Lab #1 VB#1: Determination of the Empirical Formula of Silver Oxide
Introduction:
There is an official database that keeps track of the known chemical compounds that exist in nature or have been synthesized in the lab. This database, called the chemical abstracts database, is updated daily. Currently there are over 20 million different inorganic and organic compounds that have been recognized. How is it possible to identify so many different compounds?
Background:
The composition of a chemical compound can be described in three different ways.
1. The percent composition gives the percent by mass of each element in the compound and is the simplest way experimentally to describe the composition of a substance. According to the law of definite proportions, the elements in a given compound are always present in the same proportion by mass, regardless of the source of the compound or how it is prepared.
2. The empirical formula describes the composition of a compound in terms of the simplest whole-number ratio of atoms in a molecule or formula unit of the compound. Empirical means experimentally determined. When an experiment is carried out to determine the composition of a compound, only the empirical formula can be obtained from this information. The empirical formula gives the ratio of atoms in a compound and does not necessarily represent the actual number of atoms in a molecule. It is possible for different compounds to share the same empirical formula.
3. The molecular formula of a compound tells us the actual number of atoms in a single molecule of a compound. While more than one molecule can have the same empirical formula, no two molecules have the same molecular formula and each molecule has very different properties; however, the molecular formula is a simple multiple of the empirical formula. In order to find the molecular formula of a compound whose empirical formula is known, the molar mass of the compound must also be determined.
Experiment Overview:
In this experiment you will determine the percent composition and empirical formula of silver oxide. Silver oxide decomposes into silver metal and oxygen when strongly heated. Heating silver oxide cause the oxygen to be driven off, leaving only silver metal behind. According to the law of conservation of mass, the total mass of the products of a chemical reaction must equal the mass of the reactants:
Mass of silver oxide = mass of silver metal + mass of oxygen
If both the initial mass of silver oxide and the final mass of the silver metal are measured, the decrease in mass must correspond to the mass of oxygen that combined with silver.
Pre-lab Questions (answer on a separate sheet of paper - turn in post-lab calcs)
A piece of iron weighing 85.65 g was burned in air. The mass of the iron oxide produced was 118.37g.
1. Use the molar mass of iron to convert the mass of iron used to moles.
2. According to the law of conservation of mass, what is the mass of oxygen that reacted with the iron?
3. Calculate the number of moles of oxygen in the product.
4. Use the ratio between the number of moles of iron and number of moles of oxygen to calculate the empirical formula of iron oxide. Note: fractions of atoms do not exist. In the case where the ratio of atoms is a fraction, the ratio should be simplified by multiplying all the atoms by a constant to give a whole number ratio for all the atoms (e.g., HO1/2 should be H2O)
Lab #1 VB#1: Determination of the Empirical Formula of Silver Oxide
Data Table:
Trial #1
Trial #2
Mass of crucible and lid (g)
Mass of crucible, lid, and silver oxide (g)
Mass of crucible, lid, and silver metal (g)
Appearance of product
Post-Lab Calculations:
Complete the results table. Use the following questions to help you.
Show all work for each calculation on a separate sheet of paper (or on the back).
Mass of silver oxide (g)
Mass of silver metal produced (g)
Mass of oxygen gas produced (g)
% composition of silver
% composition of oxygen
Moles of oxygen in the silver oxide sample
Moles of silver in the silver oxide sample
Mole ratio of Ag/O in silver oxide
Empirical formula of AgxOy
1. Calculate the mass of silver oxide and the mass of silver metal produced. Use the law of conservation of mass to calculate the mass of oxygen produced with the silver.
2. What is the percent composition of silver and oxygen in silver oxide?
3. Use the molar masses of silver and oxygen to calculate the number of moles of each product.
4. Calculate the ratio between the number of moles of silver and the number of moles of oxygen in the product. What is the empirical formula for silver oxide?
5. Write a balanced chemical equation for the decomposition of silver oxide to form silver metal and oxygen gas.
6. The theoretical yield of a product in a chemical reaction is the maximum mass of product that can be obtained assuming 100% conversion of the reactant(s). Calculate the theoretical yield of silver metal in this experiment. Hint: calculate the molar mass of silver oxide.
7. The percent yield reflects the actual amount of product formed versus the maximum that might have been obtained. Use the % yield formula to calculate the % yield of silver metal produced in this experiment.
8. Discuss sources of error in this experiment that might account for a percent yield that is lower or higher than 100%. Be specific!