Chapter 9 Chemical Quantities

In order to avoid waste and to be economic, a chemist has to know how much to make, a doctor must know how much to prescribe, an engineer has to know how much fuel to mix with others, etc., etc., etc... Questions we answer here in Ch 9? How much can I get? How much do I need? The engineers who develop airbags have to make sure exactly the right amount of gas is made - no more, no less.

Ch 9 is a compilation of Chs 1-8! must name, balance, g to mol, mol to g… Notice that for the whole chapter, I’ll give you info on one thing, you’ll give me info on another but the only place they can meet is the balanced equation, the only language they speak is “molese”

9.1 Information Given by Chemical Equations The cook is a chemist! Cooking is: mixing ingredients (reactants) in a recipe (equation) to get a certain amount of food (product) Importance here? that info in a recipe can be used proportionally double amts? double product!

The chemist is a cook! A 2:1 mole ratio from a balanced equation is just that - as long as the two substances are in 2:1 number ratio (not mass ratio!) it doesn’t matter how much you use

9.2 Mole-Mole Relationships Remember! The most common number ratio in chemistry is the mole ratio!

Example How many moles of KClO3 must decompose to produce 9 moles of oxygen gas? (The other product is KCl.) 1) As always! First, get a balanced equation! 2KClO3  2KCl + 3O2 2) What is the mol ratio between KClO3 & O2?

from balanced equation! Example (cont’d) Remember this? 9 mol O2 2 mol KClO3 = 6 mol KClO3 3 mol O2 from balanced equation! Given

Example When 5 mols sodium react with oxygen gas, how many mols of Na2O are formed? 4Na + O2  2Na2O 5 mol Na 2 mol Na2O = 2.50 mol Na2O 4 mol Na

Example For this unbalanced equation, how many mols of NO do you need to make 7.5 mol H2O? NO + H2  N2 + H2O 2NO + 2H2  N2 + 2H2O it’s a 1:1 ratio! you need 7.5 mol!

Example For this unbalanced equation, how many mols of SiH4 do you need to use with 4.2 mol NH3? SiH4 + NH3  Si3N4 + H2 3SiH4 + 4NH3  Si3N4 + 12H2 it’s a 3:4 ratio! you need 3.15 mol!

9.3 Mass Calculations The study of the amount of substances consumed/produced = stoichiometry Remember that chemical equations tell you number ratios, not mass amounts! Your 3 friends now: 1) balanced chemical equations 2) mole ratios 3) math relationships

We’ll start off simple with a problem in which I give you moles of one but you are asked about mass of another… But remember: the only place they can meet is the balanced equation, the only language they speak is molese

CaC2 + 2H2O  C2H2 + Ca(OH)2 Balanced equation!!! CaC2 and water get together to make acetylene, C2H2, and calcium hydroxide. How many grams of water do you need to make 1.55 mol C2H2? Balanced equation!!! CaC2 + 2H2O  C2H2 + Ca(OH)2 g mol H2O 1.55 mol C2H2

Example cont’d mol bridge They can only talk mole language from the balanced equation! 1.55 mol C2H2 2 mol H2O 18.0 g H2O 1 mol C2H2 1 mol H2O = 55.8 g H2O mol bridge

Example How many grams of potassium chlorate (KClO3) must decompose to produce KCl and 1.45 mol oxygen gas? First! balance the equation!!! 2KClO3  2KCl + 3O2

2KClO3  2KCl + 3O2 g mol KClO3 1.45 mol

Example cont’d mol bridge They can only talk mol language from the balanced equation! 1.45 mol O2 2 mol KClO3 122.6 g KClO3 3 mol O2 1 mol KClO3 = 119 g KClO3 mol bridge

Example How many moles of copper must react with AgNO3 to produce 5.5 g silver and copper(II) nitrate? First! balance the equation!!! Cu + 2AgNO3  2Ag + Cu(NO3)2

example cont’d Cu + 2AgNO3  2Ag + Cu(NO3)2 5.5g mol Cu mol Ag

Example cont’d mol bridge They can only talk mol language from the balanced equation! 5.5 g Ag 1 mol Ag 1 mol Cu 107.9 g Ag 2 mol Ag = 0.025 mol Cu mol bridge