Mass Relationships in Chemical Reactions

Chapter 3 Mass Relationships in Chemical Reactions

3.1 Atomic Mass · the mass of the atom in atomic mass units (amu) · atomic mass unit - · Average Atomic Mass · most naturally occurring elements have more than one isotope · relative atomic masses are weighted averages (of natural abundances) of all the isotopes of an element Example: Boron exists as two naturally occurring isotopes: 20.0% 10.01 amu, 80.0% 11.01 amu

3.2 Avogadro’s Number and the Molar Mass of an Element · mole (mol) - the amount of substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 grams of the carbon-12 isotope · This number is called Avogadro’s Number = 6.022045  1023 particles · · molar mass (gram-molar mass)(atomic mass)(gram- atomic mass) - the mass of Avogadro’s number of units

1 1 mol C = 12.00 g C = 6.022  1023 atoms of C 1 mol S = 32.07 g S = 6.022  1023 atoms of S

 What is the mass of 72 atoms of sulfur?

3.3 Molecular Mass the sum of the atomic masses (in amu) in the molecule · molar mass (gram-molecular mass) - molecular mass expressed in grams - one mole of a compound - 6.022  1023 molecules

 How many moles of sucrose (C12H22O11) are contained in 8.76 g of sucrose?

3.4 Mass Spectrophotometer

2 (see figure 3.3)

3.5 Percent Composition of Compounds percent composition by mass - percent by mass of each element in a compound

 Calculate the mass % of carbon in vitamin C, C3H4O3

 Calculate the empirical formula of a compound having the following composition: 26.56% K, 35.41% Cr, 38.03% O

3  Calculate the mass of chromium contained in 548 g of potassium dichromate.

3.6 Experimental Determination of Empirical Formulas combustion analysis:

[CHX] + O2  CO2 + H2O

 An unknown compound is composed of carbon, hydrogen, and oxygen. When 5.000 g is subjected to combustion analysis, 11.879 g of CO2 and 6.075 g of H2O

4 are produced. Calculate its empirical formula.

· Determination of Molecular Formulas  A certain compound has a formula mass of approximately 42 g. Find its molecular formula if it is composed of 85.64% C and 14.36% H.

3.7 Chemical Reactions and Chemical Equations · Writing Chemical Equations reactants and products · Balancing Chemical Equations

5 3.8 Amounts of Reactants and Products stoichiometry - the quantitative study of reactants and products in a chemical reaction The coefficients of a balanced equation represent numbers of moles of reactants and products.

 What mass of sodium chloride will be produced by the decomposition of 50.0 g of sodium chlorate?

6  How many grams of sodium are required to react with excess water to produce 25.0 g of sodium hydroxide?

 How many molecules of chlorine gas are needed to react with 10.0 g of sodium to produce sodium chloride?

3.9 Limiting Reagents

 What mass of sodium sulfate would be formed by the reaction of 32.0 g of sodium hydroxide and 41.3 g of sulfuric acid?

 What is the minimum amount of P4S3 that would have to be burned to produce at least 3.00 g of P4O10 and 3.00 g of SO2?

P4S3 + O2  P4O10 + SO2

7 3.10 Reaction Yield

· actual yield - the amount of product obtained in a reaction · theoretical yield - the amount of product predicted by stoichiometry when all the limiting reagent has reacted · percent yield - the proportion of the actual yield to the theoretical yield