chemical composition - orange coast collegeocconline.occ.cccd.edu/online/avieau/chp4notestwo.pdf ·...

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Copyright © McGraw-Hill Education. Permission required for reproduction or display. 4-1

Chemical Composition

Copyright © McGraw-Hill Education. Permission required for reproduction or display.

4.1 Percent Composition: a method for expressing

composition of a sample.For any element, E, in a compound, the percent composition by mass is given by the following equation:

%" = %&''()"

%&''()'&%*+,-.//%

: A 3.67-g sample of the mineral chalcopyrite was determined to contain 1.27 g Cu, 1.12 g Fe, and 1.28 g S. What is the mass percent of copper in this compound?

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4.1 Percent Composition

• The percent composition for all the elements present in a compound must add up to 100%.

• Would this mass % differ for a different sample of this mineral?

• If you had a 100-gram sample, what mass of copper would it contain?

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: Percent Composition1. A 4.55-g sample of limestone (CaCO3) contains 1.83 g of

calcium. What is the percent Ca in limestone?

2. A sample of a copper compound weighs 1.63 g and contains 1.30 g of Cu. The other element in the compound is oxygen. What is the percent composition of this compound?

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4.2 Mole Quantities: 1 mole = 6.022 � 1023 particles

• 1 mole C = 6.022 � 1023 carbon atoms

• 1 mole H2O = 6.022 � 1023 H2O molecules= 6.022 x 1023 O atoms= 1.2044 x 1024 H atoms

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: Atoms in H2S• How many sulfur atoms are in 1 mol of H2S?• How many hydrogen atoms are in 1 mol of H2S?

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4.2 Mole Quantities• The mole unit acts as a bridge between the microscopic world

and the macroscopic world.

• One mole is defined as the number of particles (atoms, molecules, or formula units) as there are atoms in exactly 12 g of carbon-12 (6.022 � 1023 .)

• Avogadro’s number has been defined so that the mass of 1 mol of C-12 has a mass of exactly 12 grams.

• This means that the average mass of an atom of any substance in amu’s is the same numerical value as the mass of 1 mole of that substance in grams (molar mass).

The molar mass of carbon is 12.01 g/mol.

The molar mass of oxygen is 16.00 g/molThe molar mass of CO2 is: 12.01 + 2(16.00) = 44.01 g/mol

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: Atomic & Molar Mass

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H

O

Na

Ca(ClO3)2

H2O

NaCl

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: Mole Quantities

Which contains the greatest number of atoms?• 1 mole of copper or 1 mole of gold?

• 1 gram of copper or 1 gram of gold?

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: Mole Quantities

1. Convert 10.0 grams of CO2 to moles.

2. Convert 0.50 mol CO2 to grams.

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: Mole Quantities

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1. How many CO2 molecules are in a 100.0 g sample of CO2?

2. How many oxygen atoms are in a 25.0 g sample of CO2?


: Mole Quantities1. What mass of MgCl2 contains 6.022 � 1023 Cl− ions?

(MMMgCl2 = 95.21 g/mol)

2. How many molecules of aspartame (C14H18N2O5) are found in 40.0 mg? (MM = 294.34 g/mol)

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: Mole QuantitiesIf one aspirin tablet contains 0.324 g of acetylsalicylic acid, AA (C9H8O4), then how many molecules of acetylsalicylic acid are in 2 aspirin tablets? (MMAA=180.17 g/mol)

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The formula for a substance tells us about its composition:• A formula unit for an ionic compound indicates the ratio of

ions in the compound. - MgCl2 has a 1:2 ratio of Mg2+ to Cl−

• A molecular formula tells the number of atoms in a molecule and the atom ratio.

- CO2 has a 1:2 ratio of C to O atoms

: Expresses the simplest ratios of atoms in a compound• Written with the smallest whole-number subscripts

: Expresses the actual number of atoms in a compound• Can have the same subscripts as the empirical formula or

some multiple of them 4-14

4.3 Empirical & Molecular Formulas

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Figure 4.13

: Empirical FormulasWhat are the empirical and molecular formula for these molecules?

• If 2 molecules have the same EF, are they the same compound?

• Will the have the same properties?


What are the empirical and molecular formulas for copper(II) oxide?

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: Empirical Formulas

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If we know the masses of the elements in a compound, or its percent composition, we can determine its mole ratio, and therefore the compound’s empirical formula.1. Since the percent composition does not change from

sample to sample, assume any size sample. The most convenient is 100 grams so % value = mass value.

2. Convert grams to moles for each element using its molar mass as a conversion factor.

3. Without changing the relative amounts, change moles to whole numbers. Do this by dividing all by the same smallest value. If all do not convert to whole numbers, multiply to get whole numbers.

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4.3 Empirical & Molecular Formulas


1. Assume 100 g sample:100 grams chalcopyrite contains:• 30.5 g Fe• 34.6 g Cu• 34.9 g S

2. g ⟶ moles:

mol Fe = (30.5g Fe) 012344.647

89 = 0.5461 mol Fe

mol Cu = (34.6g Cu) 0 123:;.447

<= = 0.5444 mol Cu

mol S = (34.9g S) 0 123;>.?@7

A = 1.088 mol S

3. mole ratios:(0.5461 mol Fe) ÷ (0. 5444) = 1.003 mol Fe(0.5444 mol Cu) ÷ (0. 5444) = 1.000 mol Cu(1.088 mol S) ÷ (0. 5444) = 1.999 mol S

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FeCuS2

4.3 Determining Empirical Formulas

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4.3 Determining Empirical Formulas


: Determining Empirical FormulasDetermine the empirical formula for the mineral covellite, which has the percent composition 66.5% Cu and 33.5% S.

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: Determining Empirical FormulasShattuckite is a fairly rare copper mineral. It has the composition 48.43% copper, 17.12% silicon, 34.14% oxygen, and 0.31% hydrogen. Calculate the empirical formula of shattuckite.

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Benzene and acetylene have the same empirical formulas but different molecular formulas.

• How much greater in mass is benzene than acetylene?

• How much greater in mass is each of these than the empirical formula?

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4.3 Determining Molecular Formulas

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What are the ratios of masses for the molecular and empirical formulas?

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To determine a molecular formula, the problem must provide another piece of experimental data, such as a molar mass, MM.

To find the molecular formula:1. Find the empirical formula.2. Divide the empirical formula’s molar mass into the given

experimental molar mass.BBCDEFGHEIJ KDJLHEI

BBMLNOJOGIE KDJLHEI= # Empirical Units

3. Multiply the EF by the EU to get the MFPQRSTSUVW 8XTQ=WV Y PQRSTSUVW Z[S\ = ]XW9U=WVT 8XTQ=WV

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A compound was determined to have an empirical formula of CH2. Its molar mass was determined to be 42.12 g/mol. What is the molecular formula for this compound?

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: Determining Molecular Formulas


Potassium persulfate is a strong bleaching agent. It has a percent composition of 28.93% potassium, 23.72% sulfur, and 47.35% oxygen. The experimental molar mass of 270.0 g/mol. What are the empirical and molecular formulas of potassium persulfate?

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: Determining Molecular Formulas

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If you know the formula for a compound, you can determine the mass percent composition. • Assume you have 1 mole of compound, and convert moles of

the element and compound to grams.

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4.3 Determining Percent Composition

: What is the percent sodium in Na2O?


4.4 Chemical Composition of Solutions: a homogeneous mixture of

a solute and a solvent.

• This is a solution being prepared by adding the CuSO4 (solute) to water (solvent).

• The composition of the solution depends on the relative amounts of the solute and solvent.

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4.4 Chemical Composition of Solutions• Which aqueous CuSO4 solution has the greatest concentration

(most concentrated)?

• Which is most dilute?

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4.4 Chemical Composition of SolutionsSolution concentration is often expressed as the mass percent of solute:

^9TU9[\]V__AXW=\9= QV___XW=\9QV___XW=\SX[

Y100%

: What is the mass percent of NaCl in a solution that is prepared by adding 10.0 g NaCl to 50.0 g water?

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4.4 Chemical Composition of Solutions: concentration of a solution as moles of solute per

liter of solution. ]XWVTS\b = 123cdd23efcgd23efh2i

: 6.25 grams of CuSO4�5H2O is added to a 250-mL volumetric flask. Water is added to for a total volume of 250.0 mL. What is the molarity of this solution?

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: Molarity1. How many moles of NaCl are in 1.85 L of a 0.25 M NaCl

solution?

2. Bluestone is copper(II) sulfate pentahydrate, CuSO4•5H2O (MM = 249.69 g/mol.) A sample of pond water was found to have a concentration of 6.2 × 10-5 M copper(II) sulfate. If the pond has a volume of 1.8 × 107 L, then what mass of bluestone did the farmer add to the pond as an algaecide?

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4.4 Chemical Composition of SolutionsSuppose you want to dilute a 0.25 M solution to a concentration of 0.025 M. How can you do this?

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4.4 Chemical Composition of Solutions

: adding solvent to a solution to result in a lower concentration solution.Molesinitial= Molesfinal

Moles = M•V = mol/L � LMinitialVinitial = MfinalVfinal

: What is the concentration of a solution prepared by adding water to 25.0 mL of 6.00 M NaOH to a total volume of 500.0 mL?

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: Dilution1. If 42.8 mL of 3.02 M H2SO4 solution is diluted to a final

volume 500.0 mL, what is the molarity of the diluted solution of H2SO4?

2. What volume of 0.361 M KBr solution is needed to prepare 500.0 mL of 0.150 M KBr?

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