STOICHIOMETRY

What is stoichiometry?

• Stoichiometry is the quantitative study of reactants and products in a chemical reaction.

What You Should Expect

• Given : Amount of reactants • Question: how much of products can be

formed.• Example

• 2 A + 2B 3C• Given 20.0 grams of A and sufficient B, how

many grams of C can be produced?

What do you need?

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

Note: This type of problem is often called "mass-mass."

Steps Involved in Solving Mass-Mass Stoichiometry Problems• Balance the chemical equation correctly• Using the molar mass of the given substance,

convert the mass given to moles. • Construct a molar proportion (two molar ratios

set equal to each other) • Using the molar mass of the unknown substance,

convert the moles just calculated to mass.

Mole Ratios A mole ratio converts moles of one compound in a balanced chemical equation into moles of another compound.

ExampleReaction between magnesium and oxygen to form magnesium oxide. ( fireworks)

2 Mg(s) + O2(g) 2 MgO(s) Mole Ratios:

2 : 1 : 2

Practice Problems 1) N2 + 3 H2 ---> 2 NH3

Write the mole ratios for N2 to H2 and NH3 to H2.

2) A can of butane lighter fluid contains 1.20 moles of butane (C4H10). Calculate the number of moles of carbon dioxide given off when this butane is burned.

Mole-Mole Problems Using the practice question 2) above: Equation of reaction 2C4H10 + 13O2 8CO2 + 10H2O Mole ratio

C4H10 CO2

1 : 4 [ bases] 1.2 : X [ problem]

By cross-multiplication, X = 4.8 mols of CO2 given off

Mole-Mass Problems

• Problem 1: 1.50 mol of KClO3 decomposes. How many grams of O2 will be produced? [k = 39, Cl = 35.5, O = 16]2 KClO3 2 KCl + 3 O2

Three steps…Get Your Correct Answer

• Use mole ratio• Get the answer in moles and then• Convert to Mass. [Simple Arithmetic]

Hello!If you are given a mass in the

problem, you will need to convert this to moles first. Ok?

Let’s go!

2 KClO3 2 KCl + 3 O2

2 : 31.50 : XX = 2.25molConvert to mass2.25 mol x 32.0 g/mol = 72.0 grams

Cool!

Try This:• We want to produce 2.75 mol of KCl. How many

grams of KClO3 would be required?

Soln

KClO3 : KCl

2 : 2X : 2.75X = 2.75mol

In mass: 2.75mol X 122.55 g/mol = 337 grams zooo zimple!

Mass-Mass ProblemsThere are four steps involved in solving these problems:

• Make sure you are working with a properly balanced equation.

• Convert grams of the substance given in the problem to moles.

• Construct two ratios - one from the problem and one from the equation and set them equal. Solve for "x," which is usually found in the ratio from the problem.

• Convert moles of the substance just solved for into grams.

Mass-Volume Problems

Just follow mass-mass problem to the penultimate level

Like this:There are four steps involved in

solving these problems:• Make sure you are working with a

properly balanced equation. • Convert grams of the substance

given in the problem to moles. • Construct two ratios - one from the

problem and one from the equation and set them equal. Solve for "x," which is usually found in the ratio from the problem.

• Convert moles of the substance just solved for into Volume.

Conversion of mole to volume

No of moles = VolumeMolar volume

Can you remember a similar equation?

Molar volume

The molar volume is the volume occupied by one mole of ideal gas at STP. Its value is: 22.4dm3

Practice ProblemsCalculate the volume of carbon dioxide formed

at STP in ‘dm3' by the complete thermal decomposition of 3.125 g of pure calcium carbonate (Relative atomic mass of Ca=40, C=12, O=16)

Solution: Convert the mass to mole:Molar mass of CaCO3

= 40 + 12 + (16 x 3) = 100gmol-1

Mole = mass/molar mass3.125/100 = 0.03125mol

Practice ProblemsAs per the equation,

Mole ratio 1 : 1problem 0.03125mol X

X = 0.03125mol of CO2

Convert mole to volume [slide 17]

Volume = (0.03125 x 22.4)dm3

= 0.7dm3