Mathematical Mathematical ChemistryChemistry

Chemical MathematicsChemical Mathematics

Chapter 7 Homework (II)Chapter 7 Homework (II)

Due Monday, November 3Due Monday, November 3rdrd

Pgs. 236-238Pgs. 236-238 Problems 30, 33, 40, 45, 46, 47, Problems 30, 33, 40, 45, 46, 47,

5050

Molar MassMolar Mass

We will learn how to…We will learn how to…

Calculate the molar Calculate the molar massmass

Calculate the percent Calculate the percent composition by mass of composition by mass of a compound. a compound.

A A reallyreally important formula! important formula!

MM

)g(massmolesofnumber

Practice ProblemsPractice Problems

What is the molar mass of Copper What is the molar mass of Copper (II) nitrate.(II) nitrate.

187.566 g187.566 g What is the mass in grams of 6.25 What is the mass in grams of 6.25

mole of Copper (II) nitrate.mole of Copper (II) nitrate. 1170 g.1170 g.

(cont.)(cont.)

How many molecules are How many molecules are contained in:contained in:

25.0 g H25.0 g H22SOSO44??

125 g. of sugar (C125 g. of sugar (C1212HH2222OO1111))

1.53 x 1023 molecules of Sulfuric Acid2.20 x 1023 molecules of Sugar

PercentPercentCompositionComposition

Percent CompositionPercent Composition

The percent composition The percent composition of a compound is the of a compound is the

percent by mass of each percent by mass of each element in the element in the compound. compound.

Percent CompositionPercent Composition

elementof%

100xcompoundofmassmolar

elementofmass

PracticePractice

Find the percent Find the percent composition to 4 composition to 4

significant figures...significant figures...

PbClPbCl22

74.50% Pb and 25.50% Cl74.50% Pb and 25.50% Cl

What’s the percent water in…What’s the percent water in…

ZnSOZnSO44•7H•7H22OO

43.85%43.85%

Find the percent Find the percent composition of composition of

(NH(NH44))22COCO33??

N=29.15%, H=8.39%, N=29.15%, H=8.39%, C=12.50%, O=49.95%C=12.50%, O=49.95%

Practice Problem 7-9Practice Problem 7-9

Ibuprofen, CIbuprofen, C1313HH1818OO22, is the active , is the active ingredient in many nonprescription ingredient in many nonprescription pain relievers. Its molar mass is pain relievers. Its molar mass is

206.29 g/mol.206.29 g/mol. If the tablets in a bottle contain a If the tablets in a bottle contain a

total of 33 g. of ibuprofen, how many total of 33 g. of ibuprofen, how many moles of ibuprofen are in the bottle?moles of ibuprofen are in the bottle?

0.16 mol

Practice Problem 7-9Practice Problem 7-9

Ibuprofen, CIbuprofen, C1313HH1818OO22, is the active , is the active

ingredient in many nonprescription ingredient in many nonprescription pain relievers. Its molar mass is pain relievers. Its molar mass is

206.29 g/mol.206.29 g/mol. How many molecules of ibuprofen How many molecules of ibuprofen

are in the bottle?are in the bottle?

9.6 x 1022 Molecules

Practice Problem 7-9Practice Problem 7-9

Ibuprofen, CIbuprofen, C1313HH1818OO22, is the active , is the active

ingredient in many nonprescription ingredient in many nonprescription pain relievers. Its molar mass is pain relievers. Its molar mass is

206.29 g/mol.206.29 g/mol. What is the total mass in grams of What is the total mass in grams of

carbon in 33 g. of ibuprofen?carbon in 33 g. of ibuprofen?

25 g

7.4 Determining Chemical 7.4 Determining Chemical FormulasFormulas

The simplest formula or The simplest formula or empirical formula consists of empirical formula consists of the symbols for the elements the symbols for the elements

combined with subscripts combined with subscripts showing the smallest whole-showing the smallest whole-number ratio of the atoms.number ratio of the atoms.

ExampleExampleDiborane has a molecular Diborane has a molecular

formula and in reality exists as formula and in reality exists as BB22HH66..

The empirical formula is BHThe empirical formula is BH33..Both are 78% Boron and 22% Both are 78% Boron and 22%

Hydrogen.Hydrogen.

Simplest Formula (from Simplest Formula (from weight percents.)weight percents.)

Step 1: Take percentages and Step 1: Take percentages and convert to a total of 100 g.convert to a total of 100 g.

Diborane is 78% by weight Boron Diborane is 78% by weight Boron and 22% by weight hydrogen.and 22% by weight hydrogen.

100 g would be 78 g. of Boron 100 g would be 78 g. of Boron and 22 g. of Hydrogen.and 22 g. of Hydrogen.

Simplest Formula (cont.)Simplest Formula (cont.)

Step 2: Determine moles.Step 2: Determine moles.

Bmol1.7g11

Bmol1xBg78

Hmol22Hg0.1

Hmol1xHg22

Simplest Formula (cont.)Simplest Formula (cont.)

Step 3: Determine Step 3: Determine simplest whole number simplest whole number molar ratio.molar ratio.

Divide through by the Divide through by the smallest number.smallest number.

Simplest Formula (cont.)Simplest Formula (cont.)

7.1 mol of Boron; 22 mol H7.1 mol of Boron; 22 mol H

1.0 mol B : 3.1 mol H1.0 mol B : 3.1 mol HEmpirical formula is BHEmpirical formula is BH33

1.7

22:

1.7

1.7 HmolBmol

Practice ProblemPractice Problem

Analysis shows a compound to Analysis shows a compound to contain 26.56% potassium, contain 26.56% potassium, 35.41% chromium, and 38.03 % 35.41% chromium, and 38.03 % Oxygen by weight. Find the Oxygen by weight. Find the simplest formula for this simplest formula for this compound.compound.

KK22CrCr22OO77..

Molecular FormulaMolecular Formula

The simplest or empirical formula The simplest or empirical formula may or may not the correct may or may not the correct molecular formula.molecular formula.

Is the correct (real) formula BHIs the correct (real) formula BH33, ,

BB22HH66, B, B33HH99 ? ? Need to know the molecular mass Need to know the molecular mass

of the formula.of the formula.

Molecular Formula (cont.)Molecular Formula (cont.)

If the formula mass of the If the formula mass of the empirical formula equals empirical formula equals

the known molecular the known molecular mass, then it is the mass, then it is the molecular formula.molecular formula.

Molecular Formula (cont.)Molecular Formula (cont.)

If not, there is a whole number If not, there is a whole number multiplier, x, for all of the multiplier, x, for all of the subscripts. subscripts.

massmolecular

massformulaempirical X )()(

Empirical formula is BHEmpirical formula is BH33

The BHThe BH33 has a formula has a formula

mass of 13.83 u.mass of 13.83 u.The known formula The known formula mass for Diborane is mass for Diborane is 27.67 u.27.67 u.

000.283.13

67.27

u

ux

(BH(BH33)(2) = B)(2) = B22HH66

Determine the molecular Determine the molecular formula of a compound formula of a compound

having the simplest having the simplest formula of CH and a formula of CH and a

formula mass of 78.110 u.formula mass of 78.110 u.

ReviewReview4.04 g of N combine with 4.04 g of N combine with

11.46 g O to produce a 11.46 g O to produce a compound with a formula compound with a formula mass of 108.0 u. What is mass of 108.0 u. What is the molecular formula?the molecular formula?

NN22OO55