DIMENSIONAL ANALYSIS

(Also called Factor labeling)

-This is a problem solving method that focuses on units.

-It uses conversion factors that are equivalents like 2 pints = 1 quart and 60 seconds = 1 minute.

-MOST conversion factors have unlimited numbers of significant figures.

-You must practice using dimensional analysis – we will use it all year and failure to use it usually results in a wrong answer.

-THE UNITS MUST CANCEL – YOU MUST BE LEFT WITH THE UNITS YOU ARE SEEKING. WATCH SGINIFICANT FIGURES.

PRACTICE:

  1. 262 days = ? seconds
  1. 20,000.0 s = ? years
  1. 25 m/s = ? km/hr

SC2. Students will relate how the Law of Conservation of Matter is used to determine chemicalcomposition in compounds and chemical reactions.

  1. Apply concepts of the mole and Avogadro’s number to conceptualize and calculate

• Mass, moles and molecules relationships,

THE MOLE

CHAPTER 10

Introduction

Matter is made up of particles. A representative particle denotes what type of particle it is (atom, ion, molecule, etc.)

Individual (molecular) (ionic)

atoms MoleculesFormula units

Fe H2O NaCl

He N2 CaCl2

You should be able to tell how many atoms are in each chemical formula. For instance:

Fe - single atom

H2O- single molecule (2 H atoms, 1 O atom = 3 atoms

CaCl2- single formula unit (1 Ca atom, 2 Cl atoms = 3 atoms)

Mg(OH) 2-

ATOMIC MASS, FORMULA MASS,

and MOLECULAR MASS

ATOMIC MASS

This isthe mass of an atom in atomic mass units (amu). It is the same as the average mass of the atoms of an element found on the periodic table.

Example: C - single atom – 12.01 amu

MOLECULAR MASS

This is the mass found by adding the atomic masses of the elements found in the molecule (molecular compound).

Example: H2O – molecular compoundH (2 x 1.01amu) + O (1x16.00amu) = 18.02 amu

FORMULA MASS

This ismass found by adding up the atomic masses of the elements found in a formula unit (ionic compound).

Example: NaCl – ionic compoundNa (1 x 22.99amu) + Cl (1 x 35.45amu) = 58.44 amu

Different atoms have different masses; THEREFORE different compounds have masses dependent upon the masses of the atoms that make them up.

PRACTICE:

Calculate the formula or molecular mass of the following compounds:

A. H2SO4C. Al(NO3) 3

B. C3H5N3O3D. Fe(CH3COO)3

THE MOLE

The MOLE (mol) is the unit of measurement used to describe the amount of matter in a particular substance. (Just like dozen, gross)

1 mole = 6.0213672 x 1023 particles

= 6.02 x 1023 particles

This is a rounded number

(particles are atoms, formula units, or molecules)

It is the number of particles in exactly 12 grams of pure Carbon-12. Also known as Avogadro’s number.

MOLES TO MOLES

These kinds of problems involve making a comparison of one item to another. In one formula unit of Ca(OH)2 there are two atoms of O. If you have 2.0 moles of Ca(OH)2, then you would have 2.0 x 2 for the moles of O.

2.0 moles Ca(OH)2 2 mole O = 4.0 mol O

1 mol Ca(OH)2

PRACTICE

Always start with the number you are given and then find the conversion factor that gets you to the desired outcome.

How many moles of oxygen are in 3.4 moles of H2SO4?

MOLES AND MASS

The mole is a conversion factor that relates the number of particles of matter to the matter’s mass. Molar mass = mass (g) of one mole of atoms. We will use dimensional analysis (factor label) to make mole/mass conversions.

Example:All have 6.02x1023 atoms

1 mole He = 4.00 g He1 mole Na = 22.99 g Na

1 mole Mg = 24.31 g Mg1 mole Al = 26.98 g Al

Use the conversion factor: atomic

molecular MASS (g) = 1 MOLE = MOLAR MASS

formula

PRACTICE

A. Calculate the mass in grams of 2.0 moles of Na.

??g = 2.0 mol Na 22.99 g Na= g Na

1 mol Na

B. Calculate the mass in grams of 0.500 moles of CO2

You can also calculate the number of moles in a given mass.

C. How many moles are in 50.0 g of H2?

?? moles = 50.0 g H2 1 mole = mol H2

2.02g H2

D. How many moles are in 75.0 g of NaOH?

MOLES AND PARTICLES

We use factor label to make mole/particles conversions.

Use the conversion factor: 1 mole = 6.02 x 1023 particles

(particles are atoms, formula units, or molecules)

PRACTICE:

E. Determine the number of molecules in 2.23 moles of nitrogen, N2.

?? molecules = 2.23 mol N2 6.02x1023 molecules N2 = molecules N2

1 mol N2

  1. Determine the number of atoms in 16.0 moles of sulfur.

G. Determine the number of moles in 4.36 x 1024 atoms of silver, Ag

MASS AND MOLE AND PARTICLE

You can also convert mass to particles by going through moles.

1 mole 1 mole

PARTICLES MOLEMASS

(atoms or 6.02 x 1023 molar mass (from

molecules) periodic table)

  1. Determine the number of atoms in 16.0 g of sulfur.

?? atoms = 16.0 g S 1 mol S 6.02x1023 atoms S = atoms S

32.06 g S 1 mol S

I. Calculate the number of grams in 1.20 x 1023 atoms of calcium.

PERCENT COMPOSITION

Chocolate Chip Cookies:

450.0g sugar20.0mg saltLooking at the ingredients, what

450.0g Crisco20.0mg baking sodapercentage of the total

2 eggs (75.0g)20.0mg vanillacomposition is eggs?

900.0g flour

To Calculate:

  1. Get the total amount (add up all the ingredients).

Total = 450.0g + 450.0g + 75.0g + 900.0g + 0.002g + 0.002g + 0.002g = 1875.006g = 1875.0g

  1. Find the percentage of the total that is eggs.

part x 100 75.0g x 100 = 4.00 %

total1875.0

Practice:

  1. What is the mass percent of oxygen in H2SO4?
  1. A 200.0 g orange contains 20 seeds. Each seed has a mass of 950.0 mg. What % of the orange, by mass, are seeds?