Analysis of an aluminum zinc alloy answers

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ANALYSIS OF AN ALUMINUM-ZINC ALLOY

I. PURPOS E There are rwo parts for this experiment, in the fi rst parr you will measure the hydrogen gas evolved from the reac- tion of zinc and aluminum wi th strong acids. In the ec- ond part, the percent composition of an aluminum-zinc alloy will be determined.

II. BACKGROUND

PART I Many metals will reacr with srrong acid ro produce hy- drogen gas and a solution of rhc salr of tht: meral. Both aluminum (t:quation l ) and zinc (equation 2) react wi th hydrochloric acid in rhis way as shown below:

2 Al (s) + 6 HCI (11q) 2 AICl 1 (nq) + 3 H! (g) Zn (s) + 2 HCl (flq)- ZnCl1 (1u1) + H! (g}

(1)

(2)

In this experiment, you will de1crmirn: the mob of H , gas produced in the reaction of ,dumin11m Jnd Linc with hydrochloric acid (HCl). Using ,1 gas burei, you will first measure the volume of hydrogen gas (V1,_) released during the reaction and then converr this qu,rntity ro moles of H

2 gas using the ideal gas law.

The calculations in this experiment are complicated by the fact that the hydrogen gas is collected over water, so the total pressure (P,) of gas collected is acrually the sum of the partial pressure of H 1 ( />11) pl us the partial pre sure of water vapor (P

111 J that is also present (equation 3):

p = p + p (3) fOI Hl 11,0

As can be: seen in the diagram of the experimental appara- tu in Figure 9- l , the atmospheric pressure (P ) exactly balances the pressure of the gas collected (I' ·')

1111

and the lo

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EX PERIMENT 9

pressure due to the water column (Pu) being supported above the level of the water

surface (equation 4):

P = P +P".' atm r01 (4)

Substituting P from 3 in 4: IOI

(5) patm =PH. + PH,0 + p"'

A column of water I 026 cm high exer;s a pr;ssure of I atm. So the pressure ~f the

water column (Pu) in the gas buret can be calculated using equation 5. where h is the height of water in the column in centimeters.

I atm X h p" ' = 1026cm

(6)

Rearranging the terms and substituting equation 6 into equation 5, gives an e~pres-

sion (equation 7) for calculating rhe partial pressure of the hydrogen gas that 1s col-

lected from the reaction.

_ p _ ( I arm X h ) _ p P H, - .um 1026cm H,O

(7)

The moles of hydrogen gas collected from the reaction of either aluminum or zinc

with H Cl can rhen be calculated from the ideal gas law (equation 8):

Where:

n = Number of hydrogen mole~ H,

P = Pressure of hydrogen (arm) 1-1,

V = Volume of hydrogen (L) 11,

O tt = P,R~II,

R = Gas constant (0.082 1 atm-L/mol-K)

T = Temperature (K)

PART II

(8)

In this second part, you will perform the same procedure using an aluminum-zinc

afloy, a solid mixture of rhe two metals. The amount of hydrogen gas produced from the aluminum-zinc alloy will be the sum of that generated by the reactions of alumi-

num and zinc with HCI (equations 1 and 2) . Assuming char the percent yield of these

reactions is very close to 100%, measuring the volume of hydrogen gas produced will allow you to determine the composition of the aluminum-zinc alloy (i .e., the mass percent of aluminum and of zinc in the alloy).

T he reaction of one gram of zinc with H Cl can generate up to 0.0153 moles of H gas; the analogous reaction with one gram of aluminum can produce a maximu~ of 0.0556 moles of Hr (You should verify these calculated amounts on your own to understand the conversion equations.) The moles of hydrogen gas (n ) produced when an aluminum-zinc alloy reacts with hydrochloric acid can therefore6e expressed

as shown in equation 9, where mass Al and mmszn are the masses of each metal in the alloy (in grams) .

ANALYSIS OF AN ALUMINUM-ZINC ALLOY

nH 2 = (0.0556)(massAJ) + (0.0153)(massz.,)

The coral mass is:

massalloy = massZn + massAl Therefore the mass of zinc in the alloy is equal to:

massZn = mass alloy - mass AJ Substituting 11 in 9:

(9)

(10)

(11)

n H 1 = (0.0556)(massAl) + (0.0 I 53)(massalloy - mass Al) (12)

Reducing terms:

nH 2 = (0.0403)(massA1) + (0.0 I 53)(mass2u0) (13)

Solving for mass Al gives equation 14, which relates the mass of aluminum to the mea- surable quantities, n11/ and mass2 110Y.

nH, - (0.0 I 53)(mass.ii.,,) massAI = 0.0403 (14)

The mass percent of aluminum in che alloy can then be c:ilculated:

massAJ 00 massA1% = mass.ii.. X I (15)

You can therefore determine che composition of an :iluminum-zinc alloy by measuring the moles of hydrogen produced from a known mass of the alloy when it is completely reacted with hydrochloric acid.

Ill. SAFETY PRECAUTIONS • Safety gkl.Sses are always required as long as anyone in the lab is still performing

laboratory work!

• An apron or l11b coat and closed-toe/closed-heel shoes are required.

•

No food or drinks in the !tzb at any time.

Highly concentrated (6 M) HCI solution can cause severe bums; you will need gloves for this lab.

Be especially careful while handling chis solucion to avoid accidents (splash- c.:~ or spills). If you come in contact with chis solution, wash it off with water immediately.

You should also thoroughly wash your hands after finishing che experiment.

Dispose of all waste chemicals in the hazardous waste boccies in the hoods.

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80 EXPERIMENT 9

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IV. MATERIALS AND REAGENTS

-;-1 Equipment QT Reagents I (communal equipment) -- - -

1 100 ml Gas buret 10 mL X 6 6 M Hydrochloric Acid

Gas transfer tube ~0.120 g Zinc (mesh)

Thermometer ~0.030 g Aluminum (turnings)

Stopper, for gas burets ~0.120 g Aluminum-Zinc Alloy Unknown (assign from your TA)

Ruler ~60 ml/ group WASTE

V. EXPERIMENTAL PROCEDURE Students will work in pairs on this experiment for part A and B, but each will do a dif- ferent unknown. Pick up a 100 mL gas buret and a gas transfer tube from the plastic tubs in the front of the room. Also, you will need four clean and dry 6-inch rest tubes from your locker.

A. REACTION OF ALUMINUM WITH HYDROCHLORIC ACID I . Set up a support stand with a buret clamp and attach the gas buret right away to

avoid breakage.

2. Fill a 400 mL beaker with about 300 ml of cap water. Fill the gas buret com- pletely with water, place a small stopper in the end of the buret and submerge the open end of the buret under the surface of the water in the beaker. Remove the stopper while submerged to minimize the amount of air initially in the buret.

3. Put one finger over the rubber stopper end of the gas transfer tube and insert into the water such chat the other end is inside the gas buret (see Figure 9-1).

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ANALYSIS OF AN ALUMINUM ZINC ALLOY 81

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Figure 9-1. 5Nttp for the collection of gm from the re11ctions between metal and HCI

4. Weigh approximately 0.030 g of aluminum in one of the 6-inch test cubes on an analytical balance and record the mass co the nearest milligram in your laboratory

notebook.

5. Rend tlm sup entirely before proceeding. Carefully pour in about 10 mL of the 6 M HCI solmion ( Caution: Strong Acid0 into the cube. Quickly and carefully, attach rhe rest cube to the rubber stopper on rhe gas transfer cube. Make sure the tubes form rt tight sen/ with the mbber stopper of the gas transfer tube. ft should be tight enough so that the hydrogen gas being rt/eased during the reaction goes into the gas

buret.

6. You may need to gently shake the rest cube co gee all the metal in the test cube inco

the acid solution.

7. Monitor the reaction until ic is complete-when gas evolution stops and che metal is no longer visible.

MEASUREMENTS AND CALCULATIONS 8. Once the reaction has gone to completion, record the volume of H, of the buret

(i,;,,, in ml) and the height (h) of rhe water column (in cm) in rhe· burer to the suiface of the water in the beaker after all the gas evolution has stopped. Plea c

refer to Figure 9-1.

9. From your experimental data, calculate the pressure of hydrogen gas (P ) using equation 7. Assume chat the temperature of rhe hydrogen gas in the bu;;c is the

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82 EXPER IM ENT 9

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same as room temperature. Use Table 9-1 to find the value of PHO that corre-1 sponds to room temperature. Assume that the atmospheric pressure (Pa,,) in the laboratory is 0.987 atm; it never varies much because we are so close to the ocean.

I 0. Calculate the actual yield of hydrogen gas (n H) using equation 8. ]

11 . Calculate the theoretical yield from the balanced chemical reactions for pure alu- minum or pure zinc from equations I and 2, respectively.

12. Calculate the percent yield of your experimental data by dividing your actual yield (from step 1 O) by the theoretical yield (from step 11) times 100.

B. REACTION OF ZINC WITH HYDROCHLORIC ACID Follow the procedure in Part A, except use approximately 0.120 g of zinc in place of aluminum. Use your experimental data to determine the actual, theoretical, and percent yield of hydrogen gas for the reaction.

C. DETERMINATION OF MASS PERCENT OF ALUMINUM IN AN ALUMINUM -ZINC ALLOY I. For your unknown alloy, follow the procedure in Part A, steps 1-7, but you will

dilute the 6.0 M HCI to 3.0 M HCI. At least two trials of the aluminum-zinc alloy with HCI solution should be done. Measure the volume of hydrogen gas produced in both reactions. Use approximately 0.120 g of the alloy in each trial. Be sure to record which unknown alloy you used in your laboratory notebook. The molarity of the acid is listed on the bottles.

PREPARING 3.0 M HCI a. For two trials you will need to prepare 20 mL of 3.0 M HCL

b. Measure IO mL of water with your l O mL graduated cylinder; transfer it into a 50 mL beaker.

c. In the same cylinder meamre l O mL of 6 M HCL and slowly pour it into the beaker containing the water.

d. Mix it with your stirring rod.

e. Measure IO mL of your dilution (3 M) for the alloy reaction.

2. Repeat the experiment with a second sample (~0.120 g) of the alloy.

3. Follow measurements and calculations in steps 8-10 from Part A to calculate the pressure of hydrogen gas (PH) and the actual yield of hydrogen gas, (nH).

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4. Calculate the mass of aluminum in the alloy using equation 14.

5. Using the actual mass of the alloy sample and the result from step 3, calculate the percent of aluminum in the alloy.

6. Finally, calculate the percent zinc present in the alloy for the two experiments.

·,

ANALYSIS OF AN ALUMINUM ZINC ALLOY 33

VI. DISCUSSION/QUESTIONS I . Discussion: Discuss the accuracy, preci ion, systematic errors, and random error

of your data.

2. What important assumptions did you make in the alloy part to determine the

composition of the Al-Zn alloy? Clearly list your assumptions and in a sentence

for each explain why it is important.

3. From the percent yields for Al and Zn chat you obtained from Parts A and B,

discuss how rhese percent yields would affect the results for your Al-Zn alloy in

this experiment.

Table 9-1. Vapor pressure of water (P HP ) at different temperatures --------- Temperature (°C) PH20 (atm)

PHiO(atm)

18.0 0.0204 22.5 0.0269

18.5 0.0210 23.0 0.0277

19.0 0.0217 23.5 0.0287

19.5 0.0224 24.0 0.0294

20.0 0.0231 24.5 0.0307

20.5 0.0239 25.0 0.0313

21.0 0.0245 26.0 0.0332

21.5 0.0254 27.0 0.0352

22.0 0.0261 28.0 0.0373

REFERENCES Chemistry and Biochemistry Department. "Manual for CHEM 200/202." Laboratory

Manual. San Diego State Universiry. San Diego. 2013. Print.

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