Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
An in-depth explanation of measuring moles and masses in chemistry, including Avogadro's number, conversion factors for moles and particles, and calculating percent composition and empirical formulas. It includes numerous examples and practice problems.
Typology: Study notes
1 / 70
This page cannot be seen from the preview
Don't miss anything!
I. Measuring Matter
Chemists need a convenient methodfor counting accurately the numberof atoms, molecules, or formula unitsin a sample of a substance.
I. Measuring Matter
Through years of experimentation, it hasbeen established that a mole of anythingcontains
23
representative
particles.
A. Measuring Moles
The number 6.022 136 7 x 10
23
is called
in honor of the
Italian physicist and lawyer AmedeoAvogadro who, in 1811, determined thevolume of one mole of a gas.
A. Measuring Moles
One-mole quantities ofthree substances areshown, each with adifferentrepresentative particle.
A. Measuring Moles
The representativeparticle in a mole ofwater is the watermolecule.
A. Measuring Moles Therepresentativeparticle in amole ofcopper is thecopper atom.
Suppose you want to determine howmany particles of sucrose are in 3.50moles of sucrose. You know that onemole contains 6.02 x 10
23
representative particles.
B. Converting Moles to Particles
-^ Therefore, you can write a conversionfactor, Avogadroās number, that relatesrepresentative particles to moles of asubstance.
You can find the number ofrepresentative particles in a number ofmoles just as you found the number ofroses in 3.5 dozen.
B. Converting Moles to Particles
-^ For sucrose, the representative particleis a molecule, so the number ofmolecules of sucrose is obtained bymultiplying 3.50 moles of sucrose bythe conversion factor, Avogadroāsnumber.
Now, suppose you want to find out howmany moles are represented by a certainnumber of representative particles.
B. Converting Particles to Moles
-^ You can use the inverse of Avogadroāsnumber as a conversion factor.
Example: ā¢^ Zinc is used as a corrosion-resistantcoating on iron and steel. It is also anessential trace element in your diet.
B. Converting Particles to Moles
-^ Calculate the number of moles thatcontain 4.50 x 10
24
atoms of zinc (Zn).
The relative scale of atomic massesuses the isotope carbon-12 as thestandard.
II. Mass and the Mole
Each atom of carbon-12 has a mass of12 atomic mass units (amu).
-^ The atomic masses of all otherelements are established relative tocarbon-12.
For example, an atom of hydrogen-1 has amass of 1 amu.
II. Mass and the Mole
The mass of an atom of helium-4 is 4 amu.
-^ Therefore, the mass of one atom ofhydrogen-1 is one-twelfth the mass of oneatom of carbon-12. -^ The mass of one atom of helium-4 is one-third the mass of one atom of carbon-12.
You know that the mole is defined as thenumber of representative particles, orcarbon-12 atoms, in exactly 12 g of purecarbon-12.
II. Mass and the Mole
Thus, the mass of one mole of carbon-12atoms is 12 g. What about other elements?
-^ Whether you are considering a single atomor Avogadroās number of atoms (a mole),the masses of all atoms are establishedrelative to the mass of carbon-12.
II. Mass and the Mole
The mass in grams of one mole of anypure substance is called its
molar mass
The molar mass of any element isnumerically equal to its atomic mass andhas the units g/mol.
