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Mathematical Chemistry
Chemical Mathematics
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Chapter 7 Homework (II) Due Monday, November 3rd Pgs. 236-238
Problems 30, 33, 40, 45, 46, 47, 50
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Molar Mass
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Calculate the molar mass
We will learn how to… Calculate the molar mass Calculate the percent composition by mass of a compound.
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Formula Mass The formula mass of any compound or polyatomic ion is the sum of the average atomic masses of all the atoms represented in the formula.
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Formula mass of H2O is u. The mass of a single water molecule is u. The mass of a mole of water (6.022 x 1023 molecules of water) is g.
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Sample Problem Find the molar mass of potassium chlorate, KClO3. H2SO4
Ca(NO3)2 PO43- g/mol g/mol g/mol 94.97 g/mol
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One mole equals two things…
One molar mass (MM) of a compound. Avogadro’s number of molecules.
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What is the molar mass of barium nitrate, Ba(NO3)2?
Sample Problem What is the molar mass of barium nitrate, Ba(NO3)2? g.
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Molar Mass is a Conversion Factor.
Molar mass of a compound is a conversion factor that relates mass in grams to number of moles and vice-versa.
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Conversion Factor How many eggs are there in 3 dozen? If one mole of water has a mass of 18. g, how much mass does three moles of water have?
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A really important formula!
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Practice Problems What is the molar mass of Copper (II) nitrate.
g What is the mass in grams of 6.25 mole of Copper (II) nitrate. 1170 g.
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(cont.) How many molecules are contained in: 25.0 g H2SO4?
125 g. of sugar (C12H22O11) 1.53 x 1023 molecules of Sulfuric Acid 2.20 x 1023 molecules of Sugar
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Percent Composition
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Percent Composition The percent composition of a compound is the percent by mass of each element in the compound.
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Percent Composition
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Find the percent composition to 4 significant figures...
Practice Find the percent composition to 4 significant figures... PbCl2 74.50% Pb and 25.50% Cl
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What’s the percent water in…
ZnSO4•7H2O 43.85%
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Find the percent composition of (NH4)2CO3?
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Practice Problem 7-9 Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. If the tablets in a bottle contain a total of 33 g. of ibuprofen, how many moles of ibuprofen are in the bottle? 0.16 mol
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9.6 x 1022 Molecules Practice Problem 7-9
Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. How many molecules of ibuprofen are in the bottle? 9.6 x 1022 Molecules
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Practice Problem 7-9 Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. What is the total mass in grams of carbon in 33 g. of ibuprofen? 25 g
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7.4 Determining Chemical Formulas
The simplest formula or empirical formula consists of the symbols for the elements combined with subscripts showing the smallest whole-number ratio of the atoms.
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Example Diborane has a molecular formula and in reality exists as B2H6. The empirical formula is BH3. Both are 78% Boron and 22% Hydrogen.
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Simplest Formula (from weight percents.)
Step 1: Take percentages and convert to a total of 100 g. Diborane is 78% by weight Boron and 22% by weight hydrogen. 100 g would be 78 g. of Boron and 22 g. of Hydrogen.
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Simplest Formula (cont.)
Step 2: Determine moles.
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Simplest Formula (cont.)
Step 3: Determine simplest whole number molar ratio. Divide through by the smallest number.
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Simplest Formula (cont.)
7.1 mol of Boron; 22 mol H 1.0 mol B : 3.1 mol H Empirical formula is BH3
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Practice Problem Analysis shows a compound to contain 26.56% potassium, 35.41% chromium, and % Oxygen by weight. Find the simplest formula for this compound. K2Cr2O7.
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Molecular Formula The simplest or empirical formula may or may not the correct molecular formula. Is the correct (real) formula BH3, B2H6, B3H9 ? Need to know the molecular mass of the formula.
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Molecular Formula (cont.)
If the formula mass of the empirical formula equals the known molecular mass, then it is the molecular formula.
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Molecular Formula (cont.)
If not, there is a whole number multiplier, x, for all of the subscripts.
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Empirical formula is BH3
The BH3 has a formula mass of u. The known formula mass for Diborane is u.
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(BH3)(2) = B2H6
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Determine the molecular formula of a compound having the simplest formula of CH and a formula mass of u.
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Review 4.04 g of N combine with g O to produce a compound with a formula mass of u. What is the molecular formula? N2O5
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