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Determination of the Empirical Formula for Magnesium Chloride - Prof. Zelda Ziegler, Papers of Chemistry

An experiment to determine the empirical formula for the compound formed when magnesium reacts with hydrochloric acid. Students will purify magnesium, react it with an excess of hydrochloric acid, remove water, and calculate the empirical formula. Precautions must be taken to avoid errors and ensure safety.

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Pre 2010

Uploaded on 08/19/2009

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Determination of a Formula Page 1 of 8
CH 221 – Experiment DETERMINATION OF A FORMULA
Purpose: The purpose of this experiment is to determine empirically the formula for the
compound formed when magnesium metal is allowed to react with chlorine in the form
of hydrochloric acid. Results from every group should be checked and compared with
those from at least one other group to develop your skills at communicating the results
of experiments with each other. This experiment will also allow us to use separations,
measurement, and precision evaluation skills developed in previous labs. Today, also,
we will be starting to validate the results of others.
Introduction: In determining an empirical formula, a chemist might start with a sample of
the pure compound, decompose it carefully into its component elements, and obtain the
mass of each element present. In some happy circumstances, the compound can be
made reliably, quantitatively and uniquely from its component parts, and some (or all) of
the component elements can be weighed accurately before the reaction occurs. The
experiment today is one such case. We will take a sample of magnesium, carefully
purify it, weigh it, and then react it. The compound formed will contain all of the
magnesium we started with since magnesium will be the limiting reagent. Any increase
in mass should be due to the other element, in this case that element is chlorine. The
reaction will take place in solution, so there will be much water that must be removed
before the compound can be considered pure. All the skills learned in previous labs
(removing water from a salt solution, and preparing glassware for the analysis) must be
used to guarantee success. Be sure that you take precautions to avoid errors, not just
use excuses that you did not do what you knew you should have avoided doing to
explain why your results might be unacceptable.
Magnesium is a metal that comes in many forms, but today it will be in the form of a flat
narrow ribbon of metal. Magnesium easily forms an oxide coating. The sample that
you get today will have this coating on it, which will make it look dull, as well as weigh
more. The magnesium oxide coating must be removed by polishing it with steel wool
until it looks shiny. Excess exuberance in polishing will only remove pure magnesium,
reduce the mass, and contribute to a smaller sample of product.
Once the magnesium is purified, it is weighed, and placed into a clean dry beaker, that
has also been weighed. The magnesium is then allowed to react with an excess of
hydrochloric acid in solution. Once this is done, the excess hydrochloric acid is
converted to hydrogen chloride gas and evaporates. This is gas is not pleasant or
necessary for a healthy individual. Although we will be generating small amounts of it,
we will further minimize the potential exposure to this by performing this reaction and
separation step in the hood. The water is removed by evaporation, just as it was last
week, so all of the precautions for last week’s lab still hold.
Recall the advice you recorded for yourselves last week about recovering a salt from a
water solution. Most of you indicated that heating to salt solution more slowly at the end
of the evaporation was a good idea to minimize product loss due to “popping.” It is also
a good idea to stop adding heat when solids start to form, let it cool down a bit and then
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CH 221 – Experiment DETERMINATION OF A FORMULA

Purpose: The purpose of this experiment is to determine empirically the formula for the compound formed when magnesium metal is allowed to react with chlorine in the form of hydrochloric acid. Results from every group should be checked and compared with those from at least one other group to develop your skills at communicating the results of experiments with each other. This experiment will also allow us to use separations, measurement, and precision evaluation skills developed in previous labs. Today, also, we will be starting to validate the results of others.

Introduction: In determining an empirical formula, a chemist might start with a sample of the pure compound, decompose it carefully into its component elements, and obtain the mass of each element present. In some happy circumstances, the compound can be made reliably, quantitatively and uniquely from its component parts, and some (or all) of the component elements can be weighed accurately before the reaction occurs. The experiment today is one such case. We will take a sample of magnesium, carefully purify it, weigh it, and then react it. The compound formed will contain all of the magnesium we started with since magnesium will be the limiting reagent. Any increase in mass should be due to the other element, in this case that element is chlorine. The reaction will take place in solution, so there will be much water that must be removed before the compound can be considered pure. All the skills learned in previous labs (removing water from a salt solution, and preparing glassware for the analysis) must be used to guarantee success. Be sure that you take precautions to avoid errors, not just use excuses that you did not do what you knew you should have avoided doing to explain why your results might be unacceptable.

Magnesium is a metal that comes in many forms, but today it will be in the form of a flat narrow ribbon of metal. Magnesium easily forms an oxide coating. The sample that you get today will have this coating on it, which will make it look dull, as well as weigh more. The magnesium oxide coating must be removed by polishing it with steel wool until it looks shiny. Excess exuberance in polishing will only remove pure magnesium, reduce the mass, and contribute to a smaller sample of product.

Once the magnesium is purified, it is weighed, and placed into a clean dry beaker, that has also been weighed. The magnesium is then allowed to react with an excess of hydrochloric acid in solution. Once this is done, the excess hydrochloric acid is converted to hydrogen chloride gas and evaporates. This is gas is not pleasant or necessary for a healthy individual. Although we will be generating small amounts of it, we will further minimize the potential exposure to this by performing this reaction and separation step in the hood. The water is removed by evaporation, just as it was last week, so all of the precautions for last week’s lab still hold.

Recall the advice you recorded for yourselves last week about recovering a salt from a water solution. Most of you indicated that heating to salt solution more slowly at the end of the evaporation was a good idea to minimize product loss due to “popping.” It is also a good idea to stop adding heat when solids start to form, let it cool down a bit and then

heat the mixture again to drive off the rest of the water. This helps create a Salt dome that will help you contain the salt solution that might pop out.

The mass of the compound will have all of the magnesium in it that you started with, and if you were successful in purifying it, the added mass will be due to any chlorine in the form of chloride that reacted with it to form the compound. This information will allow you to calculate the empirical formula for this compound.

At the end of the lab report are some exercises that give you an opportunity to practice these calculations. These can be completed before you come to lab.

Before you hand in your report today, see to it that another student, who is not your lab partner, checks over your report and signs it indicating that it meets (or does not meet) his or her approval for readability, precision, complete sentences, and thoroughness (consult the Rubric). Laboratory records are considered legal documents, and the stuff of primary literature. Companies have much invested in the quality and legal defensibility of these documents. In commercial laboratories, these documents are regularly checked by other experimenters for legibility, thoroughness and accountability. Laboratory notebooks are the records that help establish primacy of inventions, as well as the place where other technicians return to repeat important experiments. They should be complete, legible, and verified as such by more than one experimenter.

Today in lab, we will begin the practice of evaluating the work of others by cooperating in checking a lab report belonging to a person who is not your lab partner. If you see something that is on the rubric that is not dealt with adequately by the person whose paper you are checking, call their attention to it and discuss the situation. It could be that you are wrong, so be diplomatic! Try to come to some agreement about how the discrepancies are resolved before you initial the “Yes” blank on their report. Only if there is no other way to end the disagreement should you initial the “no” blank on their report.

Procedure:

  1. Wash, rinse, dry, cool and weigh a 400 mL beaker.
  2. Cut magnesium ribbon for a total of about 45 cm length, and polish it with steel wool^1 on both sides until it's shiny. Avoid touching the ribbon with your fingers to preclude contamination with fingerprint oils. Weigh the magnesium accurately.

(^1) Steel wool will be available today on the bench at the back (the south wall) of the lab. DO NOT POLISH YOUR RIBBON ON THE BENCH AT THE BACK OF THE LAB EVEN IF YOU SEE SOMEONE ELSE DOING THAT! It is a public dispensing point used by everyone. This is a preparation activity that should be done at your own bench. The surface at your own bench is better for this anyway.

  1. Calculate the formula of the magnesium + chlorine compound formed.
  2. Once you are sure that you are finished with the compound that you prepared, waste compound can be placed in the garbage container, or rinsed down the sink with water.
  3. Have your finished report checked and verified that it is free of precision errors (significant figure misrecordings, and calculations---consult the rubric) by someone in this lab section who is not your partner. Check someone else’s report.

CH 221- Ziegler Lab Report Formula Determination

Name__________________________ Lab Partner__________________________

Inventory approved by instructor_________ Mass First heating

Mass Second heating 400 mL beaker + dry salt 400 mL beaker (empty) dry salt

Magnesium in this sample Chlorine in this sample

  1. Show the calculations you used to determine the empirical formula of this compound.

Printed Name of the person whose report the experimenter checked____________________

Results in this report were checked by^6 _________________________________ who approves of the quality of the information recorded here ____Yes ____No

(^6) The person who checks your work can be the same one whose report you checked, but this person must not be your lab partner. To see what you are agreeing to with your printed name signature, see the underlined passage in page 2 of the introduction to the procedure.

  1. Record what you observed (saw) that led you to believe that a reaction was taking place when the acid was added to the magnesium.
  2. Below are given 3 chemical equations that might be used to thoroughly describe the chemical and physical changes that occurred in this laboratory today. Assign a name that is commonly used to to the processes that are described here, and give a description of what an experimenter might see, both before and after. Be sure to describe any solids, solutions and gases, and how they appeared in the lab today.

H O 2 ( )l → H O 2 ( g)

Mg ( )s + 2 HCl ( aq ) → MgCl 2 ( aq ) + H 2 ( g)

HCl ( aq) → HCl ( g)

Some practice problems that can be done prior to coming to lab. The information about an empirical formula is expressed in different ways. You should be able to calculate and empirical formula using both of these ways.

  1. A compound of nitrogen and hydrogen is 12.5% hydrogen, the rest nitrogen. What is the simplest formula of the compound?
  2. A sample of 13.32 g Fe combines with 5.12 g oxygen when burned in air. What is the empirical formula of the resulting compound?