# Unit 3: Chemical Equations and Stoichiometry

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Unit 3: Chemical Equations and Stoichiometry
Stoichiometry and Limiting Reagents

Stoichiometry Stoichiometry is a fancy chemistry word meaning “to calculate unknown quantities in a chemical reaction”. Or in other words: if I mix this much of “X” and this much of “Y” they will react to form this much of “Z”. Remember: the coefficients in the balanced equation are mole ratios, NOT mass ratios (this means you have to get things into moles first)

Example #1 Lithium hydroxide reacts with gaseous carbon dioxide to form solid lithium carbonate and liquid water. How many grams of lithium carbonate are produced from 1.00 g of lithium hydroxide? Step 1: Write and balance the chemical equation Step 2: Convert known grams to moles using the molar mass Step 3: Convert known moles to unknown moles using the balanced chemical equation Step 4: Convert unknown moles to grams using the molar mass

Practice Problem #1 How many grams of hydrogen are produced when g of hydrochloric acid (HCl) are reacted with copper? HCl + Cu  CuCl2 + H2

Limiting Reactants Limiting reactant: the reactant that is completely consumed in a reaction and limits the amount of product formed

Limiting Reactants Example #1
A strip of zinc metal with a mass of 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate. How many grams of silver will form? Step 1: Write and balance the chemical equation Step 2: Convert known grams to moles using the molar mass Step 3: Convert known moles to unknown moles using the balanced chemical equation NEW Step 2/3: Calculate ALL possible number of moles of unknown and determine the limiting reactant Step 4: Convert smallest number of unknown moles to grams using the molar mass

Limiting Reactants Practice #1
If 120. g of sodium carbonate react with 105 g of calcium hydroxide, how many grams of sodium hydroxide are formed? Na2CO Ca(OH)2  NaOH CaCO3

Example with Steps A strip of zinc metal with a mass of 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate. How many grams of silver will form? Step 1: Write and balance the chemical equation Zn (s) + 2 AgNO3 (aq)  2 Ag (s) + Zn(NO3)2 (aq)

Example #1-Step 2 A strip of zinc metal with a mass of 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate. How many grams of silver will form? Step 2: Convert the mass of the known chemical to moles (mass/molar mass) = moles

Example #1-Step 3 Zn (s) + 2 AgNO3 (aq)  2 Ag (s) + Zn(NO3)2 (aq)
Step 3: Determine limiting reagent (limiting reagent is the reagent that doesn’t fulfill the mole ratio of the balanced equation). Use the coefficients of the balanced equation to determine the mole ratios. Limiting Reagent

Example #1-Step 4 Use the mole ratio to find moles of unknown chemical(s). You MUST use the moles of the limiting reagent to determine the moles of the unknown.

Example #1-Step 5 Step 5: Convert moles of unknown chemical to mass
moles x molar mass = mass