# • The study of the numerical relationship between chemical quantities in a chemical reaction is called stoichiometry.

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## Chemical Reactions and Reaction Stoichiometry

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### O(g)

2 molecules of C8H18 react with 25 molecules of O2 to form 16 molecules of CO2 and 18 molecules of H2O.

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### • Chemical equations are concise representations of chemical reactions

Reactants appear on the left side of the equation.

Products appear on the right side of the equation.

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### O(g)

The states of the reactants and products are written in parentheses to the right of each compound. (g) = gas; (l) = liquid; (s) = solid; (aq) = in aqueous solution

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### Question

• 1) When the following equation is balanced, the coefficients are ________.

C8H18 + O2 → CO2 + H2O

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### Quiz

• 1) When the following equation is balanced, the coefficients are ________.

FeS2 + O2 → Fe2O3 + SO2

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### Quiz

• The formula weight of potassium dichromate (K2Cr2O7 ) is ________ amu

• The formula weight of calcium nitrate Ca(NO3)2 is ________

amu.

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### Quiz

• The mass % of C in methane (CH4) is ________?

• The mass % of H in methane (C2H6) is ________?

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### Quiz

• Calculate the percentage by mass of Pb, N, O in Pb(NO3)2

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### Avogadro’s Number and the Mole

• 6.02 × 1023 atoms or molecules is ONE Mole.

• Avogadro’s number: 6.02 × 1023

• One mole of 12C has a mass of 12.000 g.

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### Quiz

• Calculate the number of H atoms in 0.50 mol of (NH4)2SO4.

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### O

6 has a molar mass of 180.0 g/mol.

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### Quiz

• How many moles of carbon dioxide (CO2) are there in 52.06 g of carbon dioxide?

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### Empirical Formulas from Analyses

• One can determine the empirical formula from the percent composition by following these three steps.

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### Calculating an Empirical Formula

• Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid?

Sol:

In 100.00 g of ascorbic acid we have 40.92 g C, 4.58 g H, and 54.50 g O.

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### Quiz

• What is the empirical formula of a compound that contains 27.0% S, 13.4% O, and 59.6% Cl by mass?

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### Molecular Formulas from Empirical Formulas

• The number of atoms in a molecular formula is a multiple of the number of atoms in an empirical formula.

EX: The empirical formula of a compound was found to be CH. It has a molar mass of 78 g/mol. What is its molecular formula?

Solution:

• Whole-number multiple = 78/13 = 6

The molecular formula is C6H6.

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### Question

• Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H, and

27.79% O. Its experimentally determined molecular mass is 230 u.

What are the empirical and molecular formulas of dibutyl succinate?

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### Quiz

• A compound contains 40.0% C, 6.71% H, and 53.29% O by mass. The molecular weight of the compound is 60.05 amu.

The molecular formula of this compound is ________.

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### Combustion Analysis

• One technique for determining empirical formulas in the laboratory is combustion analysis, commonly used for compounds containing principally carbon and hydrogen

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### Formula by Combustion Analysis

• Isopropyl alcohol, sold as rubbing alcohol, is composed of C, H, and O. Combustion of 0.255 g of isopropyl alcohol produces 0.561 g of CO2 and 0.306 g of H2O. Determine the empirical formula of isopropyl alcohol.

Sol:

• all of the carbon in the sample is converted to CO2

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### Formula by Combustion Analysis

• all of the hydrogen in the sample is converted to H2O

The mass of the sample, 0.255 g, is the sum of the masses of C, H, and O.

Mass of O = mass of sample – (mass of C + mass of H)

= 0.255 g – (0.153 g + 0.0343 g) = 0.068 g O

39 Moles C = (0.153 g )(1 mol C/12.0 g C) = 0.0128 mol C

Moles H = (0.0343 g)(1 mol H/1.01 g H)=0.0340 mol H Moles O = (0.068 g )(1 mol O/16.0 g O)= 0.0043 mol O

The first two numbers are very close to the whole numbers 3 and 8, giving the empirical Formula C3H8O.

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### Question

• Vitamin C is essential for the prevention of scurvy. Combustion of a 0.2000 g sample of this carbon– hydrogen–oxygen

compound yields 0.2998 g CO2 and 0.0819 g H2O. What are the percent composition and the empirical formula of vitamin C?

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### Quiz

• A compound is composed of only C, H, and O. The combustion of a 0.519-g sample of the compound yields 1.24 g of CO2 and 0.255 g of H2O. What is the empirical formula of the

compound?

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### O

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Moles C6H12O6 = (1.00 g C6H12O6)(1 mol C6H12O6/ 180.0 g C6H12O6)

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### Limiting Reactants

• The reactant that is completely consumed in a reaction is called the limiting reactant. The other reactants are

sometimes called excess reactants

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### Theoretical and Percent Yields

• The quantity of product calculated to form when all of a limiting reactant is consumed is called the theoretical yield.

• The amount of product actually obtained, called the actual yield

• The percent yield of a reaction relates actual and theoretical yields

Percent yield = (actual yield/theoretical yield) * 100%

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## References

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