Chem 11 Chapter 7 - Stoichiometry

Chem 11 Chapter 7 - Stoichiometry Stoichiometry

Introduction

The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. The most common stoichiometric problem will present you with a certain amount of a reactant and then ask how much of a product can be formed. Here is a generic chemical equation: 2A + 2B → 3C

Here is a typically worded problem: Given 20.0 grams of A and sufficient B, how many grams of C can be produced?

You will need to use molar ratios, molar masses, balancing and interpreting equations, and conversions between grams and moles.

What is stoichiometry????

Here is an example;

1. If 2 apples cost $8. How much do 4 apples cost? ______

a. How much for 1 apple? ______

2. If 3 L of H2O can fill 6 buckets. How much water is in each bucket? ______

a. How much H2O will it take to fill 2 buckets? ______

b. How many buckets can be filled with 5L of H2O? ______

c. How many litters will it take to fill 14 buckets? ______

If 6 moles of Cl2(g) can react with 3 moles of Na

d. How many moles of Cl2(g) will react with 6 moles of Na? ______

e. How many moles of Cl2(g) will react with 1 mole of Na? ______

f. How many moles of Na does it take to react with 8 moles of Cl2(g)? ______

This is stoichiometry! You are using mole ratios to determine specific amounts. Chem 11 Chapter 7 - Stoichiometry Sec 7.1 – The Meaning of the Coefficients in a Balanced Equation

Can be summed up in two words – MOLAR RATIOS

Here’s an example; 2 H2 + O2 → 2 H2O

What is the molar ratio between H2 and O2? ______

What is the molar ratio between O2 and H2O? ______

What is the molar ratio between H2 and H2O? ______

the coefficients of a reaction only give the ratio in which substances react

Stoichiometry Calculations

There are many types of calculations. For EVERY type of calculation, moles must be calculated. This is called the mole bridge. The first type of problem is called Mole to Mole.

1. Mole to Mole Calculations Set up a ratio and solve for the unknown

Example 1. N2 + 3 H2 ---> 2 NH3 a. How many moles of N2 are required to react with 15 moles of H2?

b. How many molecule of NH3 are produced when 18 mol of H2 are reacted?

Example 2. Al2(CO3)3 + 3CaCl2  3CaCO3 + 2AlCl3 a. How many moles of AlCl3 would be produced if 6 moles of CaCl2 reacted with 2 moles

of Al2(CO3)3

Given : 6 moles of CaCl2 Find : moles of AlCl3

Chem 11 Chapter 7 - Stoichiometry b. How many moles of AlCl3 would be produced if 13.4 moles of CaCl2 reacted

c. How many moles of CaCO3 would be produced if 6.7 moles of CaCl2 reacted

Stoichiometry Calculations - Mass to Mole or Mole to Mass Calculations

Review: Mass to Moles

1) How many moles are in 26.0g of Carbon?

2) How many moles of Na(CO3)2 are in 256g?

3) 0.450 gram of Fe contains how many moles?

Mole - Mass Calculations You must always go to moles first – then convert using the ratio.

1. How many moles of O2 gas will be produced by the decomposition of 27.0g of HgO. 2HgO → O2 + 2Hg Chem 11 Chapter 7 - Stoichiometry Also knowing the how much product you want to get – you can determine the amount of reactant.

2. How many grams of KClO3 must be decomposed to produce KCl and 1.45 mole of O2 ? 2KClO3 → 3O2 + 2KCl

Here's the equation to use for all three problems: 2 H2 + O2 → 2 H2O

1. How many grams of H2O are produced when 2.50 moles of oxygen are used?

2. If 3.00 moles of H2O are produced, how many grams of oxygen must be consumed?

3. How many grams of hydrogen gas must be used, given the data in problem two?

4. Powder Zinc metal reacts violently with sulphur (S8) when heated to produce zinc sulphide. write a balanced equation

What is the amount of moles of S8 required to react with 25 g of Zn? Chem 11 Chapter 7 - Stoichiometry

5. Aluminium oxide is decomposed using electricity to produce aluminium metal and oxygen. write a balanced equation

How many moles of Al can be produced from 125g of aluminium oxide?

Stoichiometry Calculations - Mass to Mass Calculations

The balanced equation gives the mole ratio of the reactants and products. But in a lab it impossible to weight “moles of a reactant” so grams are used. Therefore you must calculate the amount of grams of product from a certain mass of reactant

How many grams of KCl, are produced by decomposing 118g KClO3?

Problem; there is no direct relationship form g → g, therefore you must go to moles because you can use the ratio to find a relationship between moles, mol → mol

The only difference from yesterday is that you must add 1 step at the beginning and the end to convert whatever you are calculating from g → mol or mol → g 2KClO3 → 2KCl + 3O2

118g KClO3 ? g KCl

118g KClO3 → mol KClO3 mol KCl → ? g KCl

118g KClO3 → mol KClO3 → RATIO → mol KCl → ? g KCl Chem 11 Chapter 7 - Stoichiometry

How many grams of O2 will be required to react with 14.6 g Na to form Na2O? 4Na + O2 → 2Na2O

14.6g Na → ? g O2 14.6g Na → mol Na →→→→mol O2 → ? g O2

14.6g Na → mol Na → RATIO →mol O2 → ? g O2

A propane stove produces heat by combustion of propane (C3H8) in oxygen. Write a balanced equation for this reaction

Determine the mass of O2 required to completely burn 10.0g of propane.

Sodium reacts with Oxygen to produce sodium oxide. Write a balanced equation for this reaction

Calculate the number of grams of sodium Oxide that will be produced when 115g of sodium react with oxygen.