3.1 Percent Composition - Mrs Hudson's BlogChapter 3 notes solutions.notebook 4 April 01, 2015 Oct 8 10:57 PM Calculating % Composition from a Formula Example 1: Determine the percent

Chapter 3 notes solutions.notebook

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3.1 Percent CompositionThe percentage composition of a compound

refers to the relative mass of each element

in a compound.

The Law of Definite Proportions states that the elements in a given chemical compound are always present in the same proportion by mass.

Water Hydrogen Peroxide

Sep 307:43 PM

The mass of an element in a compound can be expressed as mass percent or PERCENTAGE COMPOSITION (the relative mass of each element in a compound expressed as percent).

Calculating % Composition (2 types):

1. Given Compound Only

2. Given a Mass Sample

{Bubble gum activity}


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Example 1: A sample of a compound has a total mass of 48.72g. The sample is found to contain 32.69g of zinc and 16.03g of sulfur. What is the percent composition of the compound? Need to take the mass of zinc and divide it by the total mass

Need to take the mass of sulfur and divide it by the total mass

Oct 810:52 PM

Example 2: A chemical has a total mass of 156.87g and is made up of C, O, and N. The sample contains 83.25g of carbon and 36.55g of O. Calculate the percent composition of each element.

*Don't worry about sig figs...just put the percents to two decimal places!


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Practice Problems

page 82 #14

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Practice Problems

page 82 #14


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Calculating % Composition from a FormulaExample 1: Determine the percent composition of aluminum hydroxide, Al(OH)3.

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Example 2:a) Calculate the percent composition of iron (III) oxide.

b) Calculate the mass of iron that could be extracted from a 2500.0g sample of iron (III) oxide.

Practice Problems

page 85 #58


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3.2 Empirical Formula

The formula of a compound expressed as the smallest possible whole number ratio of subscripts of the elements in the formula.

Molecular Formula Empirical Formula

C2H4

C6H12O6

H2O

C2H6

H2O2

C3H4

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Can different compounds have the same empirical formula?

Example: Benzene C6H6Ethyne C2H2


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Note: Ionic compounds DO NOT have molecular formulas.

*They exist as a ratio of ions.*The empirical formula of an ionic compound is usually the same as its chemical formula.

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Steps to Finding Empirical Formula

Percent to massMass to moleDivide by smallMultiply til'whole


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Example 1: Calculate the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen.

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What would you multiply by to get a whole number?


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Example 2: Determine the empirical formula of a compound that has 80.2% carbon, 9.62% hydrogen and 10.2% oxygen.

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Example 3: Calculate the empirical formula of a compound containing 11.2g of iron and 21.3g of chlorine.


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Example 4: Calculate the empirical formula of a compound that is 85.6% carbon and 14.4% hydrogen.

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Practice Problems

page 89 #912page 91 #1316page 94 #2,4,6,7


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Exit Pass

What is the empirical formula of a compound that contains 67.6% mercury, 10.8% sulfur, and 21.6% oxygen?

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Determining the Empirical Formula of a Hydrate

*Many ionic compounds crystallize from a water solution with water molecules incorporated into their structure. This is called a hydrate.


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Every hydrate has a specific number of water molecules bonded to each formula unit.

MgSO4 7H2O

CaSO4 2H2O

Ba(OH)2 8H2O

CaCl2 6H2O

Fe(OH)2 3H2O *You need to remember the prefixes!

Oct 218:01 AM

Calculations

Example 1: A 50.0g sample of a hydrate of barium hydroxide, Ba(OH)2, contains 27.2g of Ba(OH)2. Find the value of x in Ba(OH)2 xH2O.


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Example 2: A 3.34g sample of a hydrate has the formula SrS2O3 xH2O and contains 2.30g of SrS2O3. Find the formula of the hydrate.

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Example 3: The mass of water in a hydrate of MnCl2 xH2O is 36.4g.The total mass of the hydrate is 100g. What is the empirical formula of the hydrate?


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Example 4: A hydrate of zinc nitrate has the formula Zn(NO3)2 xH2O. If the mass of 1 mol of the anhydrous zinc nitrate is 63.67% of the mass of 1 mol of the hydrate, what is the value of x?

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Example 5: A 9.00g sample of calcium phosphate tetrahydrate was thoroughly heated to remove all the water of hydration. What is the mass of the anhydrous calcium phosphate?


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Practice Problems

page 108 #21

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3.3 Molecular Formula

Suppose a suspect in a theft investigation is a researcher in a biology laboratory. The suspect works with formaldehyde, CH2O. Police officers find traces of a substance at the crime scene and send samples to the Centre for Forensic Science. The forensic analysts find that the substance contains a compound that has an empirical formula CH2O. Will this evidence help to convict the suspect?


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Name Molecular Formula Empirical Formula

formaldehyde CH2O CH2O

acetic acid C2H4O2lactic acid C3H6O3

erythrose C4H8O4

ribose C5H10O5

glucose C6H12O6

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Determining a Molecular Formula

Steps:

1. Find EF2. Find molar mass of EF3. Molar mass of molecular formula (usually given in question)

Molar mass of EF4. Multiply EF by answer to #3


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Example 1: The empirical formula of ribose is CH2O, with a molar mass of 150g/mol. What is the molecular formula?

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Example 2: Determine the molecular formula of capsaicin if it contains 71% carbon, 8% hydrogen, 15.8% oxygen and 14% nitrogen with a molar mass 304g/mol.


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Example 3: The empirical formula of a certain dioxin is C6H2OCl2. If the molar mass of the dioxin is 322g/mol, what is the molecular formula?

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Practice Problems

page 97 #17,18,19


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The CarbonHydrogen Combustion Analyzer

CxHy + O2 CO2 + H2O

A large number of important chemicals are composed of H, C, and O. The carboncombustion analyzer is a useful instrument for analyzing these chemicals. (see diagram on page 99)

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After the combustion, all the carbon in the sample is contained in the carbon dioxide and all the hydrogen in the sample is contained in the water.


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Example 1: A 1.000g sample of a pure compound containing only carbon and hydrogen was combusted in a carbonhydrogen combustion analyzer. The combustion produced 0.6919g of water and 3.338g of carbon dioxide. Determine the empirical formula.

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page 101 #22 Practice page 101 #21


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• Work on EF of a hydrate lab• If completed, start chapter 3 worksheet

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Review

• Worksheet on EF and MF

• page 107108#6#11#12#14

• Combustion of a 1.086g sample containing carbon and hydrogen is burned in a carbonhydrogen combustion analyzer. 2.76g of CO2 and 2.97g of H2O is produced. What is the empirical formula of the compound? C4H21

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3.1 Percent Composition - Mrs Hudson's BlogChapter 3 notes solutions.notebook 4 April 01, 2015 Oct 8 10:57 PM Calculating % Composition from a Formula Example 1: Determine the percent