Stoichiometry

Overview of the Atom• Protons positively charged particles in

the nucleus of the atom• Electrons negatively charged particles

found in orbitals, or electron “clouds,” outside the nucleus of the atom• Neutron neutrally charged particles

found in the nucleus of the atom

Remember: Atomic number (Z)is _______. Mass number is ____.

Atomic Mass Unitamount defined as being exactly 1/12 the mass of a carbon-12 atom

  Mass Location Charge Discovered by

Protons 1 nucleus + Rutherford

Electrons 0 orbital - J.J. Thomsonneutrons 1 nucleus 0 Chadwick

Carbon-12: has 6 protons and 6 neutrons, accounting for its mass number of 12. Thus carbon is arbitrarily assigned a mass of 12 atomic mass units (meaning protons and neutrons are assigned values of 1 amu)

Likewise, a hydrogen atom, which has only 1 proton and no neutrons would have a mass number of 1 amu.

What does the periodic table tell us about the atom??

The number of protons in a neutral atom is given (atomic number!! Z)Neutral means have NO net charge, thus the total “positives” (protons) in a neutral atom MUST be equal to the number of electrons.**An atom cannot and will not lose protons EXCEPT in cases of radioactive decay!! When an atom loses a proton, it becomes an atom of a completely different element!!

Protons p+

Electrons (e-)Electrons are involved in chemical bonding. When elements are bonded, electrons are shared or “donated” between forming bonds between elements.

chemical bonding: attraction of the positively charged nucleus of one atom to the negatively charged electrons in the valence (outer shell) of

a nearby atom

These bonds account for the properties of a substance, thus electrons directly affect the properties of a substance.

Atoms that are symbolized as being positively or negatively charged have either GAINED or LOST electrons.

Na+ , Cl-, O2- , B3+

IonAtom that has gained or lost electrons

Cation: positively chargedNa+, Al3+, H+

Anion: negatively chargedO2-, Cl-

Neutrons (n0)

neutrally charged (NO CHARGE!!!)

Mass number – Atomic Number = number of neutrons in an

element

-can also be involved in radioactive decay

Isotopes• Atoms of a given element having a differing number of neutrons

When given an isotope, the atomicmass will be given.

Example, the isotope Carbon-14

This isotope can also be designated as

CWhat is carbon-14 used for?

How many p+, n0, and e- are in a neutral carbon atom? C-14?

Elemental NotationEach “position” around the atomic symbol serves a specific purpose!!

ChargeMass number (oxidation state)

2 + 1 5

Z # of atoms in sampleH

Practice Atomic Composition  20Ne0 40Ag+ 40Ca2+ 32S2-

# of protons # of electrons # of neutrons

  39K0 56Fe2+ 22Na 88Sr2+# of protons # of electrons # of neutrons

  27Al 24Mg2+ 86Rb 31S2-

# of protons # of electrons # of neutrons

Symbol Ion Commonly Formed

Number of Electrons in Ion Z

Te 54In 49Sr Sr2+

Mg2+ 12ClF 9

Be2+ 2Br 36Al 13O

Stoichiometry

Area of chemistry that is concerned with the quantitative analysis of chemical reactions

The mole is the central concept to understanding stoichiometry.

One mole = 602,000,000,000,000,000,000,000 items!!!

Units used to count number of items present

• 24 cans = 1 case

• 144 pencils = 1 gross

• 500 sheets paper = 1 ream

• 12 eggs = 1 dozen

• Can you think of more?

Molar Mass• Mole (mol): SI unit for the amount of substance– the amount of substance that contains as many elementary

entities (atoms, molecules, or other particles) as there are atoms in exactly 12 grams of carbon-12 isotope

1 mole = 6.022 x 1023 atoms…

Avogadro’s number

The concept of a mole is like that of a dozen.

More definitions of the mole(yes, you need to learn them all…)

1 mole = 6.022 x 1023 atoms/particles/molecules

1 mole = 22.4 L gas at STP (standard temperature and pressure)

STP: 0˚C and 1 atm1 mole = average atomic mass of element (from P.T.)

1 mole = molecular mass of compound (sums of masses of constituent elements)

Avogadro’s Number• 6.022 x 1023

– Named after Amadeus Avogadro, an Italian physicist

• One mole of anything is defined as 6.022 x 1023 units of that thing

Moles of Elements in a FormulaAspirin C9H8O4

One molecule aspirin: 9 C atoms, 8 H atoms, 4 O atoms

One mole of aspirin: 9 mol C, 8 mol H, and 4 mol O

9 mol C 8 mol H 4 mol O 1 mol C9H8O4 1 mol C9H8O4 1 mol C9H8O4

Converting between Mass and Moles• To convert between mass and moles, you

will use the conversion factor involving the following equality: –1 mole of a substance = molar/

molecular mass

• The conversion will be set up so that cancelled units are on the bottom and desired units are on the top

Converting between Moles and Mass• Zinc (Zn) is a silvery metal that is used to form brass

(with copper) and plate iron to prevent corrosion. (a) How many grams of Zn are in 0.356 mol Zn?

(b) How many moles of Zn are in 668 g of Zn?

Your turn!

• Calculate the number of grams of helium (He) in 1.61 moles of helium.

• Calculate the number of moles of magnesium (Mg) in 0.317 g of Mg.

Calculating the number of atoms, particles, molecules…

• In order to calculate the number of atoms, you must first convert to moles if that is not the units you are given

• 1 mole = 6.022 x 1023 molecules, atoms, particles

Calculating the mass of a single atom

• Silver (Ag) is a precious metal used mainly in jewelry. What is the mass of one Ag atom?

• What is the mass of one iodine (I) atom?

Converting mass in grams to # of atoms• Sulfur (S) is a nonmetallic element. Its presence in

coal gives rise to the acid rain phenomenon. How many atoms are in 16.3 g of S?

• Calculate the number of atoms in 0.551 g of potassium (K).

Molecular Mass• Sum of the atomic masses (in amu) in the molecule– amu: atomic mass unit– 1 amu = approximately 1/12 of carbon-12 atom• Arbitrary unit!!

• Molecular mass of H2O = 2 (atomic mass of H) + atomic mass of O

Calculating Molecular Mass

• Sulfur dioxide (SO2)

• Ascorbic acid, or vitamin C (C6H8O6)

• Methanol (CH4O)

Calculating # moles in a compound• Methane (CH4) is the principal component of

natural gas. How many moles of CH4 are present in 6.07 g of CH4?

• Calculate the # of moles of chloroform (CHCl3) in 198 g.

Calculate the # of atoms in a compound• How many hydrogen atoms are present in 25.6 g of

urea [(NH2)2CO], which is used as fertilizer, in animal feed, and in the manufacture of polymers? The molar mass of urea is 60.06 g.

Calculating Atoms…• How many H atoms are in 72.5 g of isopropanol,

commonly called rubbing alcohol, C3H8O?

Calculating the Number of Molecules

How many molecules of ammonia, NH3, are present in 1.75 moles ammonia?

Molar Volume

• In order to calculate volume, moles must be calculated (if not given)

• The volume of 1 mole of ANY gas at STP is 22.4 L

1 mole 22.4 L 22.4 L 1 mole

Calculating volume of gas at STPHow many moles does 15.7 dm3 of gas at STP represent? (HINT: 1 dm3 = 1 L)

Determine the number of molecules represented by 1.98 dm3 of a gas at STP.

Calculating volume of gas at STP

Convert 0.0023 moles of gas to volume at STP.