Chemical Equations and Chemical Quantities

8/13/2019 Chemical Equations and Chemical Quantities

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CHEMICAL EQUATIONS & CHEMICAL QUANTITIES

Formula Mass

The massof an individual molecule or formula unit also known as molecular massor molecular weight

mass of 1 molecule of H2O

= 2(1.01 amu H) + 16.00 amu O = 18.02 amu

Molar Mass of Compounds The relative masses of molecules can be calculated from atomic masses

Formula Mass = 1 molecule of H2O

= 2(1.01 amu H) + 16.00 amu O = 18.02 amu

1 mole of H2O contains 2 moles of H and 1 mole of O somolar mass = 1 mole H2O

= 2(1.01 g H) + 16.00 g O = 18.02 g

so the Molar Mass of H2O is 18.02 g/mole

Practice How many moles are in 50.0 g of PbO2? (Pb = 207.2, O = 16.00)


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Example: Find the number of CO2molecules in 10.8 g of dry ice

Practice How many formula units are in 50.0 g of PbO2? (PbO2= 239.2)

Practice What is the mass of 4.78 x 1024

NO2molecules?


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Composition of Compounds - Percent Composition

Percentage of each element in a compound by mass

Can be determined from1. the formula of the compound2. the experimental mass analysis of the compound The percentages may not always total to 100% due to rounding

Example 3.13: Find the mass percent of Cl in C2Cl4F2

Mass Percent as a Conversion Factor

The mass percent tells you the mass of a constituent element in 100 g of thecompound

the fact that CCl2F2is 58.64% Cl by mass means that 100 g of CCl2F2contains 58.64 g Cl


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This can be used as a conversion factor 100 g CCl2F2: 58.64 g Cl

Practice Benzaldehyde is 79.2% carbon. What mass of benzaldehyde contains 19.8 g

of C?

Conversion Factors in Chemical Formulas

Chemical formulas have inherent in them relationships between numbers ofatoms and molecules

or moles of atoms and molecules These relationships can be used to convert between amounts of constituent

elements and molecules

like percent composition


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Practice Determine the empirical formula of magnetite, which contains 72.4% Fe

(55.85) and the rest oxygen (16.00)

Practice Determine the empirical formula of magnetite, which contains 72.4% Fe

(55.85) and the rest oxygen (16.00)

Apply the conceptual plan


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Molecular Formulas

The molecular formula is a multiple of the empirical formula To determine the molecular formula you need to know the empirical formula and

the molar mass of the compound

PracticeBenzopyrene has a molar mass of 252 g and an empirical formula of C5H3.

What is its molecular formula? (C = 12.01, H=1.01)

Molecular formula = {C5H3} x 4 = C20H12

Chemical Reactions

Reactions involve chemical changes in matter resulting in new substances Reactions involve rearrangement and exchange of atoms to produce new

molecules

elements are not transmuted during a reaction

Chemical Equations

Shorthand way of describing a reaction Provides information about the reaction

formulas of reactants and products states of reactants and products relative numbers of reactant and product molecules that are required can be used to determine weights of reactants used and products that canbe made


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Combustion of Methanemust be Balanced

To show the reaction obeys the Law of Conservation of Mass the equation mustbe balanced

we adjust the numbers of molecules so there are equal numbers of atomsof each element on both sides of the arrow

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Chemical Equations

This equation is balanced, meaning that there are equal numbers of atoms ofeach element on the reactant and product sides

to obtain the number of atoms of an element, multiply the subscript bythe coefficient

1 C 1

4 H 4

4 O 2 + 2

Symbols Used in Equations

Symbols used to indicate state after chemical (g) = gas; (l) = liquid; (s) = solid (aq) = aqueous = dissolved in water

Energy symbols used above the arrow for decomposition reactions D = heat hn = light

shock = mechanical elec = electrical


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Example 3.22: Write a balanced equation for the combustion of butane, C4H10

Practice

When aluminum metal reacts with air, it produces a white, powdery compound,

aluminum oxide

reacting with air means reacting with O2aluminum(s) + oxygen(g) aluminum oxide(s)

Al(s) + O2(g) Al2O3(s)

Reaction Stoichiometry

The coefficients in a balanced chemical equation specify the relative amounts inmoles of each of the substances involved in the reaction

2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)

2 molecules of C8H

18react with 25 molecules of O

2

to form 16 molecules of CO2and 18 molecules of H2O 2 moles of C8H18react with 25 moles of O2 to form 16 moles of CO2and 18 moles of H2O 2 mol C8H18: 25 mol O2: 16 mol CO2: 18 mol H2O


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Predicting Amounts from Stoichiometry

The amounts of any other substance in a chemical reaction can be determinedfrom the amount of just one substance

How much CO2 can be made from 22.0 moles of C8H18in the combustion ofC8H18?

2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)

2 moles C8H18: 16 moles CO2

Practice How many moles of water are made in the combustion of 0.10 moles of

glucose?


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Example 4.1: How many grams of glucose can be synthesized from 37.8 g of CO2in

photosynthesis?

Practice How many grams of O2can be made from the decomposition of 100.0 g of

PbO2?

2 PbO2(s)2 PbO(s) + O2(g)


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Percent Yield

The actual amount of product made in a chemical reaction is called the actualyield.

The theoretical yield will always be the least possible amount of product the theoretical yield will always come from the limiting reactant

The actual yield of product will always be less than the theoretical yield

The Limiting Reactant

The reactant that limits the amount of product is called the limiting reactant sometimes called the limiting reagent the limiting reactant gets completely consumed

Reactants not completely consumed are called excess reactants The amount of product that can be made from the limiting reactant is called the

theoretical yield

Practice How many moles of Si3N4can be made from 1.20 moles of Si and 1.00 moles

of N2in the reaction

3 Si + 2 N2 Si3N4?


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Practice How many grams of N2(g) can be made from 9.05 g of NH3reacting with 45.2

g of CuO?

2 NH3(g) + 3 CuO(s) N2(g) + 3 Cu(s) + 3 H2O(l)

If 4.61 g of N2are made, what is the percent yield?

Solution:


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Solutions

Homogeneous mixtures are called solutions The component of the solution that changes state is called the solute The component that keeps its state is called the solvent

if both components start in the same state, the major component is the solventSolution Concentration

Qualitatively, solutions are often described as dilute or concentrated Dilute solutionshave a small amount of solute compared to solvent Concentrated solutionshave a large amount of solute compared to solvent

Molarity

Moles of solute per 1 liter of solution

Used because it describes how many molecules of solute in each liter of solution


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Preparing 1 L of a 1.00 M NaCl Solution

Example 4.5: Find the molarity of a solution that has 25.5 g KBr dissolved in 1.75 L of

solution


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Practice What Is the molarity of a solution containing 3.4 g of NH3 (MM 17.03) in

200.0 mL of solution?

Using Molarity in Calculations

Molarity shows the relationship between the moles of solute and liters ofsolution

If a sugar solution concentration is 2.0 M, then 1 liter of solution contains 2.0moles of sugar

2 liters = 4.0 moles sugar 0.5 liters = 1.0 mole sugar

1 L solution : 2 moles sugarExample 4.6: How many liters of 0.125 M NaOH contain 0.255 mol NaOH?

Given:

Find:

Conceptual Plan:

Solution:


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Practice Determine the mass of CaCl2 (MM = 110.98) in 1.75 L of 1.50 M solution

Given:

Find:

Conceptual Plan:

Solution:

Example: How would you prepare 250.0 mL of a 1.00 M solution CuSO4 5 H2O(MM

249.69)?

Given:

Find:

Conceptual Plan:

Solution:

PracticeHow would you prepare 250.0 mL of 0.150 M CaCl2?

Given:

Find:

Conceptual Plan:

Solution:


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Dilution

Often, solutions are stored as concentrated stock solutions To make solutions of lower concentrations from these stock solutions, more

solvent is added

the amount of solute doesnt change, just the volume of solutionmoles solute in solution 1 = moles solute in solution 2

The concentrations and volumes of the stock and new solutions are inverselyproportional

M1V1= M2V2


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Example 4.7: To what volume should you dilute 0.200 L of 15.0 M NaOH to make 3.00 M

NaOH?

PracticeWhat is the concentration of a solution prepared by diluting 45.0 mL of 8.25

M HNO3to 135.0 mL?

Given:

Find:

Conceptual Plan:

Solution:

PracticeHow would you prepare 200.0 mL of 0.25 M NaCl solution from a 2.0 M

solution?

Given:

Find:

Conceptual Plan:Solution:


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Solution Stoichiometry

Because molarity relates the moles of solute to the liters of solution, it can beused to convert between amount of reactants and/or products in a chemical

reaction

Example 4.8: What volume of 0.150 M KCl is required to completely react with 0.150 L

of 0.175 M Pb(NO3)2 in the reaction 2 KCl(aq) + Pb(NO3)2(aq) PbCl2(s) + 2 KNO3(aq)?

Practice Solution stoichiometry

43.8 mL of 0.107 M HCl is needed to neutralize 37.6 mL of Ba(OH)2solution.What is the molarity of the base?

Chemical Equations and Chemical Quantities

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