition. Chapter 7 MARCH REVIEW SLIDES LISTEN AND UPDATE MISSING NOTES
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1 REVIEW Express A compound s chemical formula tells you which elements, & how much of each, are present in a compound. Formulas for covalent compounds show the elements and the number of atoms of each element in a molecule. Formulas for ionic compounds show the simplest ratio of cations and anions in any pure sample. and Moles, continued Formulas Express, continued Any sample of compound has many atoms and ions, and the formula gives a ratio of those atoms or ions. and Moles, continued Formulas Give Ratios of Polyatomic Ions Formulas for polyatomic ions show the simplest ratio of cations and anions. They also show the elements and the number of atoms of each element in each ion. For example, the formula KNO 3 indicates a ratio of one K + cation to one NO 3 anion. MARCH REVIEW SLIDES LISTEN AND UPDATE MISSING NOTES NOTE SLIDES TAKE NOTES BELLRINGER FROM 10 to 25 points for daily class participation and completion of the bellringer. On average 5 bellringers can be the equivalent of a test grade YOU CANNOT AFFORD NOT TO DO BELLRINGER. YOU CANNOT MAKE UP BELLRINGERS IF YOU ARE ABSENT
2 Bellringer 3/25/2008 Sheet of paper Convert this list of percentages to fractions. 20%; 233%; 75%; 67.5%; Convert these fractions to % 3/12; 4/5; 5/3; 21/64; 7/8; Formulas for Polyatomic Ionic Compounds REVIEW Visual Concepts and Moles, continued Formulas Are Used to Calculate Molar Masses The molar mass of a molecular compound is the sum of the masses of all the atoms in the formula expressed in g/mol. The molar mass of an ionic compound is the sum of the masses of all the atoms in the formula expressed in g/mol. Formula Mass
3 (download from website Calculating Molar Mass for Ionic Compounds TAKE NOTES -- Calculating Molar Mass of Compounds Sample Problem F Find the molar mass of barium nitrate, Ba(NO 3 ) 2. Calculating Molar Mass of Compounds Sample Problem F Solution Find the # of moles of each element in 1 mol Ba(NO 3 ) 2 : Each mole has 1 mol Ba, 2 mol N, and 6 mol O. Use the periodic table to find the molar mass of each element in the formula: molar mass of Ba: g/mol; molar mass of N: g/mol and molar mass of O: g/mol Calculating Molar Mass of Compounds Sample Problem F Solution, continued Multiply molar mass of each element by the # of moles of each element. Add masses to get total molar mass of Ba(NO 3 ) 2. mass of 1 mol Ba = g/mol = g/mol mass of 2 mol N = g/mol = g/mol + mass of 6 mol O = g/mol = g/mol molar mass of Ba(NO 3 ) 2 = g/mol
4 Objectives Determine a compound s empirical formula from its percentage composition. Determine the molecular formula or formula unit of a compound from its empirical formula and its formula mass. Calculate percentage composition of a compound from its molecular formula or formula unit. Using Analytical Data The percentage composition is the percentage by mass of each element in a compound. Percentage composition helps verify a substance s identity. Percentage composition also can be used to compare the ratio of masses contributed by the elements in two different substances. Percentage of Iron Oxides 3 Formulas and Percentage Use Analytical Data to Determine Empirical Formulas An empirical formula is a chemical formula that shows the simplest ratio for the relative numbers and kinds of atoms in a compound. An actual formula shows the actual ratio of elements or ions in a single unit of a compound. For example, the empirical formula for ammonium nitrate is NH 2 O, while the actual formula is NH 4 NO 2.
5 Empirical and Actual Formulas Using Analytical Data, continued Determining Empirical Formulas, continued You can use the percentage composition for a compound to determine its empirical formula. 1. Convert % of each element to grams. 2. Convert from g to mol using the molar mass of each element as a conversion factor. 3. Compare these amounts in mol to find the simplest whole-number ratio among the elements. Visual Concepts Percentage Determining an Empirical Formula from Percentage Sample Problem G ON EVEN PAGE Chemical analysis of a liquid shows that it is 60.0% C, 13.4% H, and 26.6% O by mass. Calculate the empirical formula of this substance.
6 Determining an Empirical Formula from Percentage Sample Problem G Solution Assume that you have a g sample, and convert the percentages to grams. for C: 60.0% g = 60.0 g C for H: 13.4% g = 13.4 g H for O: 26.6% g = 26.6 g O Determining an Empirical Formula from Percentage Sample Problem G Solution, continued Convert the mass of each element into the amount in moles, using the reciprocal of the molar mass. 1molC 60.0 g C 5.00 mol C g C = 1molH 13.4 g H 13.3 mol H 1.01 g H = 1molO 26.6 g O 1.66 mol O g O = Determining an Empirical Formula from Percentage Sample Problem G Solution, continued The formula can be written as C 5 H 13.3 O 1.66, but you divide by the smallest subscript to get whole numbers mol C mol H mol O 1.66 = = 3.01 mol C 8.01 mol H = 1.00 mol O The empirical formula is C 3 H 8 O. Using Analytical Data, continued Molecular Formulas Are Multiples of Empirical Formulas The formula for an ionic compound shows the simplest whole-number ratio of the large numbers of ions in a crystal of the compound. A molecular formula is a whole-number multiple of the empirical formula. The molar mass of any compound is equal to the molar mass of the empirical formula times a whole number, n.
7 Visual Concepts Comparing Molecular and Empirical Formulas Determining a Molecular Formula from an Empirical Formula Sample Problem H The empirical formula for a compound is P 2 O 5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound. Determining a Molecular Formula from an Empirical Formula Sample Problem H Solution Determining a Molecular Formula from an Empirical Formula Sample Problem H Solution, continued Find the molar mass of the empirical formula P 2 O 5. 2 molar mass of P = g/mol + 5 molar mass of O = g/mol molar mass of P 2 O 5 = g/mol n = experimental molar mass of compound molar mass of empirical formula 284 g/mol n = = g/mol n (empirical formula) = 2 (P 2 O 5 ) = P 4 O 10
8 Using Analytical Data, continued Can Give Percentage If you know the chemical formula of any compound, then you can calculate the percentage composition. Using Analytical Data, continued Can Give Percentage CO and CO 2 are both made up of C and O, but they have different percentage compositions. From the subscripts, determine the mass contributed by each element and add these to get molar mass. Divide the mass of each element by the molar mass. Multiply by 100 to find the percentage composition of that element.
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