Experiment Five-

Limiting Reagent

Empirical Formula of Magnesium Oxide

Procedural Tips

FAQs

Introduction/Goals

Background

Pre-lab

Additional Problems

Supplies and Chemicals

Safety Precautions

Waste Disposal Instructions

Chem 106 Lab Homepage

In this experiment you will determine the limiting reagent in a reaction and calculate the percent yield. You will also determine the simplest formula for a oxide of magnesium .

Print a copy of the Check-list for this lab. Initial each item on the Check-list as completed.

Procedural Tips:

Use of the Top Loading Balance

FAQ:

Why do I have to wear safety goggles?

Introduction/Goals:

Introduction:

The limiting reagent will be determined from masses of the the reactants used. The theoretical yield will be calculated. The actual yield and % yield will also be determined.

The simplest formula for an oxide of magnesium will be determined. The formula will be found by determining the mass and moles of oxygen that reacts with a specific amount of magnesium

Goals:

Background:

The Limiting Reagent:

Balanced chemical reactions show the stoichiometric relationships between reactants and products. In laboratory practice, the reactants are not always in a stoichiometric mixture. One reactant may be present in excess and one reactant may be completely used up in the reaction. This reactant is called the limiting reagent and determines the maximum amount of products that can be formed.

The amount of product that can be obtained from a complete reaction is the theoretical yield. The theoretical yield is calculated based on the balanced equation and the amount of reactants present. In practice, however, the theoretical yield is seldom obtained. The amount of product actually produced in the actual yield. The % yield is found by expressing the actual yield as a percentage of the theoretical yield.

Determining formula for magnesium oxide:

Magnesium metal is reacted with oxygen in the air by heating the metal strongly. The magnesium metal will also react with atmospheric nitrogen to produce magnesium nitride. The magnesium nitride is converted to magnesium oxide by heating the nitride with water. Ammonia is produced during this reaction.

The initial mass of magnesium is recorded. The mass of product (magnesium oxide) is determined. The mass of oxygen that reacted in found by difference.

From the above masses (e.g., mass of oxygen and mass of magnesium), the number of moles of oxygen and moles of magnesium that reacted may be calculated using their respective molar masses.

The empirical formula, or lowest whole number ratio of the atoms in the compound, may be found by dividing the mole amounts by the smallest number of moles and obtaining a set of whole numbers. For example,  2.4 mole of Fe was found to react with 3.6 mole of O. The empirical formula can be found as follows:

1. 2.4 mole Fe/3.6 mole O = 0.67 (or 2/3) mole of Fe : 1.0 mole O

2. Since we do not have a set of whole numbers, we must multiple all the values by a number that will convert the fraction to a whole number. In this case, we would multiply each mole by 3, thus:

3. 2/3 X 3 = 2 moles of Fe; and 1 X 3 = 3 mole of O

4. The empirical formula is Fe₂O₃.

Pre-lab:

Read the introductory material on the webpage.

Read Experiment Five in the laboratory manual.

Answer the pre-lab study questions for Experiment Five.

You are encouraged to do the Application Questions at the end of  Experiment Five before coming to lab.

Additional Problems:

1. Ibuprofen has the molecular formula C₁₃H₁₈O₂.

Calculate the molar mass of ibuprofen.

You have purchased a bottle of ibuprofen that contains 100 tablets. Each tablet is 200 mg ibuprofen. How many moles of ibuprofen are in the bottle?

2. The following reaction yielded 0.025 mole of product. How many grams of each reactant were required?

Mg + I₂ ®  MgI₂

3. Magnesium burns in oxygen to form magnesium oxide, MgO. How many grams of oxygen are required to react with 12.0 gram of Mg?

4. 0.2 moles of NaOH are reacted with 0.25 moles of H₂SO₄ in the following balanced equation: 2NaOH + H₂SO₄ ® Na₂SO₄ + 2H₂O. If the % yield is 87%, how much Na₂SO₄ in grams was produced?

Supplies and Chemicals:

Safety Precautions:

SAFETY GOGGLES MUST BE WORN AT ALL TIMES DURING THE LAB.

Be careful not to burn yourself on the hot crucible and iron ring.

Do not look directly at the burning magnesium ribbon. You can damage your eyes.

Be careful not to breath the ammonia fumes that are given off in part A.2 of the experiment.

Waste Disposal Instructions:

Place all of the solid magnesium oxide waste in the container labeled magnesium oxide. This container can be found under the hood.