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 Unit amount defined as being exactly 1/12 the mass of a carbon-12 atom MassLocationChargeDiscovered by Protons1nucleus+Rutherford Electrons0orbital-J.J. Thomson neutrons1nucleus0Chadwick 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 -, O 2-, B 3+

Ion Atom that has gained or lost electrons Cation: positively charged Na +, Al 3+, H + Anion: negatively charged O 2-, Cl -

Neutrons (n 0 ) 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 atomic mass will be given. Example, the isotope Carbon-14 This isotope can also be designated as C What is carbon-14 used for? How many p +, n 0, and e - are in a neutral carbon atom? C-14?

Elemental Notation Each “position” around the atomic symbol serves a specific purpose!! Charge Mass number(oxidation state) Z# of atoms in sample H

Practice Atomic Composition 20 Ne 040 Ag +40 Ca 2+32 S 2- # of protons # of electrons # of neutrons 39 K 056 Fe 2+22 Na 88 Sr 2+ # of protons # of electrons # of neutrons 27 Al 24 Mg 2+86 Rb 31 S 2- # of protons # of electrons # of neutrons

Symbol Ion Commonly Formed Number of Electrons in Ion Z Te54 In49 SrSr 2+ Mg Cl F9 Be 2+ 2 Br36 Al13 O

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 = x 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 = x atoms/particles/molecules 1 mole = 22.4 L gas at STP (standard temperature and pressure) STP: 0˚C and 1 atm 1 mole = average atomic mass of element (from P.T.) 1 mole = molecular mass of compound (sums of masses of constituent elements)

Avogadro’s Number x – Named after Amadeus Avogadro, an Italian physicist One mole of anything is defined as x units of that thing

Moles of Elements in a Formula Aspirin  C 9 H 8 O 4 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 C 9 H 8 O 4 1 mol C 9 H 8 O 4 1 mol C 9 H 8 O 4

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 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 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 = x 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 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 H 2 O = 2 (atomic mass of H) + atomic mass of O

Calculating Molecular Mass Sulfur dioxide (SO 2 ) Ascorbic acid, or vitamin C (C 6 H 8 O 6 ) Methanol (CH 4 O)

Calculating # moles in a compound Methane (CH 4 ) is the principal component of natural gas. How many moles of CH 4 are present in 6.07 g of CH 4 ? Calculate the # of moles of chloroform (CHCl 3 ) in 198 g.

Calculate the # of atoms in a compound How many hydrogen atoms are present in 25.6 g of urea [(NH 2 ) 2 CO], which is used as fertilizer, in animal feed, and in the manufacture of polymers? The molar mass of urea is g.

Calculating Atoms… How many H atoms are in 72.5 g of isopropanol, commonly called rubbing alcohol, C 3 H 8 O?

Calculating the Number of Molecules How many molecules of ammonia, NH 3, 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 mole22.4 L 22.4 L1 mole

Calculating volume of gas at STP How many moles does 15.7 dm 3 of gas at STP represent? (HINT: 1 dm 3 = 1 L) Determine the number of molecules represented by 1.98 dm 3 of a gas at STP.

Calculating volume of gas at STP Convert moles of gas to volume at STP.