Stoichiometric Calculations Stoichiometry Stoichiometric Calculations

A. Proportional Relationships 2 1/4 c. flour 1 tsp. baking soda 1 tsp. salt 1 c. butter 3/4 c. sugar 3/4 c. brown sugar 1 tsp vanilla extract 2 eggs 2 c. chocolate chips Makes 5 dozen cookies. I have 5 eggs. How many cookies can I make? Ratio of eggs to cookies 5 eggs 5 doz. 2 eggs = 12.5 dozen cookies

A. Proportional Relationships Stoichiometry mass relationships between substances in a chemical reaction based on the mole ratio Mole Ratio indicated by coefficients in a balanced equation 2 Mg + O2  2 MgO

What can we do with Stoichiometry? For the generic equation: RA +RB → P1 + P2 Given the… …one can find the… Amount of RA (or RB) Amount of RA (or RB) that is needed to react with it Amount of RA or RB Amount of P1 or P2 that will be produced Amount of P1 or P2 you need to produce Amount of RA &/or RB you must use

Given the equation… 2TiO2 + 4Cl2 + 3C → 2TiCl4 + CO2 + 2CO How many mol chlorine will react with 4.55 mol carbon? X mol Cl2 = 4.55 mol C (4mol Cl2/ 3mol C) 6.07 mol Cl2 will react with 4.55 mol carbon

Given the same equation… 2TiO2 + 4Cl2 + 3C → 2TiCl4 + CO2 + 2CO What mass titanium (IV) oxide will react with 4.55 mol carbon? X g TiO2 = 4.55 mol C(2 mol TiO2/3 mol C)(79.9 g TiO2/1 molTiO2) 242 g TiO2 will react with 4.55 mol carbon

And with the same equation… 2TiO2 + 4Cl2 + 3C → 2TiCl4 + CO2 + 2CO How many molecules titanium (IV) chloride can be made from 115 g titanium (IV) oxide? X m’c TiCl4 = 115g TiO2 (1mol TiO2/79.9g TiO2)(2 mol TiCl4/2 mol TiO2)(6.02 x 1023 m’c TiCl4/1 mol TiCl4) 8.7 x 1023 m’cules TiCl4 can be made from 115 g TiO2

B. Stoichiometry Steps 1. Write a balanced equation. 2. Identify known & unknown. 3. Line up conversion factors. Mole ratio - moles  moles Molar mass - moles  grams Molarity - moles  liters soln Molar volume - moles  liters gas Mole ratio - moles  moles Core step in all stoichiometry problems!! 4. Check answer.

Standard Temperature & Pressure C. Molar Volume at STP 1 mol of a gas=22.4 L at STP Standard Temperature & Pressure 0°C and 1 atm

C. Molar Volume at STP LITERS OF GAS AT STP Molar Volume MASS IN GRAMS (22.4 L/mol) MASS IN GRAMS MOLES NUMBER OF PARTICLES Molar Mass (g/mol) 6.02  1023 particles/mol Molarity (mol/L) LITERS OF SOLUTION

Stoichiometry “Pop” Quiz Iron (III) reacts with water… Write the complete balanced equation How many liters of hydrogen (@ STP) will be produced from 8 moles of iron? How many grams of water are needed to produce 68 grams of Iron (III) oxide? 5.2 x 1025 particles of iron will produce how many moles of hydrogen? For the complete combustion of butane (C4H10)… In a room at 0°C and 1 atm pressure, how many m’cules oxygen react with 632 L butane? If 316 L liters of carbon dioxide gas are produced, how much butane (in moles) was combusted? Silver nitrate reacts with sodium chloride… Write the complete balanced equation (including states of matter) How much precipitate (in g) is formed when 1.5 L of 0.10M silver nitrate is used? How many molecules of sodium chloride do you need to react with that amount of silver nitrate?

The Limiting Reactant A balanced equation for making a Big Mac® might be: 3 B + 2 M + EE  B3M2EE With… …and… …one can make… 30 M excess B and excess EE 15 B3M2EE 30 B excess M and excess EE 10 B3M2EE 30 B and excess EE

A balanced equation for making a tricycle might be: 3 W + 2 P + S + H + F  W3P2SHF With… …and… …one can make… 50 P excess of all other reactants 25 W3P2SHF 50 S 50 W3P2SHF 50 S and excess of all other reactants

Solid aluminum reacts w/chlorine gas to yield solid aluminum chloride. 2 Al(s) + 3 Cl2(g)  2 AlCl3(s) If 125 g aluminum react w/excess chlorine, how many g aluminum chloride are made? = 618 g AlCl3

If 125 g chlorine react w/excess aluminum, how many g aluminum chloride are made? = 157 g AlCl3 If 125 g aluminum react w/125 g chlorine, how many g aluminum chloride are made? 157 g AlCl3 We’re out of Cl2.

limiting reactant (LR): the reactant that runs out first Amount of product is based on LR. Any reactant you don’t run out of is an excess reactant (ER).

Example Limiting Reactant Excess Reactant(s) Big Macs buns meat tricycles pedals W, S, H, F Al / Cl2 / AlCl3 Cl2 Al

How to Find the Limiting Reactant For the generic reaction RA + RB  P Assume that the amounts of RA and RB are given. Should you use RA or RB in your calculations?

How to find LR steps… 1. Calc. # of mol of RA and RB you have. 2. Use the mole ratio from the balanced equation to calculate the required amount of the other reactant. 3. If resulting amount is less than what is given, that reactant is the LR.

% Yield 6 Na(l)+ 1 Al2O3(s)  2 Al(l) + 3 Na2O(s) molten + solid  molten + solid sodium aluminum aluminum sodium oxide oxide Al3+ O2– Na1+ O2– ___Na(l) + ___Al2O3(s)  ___Al(l) +___Na2O(s) 6 Na(l)+ 1 Al2O3(s)  2 Al(l) + 3 Na2O(s) Find mass of aluminum produced if you start w/575 g sodium and 357 g aluminum oxide. = 189 g Al

% Yield This amount of product is the theoretical yield. amt. we get if reaction is perfect found by calculation Now suppose that we perform this reaction in the lab and get only 172 grams of aluminum. Why? couldn’t collect all Al not all Na and Al2O3 reacted some reactant or product spilled and was lost

% yield can never be > 100% % Yield Calculation: % yield can never be > 100%

Find % Yield for the Previous Problem