Quantitative Chemistry

Chapter 3

Objectives Learning objective 1.2 The student is able to select and apply

mathematical routines to mass data to identify or infer the composition of pure substances and/or mixtures.

Learning objective 1.3 The student is able to select and apply mathematical relationships to mass data in order to justify a claim regarding the identity and/or estimated purity of a substance.

Learning objective 1.4 The student is able to connect the number of particles, moles, mass, and volume of substances to one another, both qualitatively and quantitatively.

Learning objective 1.14 The student is able to use data from mass spectrometry to identify the elements and the masses of individual atoms of a specific element

Objectives, Part 2 Learning objective 3.1 Students can translate among

macroscopic observations of change, chemical equations, and particle views.

Learning objective 3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/or to analyze deviations from the expected results.

Learning objective 3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.

Learning objective 3.6 The student is able to use data from synthesis or decomposition of a compound to confirm the conservation of matter and the law of definite proportions.

Atomic MassAtoms are so small, it is difficult to discuss how

much they weigh in grams.

Use atomic mass units.An atomic mass unit (amu) is 1/12 the mass of a

carbon-12 atom. (Adopted in 1961)

The decimal numbers on the table are atomic masses in amu.Sometimes abbreviated as u.

Decimals??Because they are based on averages of atoms

and of isotopes.

Can figure out the average atomic mass from the mass of the isotopes and their relative abundance.

Add up the percent as decimals times the masses of the isotopes.

Mass Spectrometer

Mass Spec Data

The MoleThe mole is a number.

A very large number, but still, just a number.

6.022 x 1023 of anything is a mole

The number of atoms in exactly 12 grams of carbon-12.

Molar MassMass of 1 mole of a substance.

Often called molecular weight.

To determine the molar mass of an element, look on the table.

To determine the molar mass of a compound, add up the molar masses of the elements that make it up.

Find the Molar Mass

Why not both?

Percent CompositionPercent of each element a compound is

composed of.

Find the mass of each element, divide by the total mass, multiply by a 100.

Find the % Comp.

Working Backwards…From percent composition, you can determine

the empirical formula.

Empirical Formula - the lowest ratio of atoms in a molecule.

Based on mole ratios of the constituent elements.

A sample is 59.53% C, 5.38%H, 10.68%N, and 24.40%O, what is its empirical formula?

Pure O2 in CO2 is absorbed

H2O is absorbed

Sample is burned completely to form

CO2 and H2O

Try This!A 0.2000 gram sample of a compound (vitamin

C) composed of only C, H, and O is burned

completely with excess O2 . 0.2998 g of CO2

and 0.0819 g of H2O are produced. What is the

empirical formula?

Empirical To Molecular Formulas

Empirical is lowest ratio.

Molecular is actual molecule.

Ratio of empirical to molar mass will tell you the molecular formula.

Must be a whole number because...

ExampleA compound is made of only sulfur and oxygen.

It is 69.6% S by mass. Its molar mass is 184 g/mol. What is its formula?