Lab 2 - lab 2 PDF

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Mombourquette Title: GRAVIMETRIC ANALYSIS Purpose: The purpose of this experiment is to determine the composition of an unknown compound using gravimetric analysis.

Introduction: This lab uses the methods of gravimetric analysis and stoichiometric calculations in order to determine which compound an unlabeled bottle of material is. Gravimetric analysis is used to discover the masses of each potential compound and to attain the change in mass. After calculating the mass of the unlabeled chemical, we must dissolve the unknown compound and mix it with an excess of silver nitrate so the compound reacts with silver nitrate to completion. After we mix silver nitrate and our unknown compound we separate the mixed solution through filtration, and dry the precipitate in the oven. This allows us to determine the actual mass of the unknown compound and silver nitrate solution. Finally, through stoichiometry we can find the relationship between the three possible compounds and through mole calculation we can compare which compound’s theoretical mass is closest to the experimental mass of the unknown.

Materials:

• • • • • • • • • • • • • • •

Lab Coat Goggles Gloves Magnetic Stirrer Magnet 3 grams of unlabeled chemical (XCl2) Demineralized Water Weighing Dish Analytical Balance Spatula Conical Flask 250mL Measuring Cylinder 100mL 100 mL Silver Nitrate Solution Paper Filter Rubber cones

• • • • • • •

Bücher Funnel Watch Glass Vacuum Valve Vacuum Tube Conical Flask 500 mL Oven Rubber Cone

Procedure: • Put on lab coat, safety googles, and put on gloves. • Place your weighing dish on the analytical balance, close the balance and tare the dish. • Open the analytical balance and proceed to measure out 3 grams of your unknown chemical onto the dish close the balance to ensure the measurement is precise before copying down the initial mass of the unknown chemical into your lab journal. • Pickup the conical flask and place it on the magnetic stirrer. The magnetic stirrer allows the liquid to be stirred evenly and constantly as the mixture is heated up. • Transfer the contents of your weighing dish into the conical flask. • Measure 50 mL of demineralized water in a measuring cylinder. • Pour the the 50 mL of demineralized water into the conical flask, drop in your magnet and turn on the magnetic stirrer. • The unknown substance is a chloride and in order for all of the chloride ions to react there must be an excess of Silver Nitrate in the solution. Refer back to the original balanced equation and use stoichiometry to ensure that there is an excess of silver nitrate for every possible case of the unknown substance. • After determining the amount of Silver Nitrate needed (100 mL), measure it out and transfer the Silver Nitrate into the conical flask solution. • Next, prepare your station for a suction filtration, place a rubber cone on top of your conical flask, attach funnel, and the in-house vacuum system. • Turn on the vacuum by twisting the knob • Place a sheet of filter paper inside the funnel, and squirt some water to ensure the filter paper sticks. • Filter your product. • To Dry the precipitate you must first, tare your watch glass using the analytical balance. • Take your watch glass out of the balance and place it on the workbench. • Wash your filter cake with demineralized water. • Transfer your filter cake from the funnel to the watch glass. • Place your watch glass in the oven and wait for it to dry. • Once dried, remove the watch glass from the oven and place it on the balance. Record your experimental yield in your lab journal

• Begin calculations to find the theoretical yield and compare the answers to your experimental yield, whichever compound is the most accurate to your experimental yield is the unknown substance. • You have now completed the lab.

Observations: You will notice as silver nitrate is added to the unknown compound and distilled water, the clear solution will become opaque. This is the precipitation caused by silver chloride. Mass of XCl2 = 3.0456g Mass of 2AgCl(s) = 8.5341g

Results: Main Equation: XCl2 + 2 AgNO3 => X(NO3)2(aq) + 2AgCl(s) m(g) = 3.0456g. 8.5341g Compound discovered as MgCl2 experimental yield of 9.169g Percentage yield: Y= (experimental yield / theoretical yield) * 100% Y= (9.169/8.5341) * 100% Y= 93.075% Table 1: Calculations of the three possible compounds X=Be

X=Mg

X=Ca

Equations

BeCl2+2 AgNO3 → Be (NO3)2+2 AgCl

MgCl2+2 AgNO3 → Mg (NO3)2+2 AgCl

CaCl2+2 AgNO3 →Ca (NO3)2+2 AgCl

m(g) =

𝑚(𝐵𝑒𝐶𝑙2)=3.0456𝑔

𝑚(𝑀𝑔𝐶𝑙2)=3.0456𝑔

𝑚(𝐶𝑎𝐶𝑙2)=3.0456𝑔

𝑚(𝐴𝑔𝐶𝑙)=𝑀∗𝑛

𝑚(𝐴𝑔𝐶𝑙)=𝑀∗𝑛

𝑚(𝐴𝑔𝐶𝑙)=𝑀∗𝑛

𝑚(𝐴𝑔𝐶𝑙)=143.32∗0.0 76 𝑚(𝐴𝑔𝐶𝑙)=10.951𝑔

𝑚(𝐴𝑔𝐶𝑙)=95.211∗0.0 64 𝑚(𝐴𝑔𝐶𝑙)=9.169𝑔

𝑚(𝐴𝑔𝐶𝑙)=143.32∗0.0 55 𝑚(𝐴𝑔𝐶𝑙)=7.866𝑔

𝐵𝑒𝐶𝑙2=79.718𝑔/𝑚𝑜𝑙

𝑀𝑔𝐶𝑙2=95.211𝑔/𝑚𝑜𝑙

𝐶𝑎𝐶𝑙2=110.98𝑔/𝑚𝑜𝑙 2

2 𝐴𝑔𝐶𝑙=143.32𝑔/𝑚𝑜𝑙

𝐴𝑔𝐶𝑙=143.32𝑔/𝑚𝑜𝑙

M(g/mol) =

2 𝐴𝑔𝐶𝑙=143.32𝑔/𝑚𝑜𝑙

n(mol) =

X=Be

X=Mg

𝑛(𝐵𝑒𝐶𝑙2)=𝑚𝑀 𝑛(𝐵𝑒𝐶𝑙2)=3.045679.7 18 𝑛(𝐵𝑒𝐶𝑙2)=0.038𝑚𝑜𝑙 𝑛(𝐴𝑔𝑁𝑂3)=0.076𝑚𝑜𝑙

𝑛(𝐴𝑔𝐶𝑙)=𝑚𝑀 𝑛(𝐴𝑔𝐶𝑙)=𝑚𝑀 𝑛(𝐴𝑔𝐶𝑙)=10.951143.3 𝑛(𝐴𝑔𝐶𝑙)=9.169143.32 2 𝑛(𝐴𝑔𝐶𝑙)=0.076𝑚𝑜𝑙 𝑛(𝑀𝑔𝐶𝑙2)=𝑚𝑀 𝑛(𝑀𝑔𝐶𝑙2)=3.045695. 211 𝑛(𝑀𝑔𝐶𝑙2)=0.032𝑚𝑜𝑙

X=Ca

𝑛(𝐴𝑔𝐶𝑙)=0.064𝑚𝑜𝑙 𝑛(𝐶𝑎𝐶𝑙2)=𝑚𝑀 𝑛(𝐶𝑎𝐶𝑙2)=3.0456110. 98 𝑛(𝐶𝑎𝐶𝑙2)=0.027𝑚𝑜𝑙

𝑛(𝐴𝑔𝑁𝑂3)=0.064𝑚𝑜𝑙 𝑛(𝐴𝑔𝑁𝑂3)=0.055𝑚𝑜𝑙 𝑛(𝐴𝑔𝐶𝑙)=𝑚𝑀 𝑛(𝐴𝑔𝐶𝑙)=7.866143.32 𝑛(𝐴𝑔𝐶𝑙)=0.076𝑚𝑜𝑙

Conclusion: The results from this lab and stoichiometric calculations have exhibited that the unknown compound is MgCl2. This is because MgCl2 had the closest experimental yield to the theoretical yield of the compounds studied.

References:

Anne Marie Helmenstine, P. (n.d.). What Is Stoichiometry? Retrieved from https:// www.thoughtco.com/definition-of-stoichiometry-605926

Khan Academy. 2020. Gravimetric Analysis Intro: Volatilization Gravimetry (Article) | Khan Academy. [online] Available at: [Accessed 24 September 2020].

Labster. 2020. Labster. [online] Available at: [Accessed 24 September 2020]. - Molecular Weights of BeCl2, MgCl2, and CaCl2 - Mass of XCL2 - Mass of AgC...