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Week 2 CHEM 1310 - Sections L and M 1

Wed | Aug 29, 2007

Chapter 3: Stoichiometry– Focus on Sections 3.2 - 3.9– Chemical Formulas– Moles and Related Calculations

Bring PRS units on Friday, Aug 31

WebAssign HW due Thurs before midnight

Week 2 CHEM 1310 - Sections L and M 2

Mole

Avogadro’s Number, N0The number of atoms inexactly 12 grams of 12C.

6.022 x 1023 entities x mol-1

Mole (mol)The amount of a substance

containing as manyelementary particles asthere are in 12 g of 12C.

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Week 2 CHEM 1310 - Sections L and M 3

Counting in Chemistry

1 mole = different masses of elements

Week 2 CHEM 1310 - Sections L and M 4

Molar MassGiven: Molecular FormulaFind: Molecular Weight (i.e. Molar Mass)

Sulfuric Acid: H2SO4

Solution:(1) Multiply the atomic mass by the # of each atom type(2) Sum the resulting weights

H (1.0079 g/mol) x 2 = 2.0158 g/molS (32.065 g/mol) x 1 = 32.065 g/molO (15.999 g/mol) x 4 = 63.996 g/mol

SUM: 98.077 g/mol

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Week 2 CHEM 1310 - Sections L and M 5

Calculations

Given: Moles of an element or compoundFind: Weight of element or compound

EXAMPLE What is the weight of 0.03 mol of gold?

Atomic Mass of Au: 196.97 g/mol

0.03 mol x 196.97 gmol

= 6 g of Au

Week 2 CHEM 1310 - Sections L and M 6

Calculations Involving Moles

Given: Weight of a moleculeFind: Number of atoms

EXAMPLE How many atoms are contained in 1.5 g of NaOH?

FW of NaOH: 40 g/mol

1.5 g x 1 mol40 g

x 6.022 x 1023 atoms1 mol

= 2.3 x 1022 atoms

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Week 2 CHEM 1310 - Sections L and M 7

Chemical Formulas

Empirical Formula: a chemical formula of a compoundthat expresses the relative chemical amounts of itselements in terms of the smallest integers.

Molecular Formula: a chemical formula that specifiesthe actual number of atoms of each element in onemolecule.

Empirical FormulaCH2O

Molecular FormulaC6H12O6

(Glucose)

Week 2 CHEM 1310 - Sections L and M 8

Given: Mass CompositionFind: Empirical Formula

EXAMPLEA sample of a tin and chlorine compound with a

mass of 2.57g was found to contain 1.17 g of tin. What is the compound’s empirical formula

Atomic Mass of Tin: 118.710 g/molAtomic Mass of Chlorine: 35.453 g/mol

Calculations with Formulas

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Week 2 CHEM 1310 - Sections L and M 9

Given: Empirical Formula and Molecular MassFind: Molecular Formula

EXAMPLEThe empirical formula of squalene is C3H5.

Its molar mass is 410.7 g/mol.

What is the molecular formula of squalene?

Calculations with Formulas

Week 2 CHEM 1310 - Sections L and M 10

More Calculations with Moles

Given: Amount of a compoundFind: Amount of an element in compound

Compute the mass (in grams) oflithium in 65.4 g of Li2CO3

(lithium carbonate).

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Week 2 CHEM 1310 - Sections L and M 11

More Complexity…

Given: Amount of an element within a compoundFind: Amount of a different element in compound

EXAMPLEChlorophyll, the green pigment in plants,

has the formula C55H72MgN4O5

If 0.0011g of Mg is available to a plant cell forchlorophyll synthesis, how many grams of carbon

will be required to completely use up themagnesium?

Week 2 CHEM 1310 - Sections L and M 12

Density & MW

Given: Density and Molecular WeightFind: Molar Concentration (mol/L)

Density of NaCl: 2.16 g/cm3

FW of NaCl: 58.443 g/molcm3 = mL = 1 x 10-3 L

1 mol58.443 g

x 2.16 gmL

x 1000 mLL

= 36.96 M NaCl

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Week 2 CHEM 1310 - Sections L and M 13

Dilutions, Proportions

M1 x V1 = M2 x V2

What volume of 0.1M NaCl is needed to make0.2 L of 10 mM NaCl?

(0.1M) x V1 = (10 x 10-3M) x (0.2L)

V1 = 0.02L or 20 mL

Remember this!

Week 2 CHEM 1310 - Sections L and M 14

On Friday

Complete Chapter 3

– Chemical Equations

– How to balance chemical equations

– Calculations involving reactants and products