Stoichiometry …the basis of chemical calculations
What the heck is Stoichiometry • Stoichiometry is the study of quantities of materials produced and consumed in chemical reactions. Or, in a way normal people (not honors students) could understand… …solving problems using relationships found in balanced, chemical equations!
How could this be possibly used? 2 Na. N 3 → 2 Na + 3 N 2 To get the correct amount of N 2 we must start with the correct amount of Na. N 3. Stoich is used to calculate this amount.
The MOLE Concept –How to count things that are too small to see • A mole is a counting number used by chemists …Kind of like 1 dozen eggs – 12 eggs A mole is the name of a number! The mole is defined as the number of Carbon atoms in exactly 12 grams of pure Carbon – 12 (think the graphite in a pencil!) It was determined that 1 mole = 6. 02 x 1023 units The units are usually atoms or molecules This number is called AVOGADRO’S NUMBER!
A Brief History In 1811 Amadeo Avogadro proposed that the number of particles in a given volume of a gas (at a specific temperature and pressure) is always the same, regardless of the identity of the gas. In 1865, Loschmidt estimated a value for what Specific number this would be. Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e di Cerreto
Just How Big is This Number? • How big is 6. 02 x 1023 ? A huge number! • 1 mole of marbles would cover the entire earth a depth of 50 miles! • 1 mole sheets of paper would go to the moon and back 80 billion times. • 1 mole of pennies- each of us on earth could spend 1 million dollars each hour (day and night) and have ½ of it left if we died at age 70! (Much too big a number to comprehend)
So what is the BIG DEAL about a mole? We know that 1 mole of anything is 6. 02 x 1023 anythings. So 1 mole of atoms of a given element would be 6. 02 x 1023 atoms of that element.
We also know that the mass of an atom is given in amu’s. (The mass of a N atom is 14 amu). The true wonderfullness of the mole concept is that: If we have 6. 02 x 1023 atoms (1 mole) of an element, the mass of that element will be equal to it’s atomic mass IN GRAMS! So 1 mole of N has a mass of 14 g. And since atomic masses are relative to each other, this works for ALL ELEMENTS!
Comparisons of 1 mole of different elements Aluminum 6. 02 x 1023 atoms 26. 98 grams Gold 6. 02 x 1023 atoms 196. 97 grams Boron 6. 02 x 1023 atoms 10. 81 grams
Essentially, this makes the periodic table a huge list of conversion factors!!! N - 14. 01 g / 1 mol Si - 28. 09 g / 1 mol CFe Au This is why we love the periodic table!
You have 10 g of Al. Calculate the number of moles AND the number of atoms. • Remember: the mass of 1 mole of Al equals 26. 98 or 27 grams (the at. wt. )
Mole Map 6. 02 x 1023 Atomic mass grams 1 mole Atoms, molecules, ions
A silicon chip used in your cell phone has a mass of 5. 68 mg (which is 0. 00568 g). How many Si atoms are present in this chip?
Co is a metal added to steel (a mixture) to help the steel resist corrosion. You have 5. 00 x 1020 atoms of Co. Calculate: a. The number of moles: b. The mass of Cobalt:
Molecular or Formula Weights • This concept can be extended to compounds. • Since a molecule is made up of a specific combination of atoms, if we add up the masses of the atoms, we will get the mass of the compound!!! • Find the formula weight of CH 4 (a byproduct of Chipotle)
Calculate the formula weight of Na. Cl
Calculate the formula weight of C 10 H 6 O 3 , a natural herbicide (weed killer) and dye in Rit Dye.
Now, nerds of the world…calculate the number of moles of C 10 H 6 O 3 in 0. 0156 g
Isopentyl acetate (C 7 H 14 O 2) is the compound responsible for the scent of bananas. Bee’s also release this same compound when they sting in order to attract other bees (to the buffet) and join in the attack. They release 1 microgram in each bee sting. 1. How many molecules of C 7 H 14 O 2 are released in one bee sting?
How many atoms of C are present in one bee sting? (a fact you have always wanted to know)
Percent Composition or Rabbit and Cow stew • Ethanol, an additive to gas (and alcoholic drinks) has the formula C 2 H 5 OH. Find the % of each element in ethanol. STEP 1: Calculate the formula weight STEP 2: Divide the total mass of the individual element by the formula weight. Times 100 to get %.
Find the mass % of each element in C 10 H 14 O
Penicillin, an antibiotic, has the formula C 14 H 20 N 2 SO 4. 1. Find the % of Nitrogen in penicillin. 2. Find the number of moles of penicillin if you give a patient a 250 mg tablet.
How many grams of potassium can be produced when 150. 6 g of potassium sulfate is completely decomposed? First, find the % of K in K 2 SO 4 Next, multiply the % (decimal) by the mass given.
How many grams of Cu are needed to make 16. 6 g of copper II phosphate? First, find the % of Cu: Second, multiply the % by the mass given:
Empirical Formulas • A empirical formula is the simplest, whole number ratio of atoms. Most formulas for ionic compounds are empirical. What is the key? If it CAN’T be reduced, it is empirical. If it CAN be reduced, it is a molecular formula. What are these? Na 2 SO 4 C 12 H 22 O 11 C 6 H 12 O 6 N 4 O 12
To find the Empirical Formula (from %) 1. Convert % data to grams (% means “parts per hundred”). 2. Convert the grams to moles (÷ by the atomic weight) 3. Divide by the smallest to the get the simplest, whole-number ratio.
The junior scientist was given 38. 67% C, 16. 23% H and 45. 1% N. Find the empirical formula.
A white powder was found near where mole died and was found to contain 43. 64% P and 56. 36% O. Find the empirical formula and name this molecule.
Molecular Formulas • A molecular formula is NOT the simplest formula (it can be reduced…but don’t do it!) • Formula: Mol. Form = X(Emp. Form) –Where X is an integer To get X, divide the Mol FW by the Emp FW
Ex: Emp Form CH 2 Mass =14 X 2 Mol Form C 2 H 4 mass =28 CH 2 3 C 3 H 6 mass = CH 2 4 C 4 H 8 mass =
A compound with an empirical formula of CH 5 N has a molecular weight of 93 g/mole. Find the molecular formula.
Find the empirical and molecular formulas (normal kids don’t get to do for a compound that is 71. 65% Cl, 24. 27% C, and 4. 07% H. The molecular formula weight is 98. 96 g/mol. this…. you’re lucky!) Four Steps to find the Molecular Formula: 1. Find the empirical formula: 2. Find the empirical formula weight:
3. Find X (X = Molec. fwt/Emp. Fwt) 4. Multiply X by the empirical formula to get the molecular formula.
Chemical Equations • CH 4 + O 2 → CO 2 + H 2 O • In a chemical reaction, atoms have been rearranged but have NOT been created or destroyed. This is why we have to balance every chemical reaction/equation. Java-Balancing! • Balance CH 4 + O 2 → CO 2 + H 2 O • Balance HCl + Na. HCO 3 → CO 2 + H 2 O + Na. Cl • When balancing equations, the formulas must NEVER be changed. We may just add coefficients.
Tips on Balancing! 1. Leave lone elements until the end. 2. If you get stuck, start over with a different element. 2. If a group (polyatomic) does not change from one side to the other, balance it as a group. 3. Temporarily use fractions to balance and then multiply to get rid of them.
Balance the following reactions: • (NH 4 )2 Cr 2 O 7 → Cr 2 O 3 + N 2 + H 2 O • C 3 H 8 + O 2 → CO 2 + H 2 O • NH 3 + O 2 → NO + H 2 O
Na 2 SO 4 + Ba(NO 3)2 → Na. NO 3 + Ba. SO 4 C 2 H 5 OH + O 2 → CO 2 + H 2 O C 4 H 10 + O 2 → CO 2 + H 2 O
Diatomic Elements N 2 , Cl 2 , O 2 , H 2 , I 2 , Br 2 , F 2 Memory aid: I Bring Clay For Our New House BOFINCH or H. Br. OFINCl Diatomic elements always combine with a like atom when they are a free element. • An arrow pointing up (↑) indicates a gas. The abbreviation ppt. or a down arrow (↓) indicates a precipitate. • •
Chemical equations indicate a… • Chemical change. Remember, they must obey the Law of Conservation of Mass – the number of atoms of the reactants MUST equal the number of atoms of products. • The coefficients from the balanced equation tell us the mole ratios of reacting substances (this will be important later!!!) • 2 K + S → K 2 S • 2 Fe. Cl 3 + 3 Ca(OH)2 → 2 Fe(OH)3 + 3 Ca. Cl 2
Five Types of Chemical Reactions • 1. Composition (or Combination) Reactions A + B → AB • Sodium and sulfur yields _______ 1. complete the word equation 2. write the correct formulas 3. Balance that bad boy!
Aluminum and iodine yields ? ?
2. Decomposition Reactions (6 types) • Generic equation AB → A + B • These are the opposite of Composition Reactions. • Here, one thing is HEATED and it breaks down into two things. • The best way to learn these six types of equations is to MEMORIZE them!!!
1. Binary Compound into Elements • Arsenic III oxide yields when heated • Water yields with electricity
2. Metal Carbonates form metal oxides and carbon dioxide • Magnesium carbonate yield when heated • Sodium carbonate yields when heated
3. Metal hydroxides yield metal oxides and water • Potassium hydroxide yields when heated • Calcium hydroxide yields when heated
4. Metal chlorates yield metal chlorides and oxygen • Potassium chlorate yields when heated
5. Metallic bromates yields metal bromides and oxygen • Calcium bromate yields when heated • Iron II bromate yields when heated
6. Acids when heated form nonmetal oxides and water • Carbonic acid when heated yields
3. Single Replacement Reactions (two types) • A. Positive Replacement (3 types) • AX + B → BX + A • The single element will replace the positive element in the compound
1. Replacement of a Metal by a more active metal • Magnesium and calcium nitrate yields
2. Replacement of hydrogen in water by a metal (to form a metallic hydroxide and hydrogen gas) • Aluminum and water yields
3. Replacement of H in an acid by a metal • Aluminum and hydrochloric acid yields
B. Negative Replacement of Halogens (1 type) • A + MB → MA + B -1 Br -1 Cl -1 F • The halogens are , , , and I-1 and they are all diatomic when they stand alone (Br 2 , Cl 2 , F 2 , and I 2 )
Hydrochloric acid and bromine yields…
4. Ionic/Exchange/Double Replacement Reactions • AB + CD → AD + CB • The product MAY produce a solid or a precipitate • PRECIPITATE: an insoluble substance formed in solution. • a liquid + liquid → solid • but only one of the products may be a precipitate
Solubility Rules • See chart in your book, the chart in your handouts section or on the back of your Chem Helper.
Sodium hydroxide + iron III chloride →
Lead II nitrate + potassium iodide →
Sodium sulfate + lead II nitrate →
5. Combustion Reactions • Combustion reactions are the burning of a fuel (usually a hydrocarbon like CH 4) with oxygen (O 2 ) to form CO 2 and H 2 O. • Methane gas (CH 4) is burned in a furnace
Propane gas (C 3 H 8) is used to cook in most campers.
First 10 Hydrocarbons • • Methane Ethane Propane Butane Pentane Hexane Heptane CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12 C 6 H 14 C 7 H 16 • Octane C 8 H 18 • Nonane C 9 H 20 • Decane C 10 H 22
MASS/MASS Problems • Five Steps: 1. Balance the equation 2. Find the formula weights of given and wanted 3. Convert the known mass to moles 4. Use the mole ratio to get from the given to the wanted 5. Convert the moles to grams of the wanted
Given grams of substance A Formula weight of A Convert grams of A to moles of A Mole Ratio Convert moles of A to moles of substance B Formula weight of B Convert moles of B to grams of B
Lithium hydroxide is used in the space shuttle to remove exhaled carbon dioxide from the living environment to form lithium carbonate and water. What mass of CO 2 can be absorbed by one Kg of lithium hydroxide?
Sodium bicarbonate is often used as an antacid because it neutralizes HCl secreted by the stomach to form sodium chloride, water and carbon dioxide. How much baking soda is required to neutralize 6. 8 g of HCl?
Summer Job: Preparing lunch at Boy Scout camp • 400 hungry boy scouts need lunch. • You have 35 loaves of bread, 17 jars of jelly and 2 jars of peanut butter. • During the sandwich-making process (the “reaction”) which “reactant” will you run out of first (bread, jelly, or pb? ) • The “reactant” that you run out of 1 st will stop the sandwich-making process. • And, 300 boy scouts are going to tie you up in the knots they just learned to tie and hang you in an oak tree (they just learned to identify trees)
Limiting Reagent Problems X + Y → XY 3 g + 4 g → both are used up completely What would happen if we reacted 3 g of X with 5 grams of Y?
Limiting Reactant The limiting reactant is the reactant that is used up first in a reaction. The other reactant that still remains is said to be “in excess”. Think PB+J sandwiches. To solve a limiting reactant problem: -Do 2 mass-mass problems, one with each reactant. -The product for each mass-mass must be the same. -The reactant that gives the least mass of product is the limiting reactant.
Ammonia is an important fertilizer used by farmers. It is produced by a famous reaction called the Haber process. Suppose 25, 000 grams of N 2 and 5000 grams of H 2 are mixed to form NH 3. Find the limiting reactant and calculate the amount of ammonia produced. First, write the balanced equation: Mass-mass 1
Mass-mass 2:
Compare the 2 results, the lower mass will be the amount produced and the reactant that gives that amount is the limiting reactant
Ammonia and copper II oxide yields nitrogen gas, copper, and water vapor. If 18. 1 g of ammonia is reacted with 90. 4 g of copper II oxide, which is the limiting reagent? How many g of nitrogen will be formed?
Wood alcohol is methanol (CH 3 OH). If hydrogen + carbon monoxide yields methanol, write the balanced equation.
If 68. 5 Kg of CO is reacted with 8. 60 Kg of hydrogen, find the limiting reagent:
Now, calculate the mass of methanol produced.
If the actual yield is 35, 700 g, calculate the % yield of CH 3 OH.
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