Lab 9 PreLab

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. It is a very mathematical part of chemistry, so be prepared for lots of calculator use.

As fascinating as it was to find out we could measure out 27.0 grams of aluminum and know it was exactly 6.02 x 1023 atoms of aluminum, chemical stoichiometry is just as amazing.

Before we let elements and compounds meet, we can predict how they are going to react, and even further, how much product (if any) gets produced.  We could also be on the other end of things and find a waste product and determine exactly how much of a particular compound or element was needed to produce the waste.  How WONDERFUL!!  This is STOICHIOMETRY!

REACTION STOICHIOMETRY involves the mass and mole relationships among reactants and products in a chemical reaction.  A balanced chemical equation is used to determine these relationships.

This laboratory exercise will focus on mass measurements for determining the following:

ACTUAL YIELD measurements – A measured value of the amount of product that is actually produced.

THEORETICAL YIELD calculations – A calculated value of the amount of product that should be produced.

PERCENT YIELD calculations – A calculated value that determines the efficiency of the reaction and is determined by dividing the actual yield by the theoretical yield and multiplying by 100.

SAMPLE DATA

A student has measured out potassium chlorate, KClO3 which undergoes a decomposition reaction.  At the end of the combustion reaction he determines that 148.2 grams of solid product remains.

Decomposition of potassium chlorate, KClO3:

mass of empty crucible 35.8666 g
mass of crucible and KClO3 38.5902 g
mass (grams) of KClO3 38.5902 g – 35.8666 g = 2.7236 g

Theoretical Yields of Possible Products:

Possible reaction 1: (producing potassium chloride as a possible product)                          2 KClO3(s)  →   2 KCl(s)  + 3 O2(g)   (already balanced)
Theoretical Mass of KCl

(predict mass of KCl produced by performing stoichiometry: convert g KClO3  to g KCl)

1.6569 g(calculation shown below)
Three Steps of stoichiometry calculation:

a) Convert 2.7236 grams of KClO3 to moles KClO3 (using the molar mass of KClO3):

l09_e01

b) Convert 0.022225 mol KClO3 to mol KCl (using the balanced chemical equation, also known as the bridge step):

l09_e02

c) Convert 0.022225 mol KCl to grams KCl (using the molar mass of KCl):

L09_E03.jpg

This is the theoretical yield of KCl in grams.

 

Possible reaction 2: (producing potassium oxide as a product)  2 KClO3(s)  →    K2O(s)  +  Cl2O(g) + 2 O2(g)   (already balanced)
Theoretical Mass of K2O

(predict mass of K2O produced by performing stoichiometry: convert g KClO3  to g K2O)

1.0468 g(calculation shown below)
Three Steps of stoichiometry calculation:

a) Convert 2.7236 grams of KClO3 to moles KClO3 (using the molar mass of KClO3):

L09_E04.jpg

b) Convert 0.022225 mol KClO3 to mol K2O (using the balanced chemical equation, also known as the bridge step):

l09_e05

c) Convert 0.011113 mol K2O to grams K2O (using the molar mass of K2O):

L09_E06.jpg

This is the theoretical yield of K2O in grams.

Test: Mass of product to determine ACTUAL YIELD

Mass of empty crucible (from first step, recorded above) 35.8666 g
Mass of crucible and product (water removed) 37.4589 g
Mass of product (Actual Yield) 37.4589 g – 35.8666 g = 1.5923 g
Possible product

(based upon comparison of ACTUAL YIELD with calculated THEORETICAL YIELDS)

KCl (because 1.5923 g is closest to 1.6569 g)
Percent Yield (Actual/Theoretical) x 100  l09_e07

OBJECTIVES

The laboratory objectives for the exercise relating to measurements are:

  1. Precisely measure the quantities of compounds.
  2. Prepared solution mix them, and record observations.
  3. Collect the product by filtration and determine its mass.
  4. Calculate the percent yield of the product.

ORIENTATION

This exercise will require the use of the following glassware and hardware:

  1. Analytical balances
  2. Crucible with lid
  3. Crucible tongs
  4. Stirring rod
  5. Bunsen burner
  6. Sparker
  7. 250 mL beaker
  8. Test tubes
  9. Disposable pipets

This exercise will use the following chemical:

  1. Sodium bicarbonate
  2. Red cabbage
  3. Water
  4. Sodium hydroxide
  5. Sodium carbonate
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