Honors Chemistry Lab 14: Determining the Formula of an Unknown Hydrate – experiment 1

Objectives:

In this lab you will determine both the identity of the anhydrous salt and the number of waters of hydration in an unknown hydrate. This lab serves as both comprehensive review and as a bridge to new material. Topics include: electron configuration, nomenclature, dimensional analysis, significant figures, single replacement reactions, balancing equations, and mole calculations. New material covered in this lab: determining the formula for a hydrate; naming your unknown hydrate sample; utilizing flame tests to identify a metal, and recognizing when a single replacement reaction occurs.

Equipment

crucible

crucible tongs

ceramic pad

50 mL beaker

metal spatula

Bunsen burner

test tube

test tube rack

Chemicals

unoxidized magnesium metal

10 mL of 6M HCl,

Introduction

When most people think of a solid substance, it almost never crosses their minds that it could be anything but completely dry (on the inside, that is). However, there are many solids that naturally incorporate water into their solid matrix when they form.. These compounds are called hydrates and are hygroscopic, or water absorbing. Each of these hydrates contains a certain number of water molecules, and it follows the law of constant composition, as do all compounds. The way the waters are notated, however, looks slightly differently.

An example of a hydrate is potassium aluminum sulfate dodecahydrate, or simply, KAl(SO4)2·12H2O. It is used commonly in deodorants and as a styptic pencil to stop bleeding from minor cuts. When it is heated, and dried, it follows the chemical equation:

In this way, the percentage of water in a sample can be found. Theoretically, the percentage in a hydrate is merely the molar mass of water divided by the molar mass of the compound. Experimentally the amount of water in any hydrate can be determined by comparing the mass of the hydrate before and after heating. potassium aluminum sulfate dodecahydrate incorporates 12 waters when it forms a solid. Its chemical formula is written as, where the “dot” refers to the fact that the water is not part of the chemical structure, but merely absorbed in the solid. These hydrates can be heated, and the water driven off from the solid. In this case the anhydrous, or “without water”, dry, form of the solid is obtained. Figure 1 is a picture of what it looks like.

Figure 1: potassium aluminum sulfate dodecahydrate Figure 2: Evaporation set up

The ionic compound is called the anhydrous salt. The number of water molecules is called the waters of hydration. For example potassium aluminum sulfate dodecahydrate has 12 waters of hydration and the anhydrous salt is potassium aluminum sulfate, When a hydrate is heated the water molecules vaporize, leaving behind the anhydrous salt.

PART I – Flame Test

You will discover the identity of the anhydrous salt by first performing a flame test. The flame test is used to visually determine the identity of an unknown metal or metalloid ion based on the characteristic color the salt turns the flame of a Bunsen burner. The heat of the flame excites the electrons of the metals ions, causing them to emit visible light. Every element has a signature emission spectrum (we will learn more about this in our light and energy unit) that can be used to differentiate between one element and another. For example, copper produces a blue flame, lithium and strontium a red flame, calcium an orange flame, sodium a yellow flame, and barium a green flame. Please view lab on our website to see colors in the chart below. Notice, sometimes the colors are very similar, thus additional laboratory tests are necessary.

Flame Test procedure: You will heat a small sample of your unknown hydrate in a flame. The color of the resulting flame will help you narrow down which salt you have. The possible salts are SrCl2, MgSO4, K2CO3, or ZnSO4. Strontium, when heated in a flame, produces a bright, vivid red color. Potassium gives a violet flame. If you get a red flame when you perform the flame test you know you have strontium chloride. If you get a violet flame you know you have potassium carbonate.

If you do not get either a red flame or a violet flame, then you will proceed to test your unknown hydrate with a single replacement reaction to determine if you have magnesium sulfate or zinc sulfate. To distinguish between the two (you only do this if you don't get a red or violet flame in the flame test).

PART II – Single Replacement Reaction (only if necessary)

During single replacement, one element replaces another element in a compound. We referred to this in our demo day presentation as “trading up”. The reaction only takes place if a trade up is involved.

Written using generic symbols, it is:

1) AX + Y ---> YX + A or 2) A + XY ---> XA + Y (this only occurs with diatomic halogens such as Cl2 and Br2

In example 1) Element Y has replaced A (in the compound AX) to form a new compound YX and the free element A. Remember that A and Y are both cations (postively-charged ions) in this example.

In example 2) Element A (which in this case is a diatomic molecule Cl2 and Br2 ) has replaced Y (in the compound XY) to form a new compound XA and the free element Y. Remember that A and Y are both anions (negatively-charged ions) in this example.

Some examples of 1) are:

Cu + AgNO3 ---> Ag + Cu(NO3)2Fe + Cu(NO3)2 ---> Fe(NO3)2 + CuCa + H2O ---> Ca(OH)2 + H2Zn + HCl ---> ZnCl2 + H2

Some examples of 2) are:

Cl2 + NaBr ---> NaCl + Br2Br2 + KI ---> KBr + I2

In single replacement, one reactant is always an element. It does not matter if the element is written first or second on the reactant side. The other reactant will be a compound.

Typically, you will be given the left-hand (reactant side) and asked to provide the products to the reaction. You need to be able to recognize single replacement reactions AND be able to break a formula apart into proper cations and anions as well as write correct formulas.

In our experiment, if you have zinc sulfate the following reaction will occur.

Mg(s)+ZnSO4(aq) -> Zn(s)+MgSO4 (aq)

Notice , sulfate traded up to become magnesium sulfate, when Magnesium replaced the zinc. You will know this reaction happens if you see a dark coating start to form on the piece of magnesium. If no reaction happens then you know you have magnesium sulfate as your anhydrous salt.

Part III Dehydrating the Hydrate (everyone will do this)

To find out how many waters of hydration are in your hydrate, you will heat it until all of the water molecules have evaporated. The difference in mass between the hydrate (before you heated it) and the anhydrous salt (after you heated it is the mass of water that was in your hydrate. Knowing the mass of water in your hydrate will let you calculate the moles of water in your hydrate.

Knowing the mass of the anhydrous salt (after heating) will let you calculate the moles of your anhydrous salt.

The ratio of moles of water to moles of anhydrous salt gives you the waters of hydration.

Procedure

1.) Obtain your unknown hydrate sample. Mrs. B and Mr. U may elect to have you and your partner run this experiment individually. If this is the case, you and your partner will have different hydrates. Each lab partner team will have samples A,B or C, D so that each lab table team (and if run individually, individual) has a different unknown hydrate. However, if this experiment is run individually, you and your partner will work concurrently, proceeding at the same pace, helping each other, and recording data and observations for each sample. Thus, you will need 2 data tables in your lab book.)

2.) Weigh your crucible. Record the mass in the data section (A1).

3.) Tare the crucible. Add about 4 grams of your unknown to the crucible and record this mass to 3 places past the decimal in the data section (A2).

4.) Set up a Bunsen burner with a ring stand, iron ring and clay triangle. (please note, we will be using the flame test burners and not our new burners (they look corroded).

5.) Gently heat the crucible with your unknown in it for 5 min by brushing the crucible with the flame using a back and forth motion. LISTEN AND WATCH CAREFULLY FOR THE HYDRATE TO POP OUT OF THE CRUCIBLE! IF IT DOES, USING YOUR CRUCIBLE TONGS REMOVE THE CRUCIBLE FROM THE Burner for A FEW SECONDS THEN PUT IT BACK ON. CONTINUE THIS UNTIL IT NO LONGER POPS.

6.) After the 5 minutes are up, continue heating for another 15 minutes without brushing. Continue to monitor the

hydrate to make sure it does not pop out of the crucible.

7.) While waiting for the hydrate to heat, perform the flame test on a sample of your hydrate.

a.) Get a small amount (10 ml) of 6M HCl in your 50 mL beaker.

b.) Take a metal spatula and dip it first in the HCL and then put it into a flame from a Bunsen burner. If you see any colors, especially yellow, in the flame, repeat until you do not see any colors coming from the spatula.

c.) Place a small amount of your unknown hydrate on the tip of the cleaned spatula and hold it in the hottest part of the flame for a minute. Record your observations in the data section.

d.) If you see a red flame your anhydrous salt is strontium chloride, if you see a violet flame your anhydrous salt is potassium carbonate and you do not have to do the next step.

e.) Only if you did you not get either a red or violet flame in the flame test, dissolve a small amount of your unknown hydrate in a couple of milliliters of distilled water in a test tube.

f.) Drop a small piece of magnesium metal into the solution in the test tube.

g.) Wait at least 5 minutes. If you see a dark colored coating form on the magnesium metal your anhydrous salt is zinc sulfate. If you do not see a dark coating form you have magnesium sulfate as your anhydrous salt. Record your observations in the data section.

8.) After the crucible has been heating for 20 minutes over a burner (5 minutes gentle heating and 15 minutes strong heating) remove the crucible from the burner with crucible tongs.

9.) Place the crucible on a ceramic pad on the lab bench.

10.) When the crucible is cool to the touch, weigh the crucible and on the balance. Record the mass to three places past the decimal in the data section (A3).

11.) Place the crucible back over the burner and heat for another 10 minutes.

12.) Using crucible tongs, remove the crucible and set it on a ceramic pad

13.) When the crucible is cool to the touch weigh the crucible. Record the mass in the data section (A4).

14.) On your calculator find (A3) – (A4). If the difference is greater than 0.01 g proceed to step 12. If the difference is 0.01 g or less stop, you are finished with the procedure. IF not, you need to drive off more water so continue heating by moving on to step 12.

15.) If the difference between (A3) and (A4) was bigger than 0.01 g place the crucible over the burner again and heat for another 10 minutes.

16.) Using crucible tongs, remove the crucible from the hot plate and set on ceramic pad.

17.) When the crucible is cool to the touch weigh the crucible and cover. Record the mass in the data section (A5).

Data Table I

DATA for Sample __________

Mass of crucible and cover__________________(A1)

Mass of hydrate before heating__________________(A2)

Flame test observations______________________________________________

Magnesium strip observations (if necessary)_________________________________________________________

______________________________________________________________________________________________

Mass of crucible and cover after first heating__________________(A3)

Mass of crucible and cover after second heating__________________(A4)

Mass of crucible and cover after third heating__________________*(A5)

*Only if necessary

Data Table 2:

DATA for Sample __________

Mass of crucible and cover__________________(A1)

Mass of hydrate before heating__________________(A2)

Flame test observations______________________________________________

Magnesium strip observations (if necessary)_________________________________________________________

______________________________________________________________________________________________

Mass of crucible and cover after first heating__________________(A3)

Mass of crucible and cover after second heating__________________(A4)

Mass of crucible and cover after third heating__________________*(A5)

*Only if necessary

Sample Problems: Hydrates may waters of hydration ranging from one to twelve. Upon heating, a hydrate decomposes and produces an anhydrous salt and water (in the form of steam).

Data Table 1: Post lab Analysis Sample___________

1. Record the identity of your anhydrous salt (SrCl2, MgSO4, K2CO3, or ZnSO4) based on the results of your flame test and magnesium strip test on the line (A6).

________________________(A6)

2. Calculate the mass of your anhydrous salt, (A4) – (A1) or (A5) –(A1) if you had to do a third heating. Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A7).

________________________(A7)

3.) Calculate the molar mass of your anhydrous salt using the molar masses on the periodic table. Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A8).

________________________(A8)

4.) Calculate the moles of your anhydrous salt in your unknown. You can find this by dividing the mass of your anhydrous salt, (A7), by it's molar mass, (A8). Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A9).

_________________________(A9)

5.) Calculate the mass of water in your unknown. You can find this by subtracting the mass of the anhydrous salt (after you drove off all of the water) from the original mass of the hydrate (when it still had the water). That is, (A2) – A(7). Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A10).

_______________(A10)

6.) Calculate the moles of water in your unknown. You can do this by dividing the mass of water in your unknown, (A10), by the molar mass of water. Make sure to use the molar masses from the periodic table. Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A11).

_______________(A11)

7.) Calculate the number of waters of hydration in your unknown. That is, find x in the formula anhydrous salt∙xH2O where “anhydrous salt” is the formula of your anhydrous salt. You do this by dividing the moles of water by the moles of the anhydrous. Round to the nearest whole number. (A11)/(A9) = x Show all of your work including units in the space below. Write your answer on the line provided (A12).

______________________(A12)

8.) Write the complete formula for your hydrate on the line below. That is, write it in the form anhydrous salt∙xH2O where for anhydrous salt you put the formula for your anhydrous salt (SrCl2, MgSO4, K2CO3, or ZnSO4) and for x you put what you got for (A12).

____________________________________________(A13)

Data Table 2: Post lab Analysis Sample___________

9.) Record the identity of your anhydrous salt (SrCl2, MgSO4, K2CO3, or ZnSO4) based on the results of your flame test and magnesium strip test on the line (A6).

________________________(A6)

10.) Calculate the mass of your anhydrous salt, (A4) – (A1) or (A5) –(A1) if you had to do a third heating. Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A7).

________________________(A7)

11.) Calculate the molar mass of your anhydrous salt using the molar masses on the periodic table. Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A8).

________________________(A8)

12.) Calculate the moles of your anhydrous salt in your unknown. You can find this by dividing the mass of your anhydrous salt, (A7), by it's molar mass, (A8). Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A9).

_________________________(A9)

13.) Calculate the mass of water in your unknown. You can find this by subtracting the mass of the anhydrous salt (after you drove off all of the water) from the original mass of the hydrate (when it still had the water). That is, (A2) – A(7). Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A10).

_______________(A10)

14.) Calculate the moles of water in your unknown. You can do this by dividing the mass of water in your unknown, (A10), by the molar mass of water. Make sure to use the molar masses from the periodic table. Show all of your work including units and significant figures in the space below. Write your answer rounded to the correct number of significant figures on the line provided (A11).

_______________(A11)

15.) Calculate the number of waters of hydration in your unknown. That is, find x in the formula anhydrous salt∙xH2O where “anhydrous salt” is the formula of your anhydrous salt. You do this by dividing the moles of water by the moles of the anhydrous. Round to the nearest whole number. (A11)/(A9) = x Show all of your work including units in the space below. Write your answer on the line provided (A12).

______________________(A12)

16.) Write the complete formula for your hydrate on the line below. That is, write it in the form anhydrous salt∙xH2O where for anhydrous salt you put the formula for your anhydrous salt (SrCl2, MgSO4, K2CO3, or ZnSO4) and for x you put what you got for (A12).

____________________________________________(A13)

Prelab questions:

1. What is meant by the following terms:

a. Hydrate

b. Anhydrous

c. Water of hydration

2. Fill out the table below listing the prefixes used for naming hydrates:

Prefix

Moles of Water

Formula

1

XY•H2O

2

XY•2H2O

3

XY•3H2O

4

XY•4H2O

5

XY•5H2O

6

XY•6H2O

7

XY•7H2O

8

XY•8H2O

9

XY•9H2O

10

XY•10H2O

3. Hydrate nomenclature:

a. Suppose you heard "trihydrate." What would you write?

b. Suppose you heard "octahydrate." What would you write?

c. Name this substance: MgSO4 . 9H2O

d. Name a hydrate with formula: Na2SO4 . 10H2O

e. Write the formula for: barium chloride dihydrate

f. Write the formula for: cobalt(II) chloride hexahydrate

4. Below are several examples of single replacement reactions. Write the products and then balance the equations.

a. ZnS + O2 --->

b. K + H2O --->

c. Fe + HCl --->

d. NaI + Br2 --->

5. Refer to the lab on line to view the colored chart of the flame test results on page 2 of this lab. Explain why you would or wouldn’t use a flame test to distinguish between compounds that were made up of the following ions:

a.

b. Copper and boron

c. Strontium and Barium

d. Indium and Selenium

6. What is the %H2O in a sample of CaCl2·2H2O?

7. An unknown sample of a hydrate was dehydrated and found to contain 13.7% water. If the molar mass of the anhydrous compound is 227 what is the chemical formula of the hydrate?

Post lab questions: Barium chloride dihydrate, BaCl22H2O, has two waters of crystallization, or two waters of hydration. Other hydrates have waters of hydration ranging from one to twelve. Upon heating, a hydrate decomposes and produces an anhydrous salt and water (in the form of steam).

BaCl22H2O (s ) BaCl2 (s) + 2 H2O (g)

1-Theoretical Percentage - Calculate the theoretical percentage of water in barium chloride dihydrate. Note, the theoretical percentage of water is found by using the hydrate formula and the molar mass determined from the periodic table. Recall % composition is part/whole.

2 – Experimental percentage - The experimental percentage of water in a hydrate is found by comparing the mass of water driven off to the total mass of the compound, expressed as a percentage.

Assume you began with 1.250 g sample of barium chloride dehydrate. After heating the mass is 1.060 g. Calculate the experimental percentage of water. Note, you must first find the grams of water that were driven off.

3. If you were explaining what the difference is between theoretical % and experimental %, what would you say. Describe these terms without using specific numbers.

4. Water of crystallization - Calculate the water of crystallization for an unknown hydrate that is found to contain 30.6 % water. The formula mass of the anhydrous salt (AS) is 245 g/mol. Note, since the unknown hydrate is 30.6 % water, subtracting this from 100 % will give you the % anhydrous salt. If you assume you had a sample of 100 g, then mass of water is 30.6 g and likewise you know the mass of the anhydrous salt.. Calculate the moles of water and the moles of the anhydrous salt (AS). Then, find the water of crystallization, by finding the mole ratio of water to anhydrous salt. Remember the water of crystallization is always a whole number, and written in the form: AS?H2O.

5.

Lab 14: Determining the Formula of an Unknown Hydrate Rubric – 81 points (data table 1 only);

97 points (2 samples and data table 2 completed)

____ 4 pts Lab is written according to basic lab instructions

____ 2 pts Rubric is included

____ 5 pts Questions answered with question clearly indicated with the answer

____ 10 pts Lab physically completed within one week of original date assigned

____ 2 ptsPrelab #1 - clearly defines each term with complete sentences

____ 2 pts Prelab #2 table completely filled in

____ 6pts Prelab #3– proper nomenclature used

____ 4 ptsPrelab #4– correct balanced equation

____ 3 ptsPrelab #5 – Explain why you would or would not use a flame test to identify the pairs listed

____ 5 pts Prelab #6 - Calculations shown, answer reported with correct sig figs, label and answer circled.

____ 5 ptsPrelab #7 – Calculations shown, answer circled.

____ 16 ptsData table 1 completely filled in with answers circled and work shown. The factor label method is utilized and proper sig figs and units are reported.

____ 16 ptsData table 2 (if assigned) completely filled in with answers circled and work shown. The factor label method is utilized and proper sig figs and units are reported.

____ 5 pts Post lab question 1 - Calculation for the theoretical percent composition of water in the sample is shown with units of measure and is calculated using lab data.

____ 5 pts Post lab question 2 - Calculation for the experimental percent composition of water in the sample is shown with units of measure and is calculated using lab data.

____ 2 ptsPost lab question 3 - Clearly states the difference between theoretical and experimental percentages without the use of numbers – an general explanation is provided.

____ 5 ptsPost lab question 4 - Calculation for the formula of the hydrate is provided.