Alternate Separation of a Mixture Lab

1. To understand different physical separation techniques

2. To separate the components of a mixture of sand (SiO2), table salt (NaCl) and ammonium chloride (NH4Cl), using various separation techniques

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3. To determine the percent composition of each component

4. To determine the percent recovery of the total mixture

Materials:
· 2 g unknown sample

· Small evaporating dish

· Large evaporating dish

· Watch glass

· Bunsen burner

· Balance

· Clay triangle

· Clay square

· Glass stirring rod

· Evaporating dish tongs

Introduction:
Matter can be generally classified either by state (liquid, gas, or solid) or by composition. In terms of composition, there are two types of matter: pure substances and mixtures. A pure substance is matter that has a fixed composition and distinct properties. Elements or compounds are pure substances (e.g. water, helium.) On the other hand, a mixture is matter that consists of two or more pure substances physically combined in varying amounts (e.g. salt water). There are two types of mixtures: homogeneous and heterogeneous mixtures. Whereas a homogeneous mixture is uniform throughout, the components of a heterogeneous mixture vary throughout and can be distinguished. For instance, granite (a type of rock) is a heterogeneous mixture.

Regardless of the type of mixture, the components of a mixture can always be separated by physical means. Some examples of physical methods of separation are defined below.

Decantation is the rough separation of a liquid from a solid. It consists of pouring the liquid out and leaving the solid in the container. No filter paper is needed in this process. As an example, sand and water can be separated by decantation.

Filtration is the fine separation of a solid material from a liquid with the help of filter paper and a funnel or other porous membrane. In this process, the solid or residue is collected on a filter paper. The liquid that passes through the filter paper is called the filtrate.

Extraction involves using a solvent to dissolve only one component of a mixture so that it can be removed from the other component(s). For example, if only one solid is soluble in water, extraction could be used to dissolve that component and then the aqueous solution can be removed from the mixture by decantation or filtration.

Distillation is a separation technique that uses the different boiling points of liquids. It consists of vaporizing a liquid substance out of a mixture of two or more liquids and condensing it into a separate container using a condenser. For example, alcohol can be separated from water in an aqueous solution by distillation because the alcohol has a lower boiling point than water.

Sublimation is the direct phase change from solid to gas. Ammonium chloride sublimates when heated. This means it can be directly converted to gas by heating.

Evaporation can be used to separate a liquid from a dissolved solid. The dissolved solid will have a higher boiling point than the liquid in the mixture. Therefore heating the solution until all of the liquid has vaporized will leave behind the solid in the bottom of the container.

In this experiment, you will use various separation techniques to separate the components of a heterogeneous mixture. Ammonium chloride sublimes when heated. This means it can be directly converted to gas by heating. This property will be used to separate it from sand and salt in the mixture. Once the ammonium chloride is removed, water is added to the remaining sample to extract NaCl. The resulting aqueous mixture is then decanted to separate the aqueous salt solution from the sand. Finally, the water is driven off from the salt solution and the wet sand.

The primary goal of the experiment is to determine the composition of the original mixture. The ratio of components in a mixture is described by the mass percent composition of each component. For example, a solution of sugar water might be 5% sugar and 95% water or it could be 40 % sugar and 60% water. The method for calculating percent composition is given in the section on calculations.

In addition to the composition of the mixture, it is also valuable to know how much material is lost during the processes used to separate the components. Some material will always be lost. However, significant losses are due to the use of poor lab technique. The ratio of the total mass of all the materials collected (after separation) to the mass of the original mixture gives the percent recovery. A percent recovery of greater than 100% indicates that some contamination remains in the sample. In this experiment, this contamination is water and can be removed by further heating. During an experiment it is important to make careful observations. Note anything unusual that occurs that may be a source of error in the lab.

Procedure:
Part I: Separation of Ammonium Chloride
1. Obtain a clean, dry small evaporating dish from your tray.Weigh it and record its mass to the proper number of sig figs.

2. Place about 2 g of the unknown mixture in the evaporating dish.

3. Weigh the evaporating dish and the sample together and record the mass to the proper number of sig figs.

4. Set up a Bunsen burner under the hood.

Note : The next steps are to be done in the HOOD.

5. Place the small evaporating dish with the sample on a stand over the burner. Do NOT cover the dish.

6. Heat gently for ten minutes.

7. Turn off the flame.

8. Carefully move the dish to a clay square and gently stir the sample with a stirring rod.

9. Relight the Bunsen burner and heat gently again for another twenty minutes. Ocassionally stir the contents with the stirring rod to help the ammonium chloride to sublimate.

10. Stop heating when no more white fumes are observed.

11. Allow the small evaporating dish and contents to cool to room temperature. Never weigh a hot object.

Lab Technique Tip: Hot containers such as glass beakers and ceramic evaporating dishes can crack if they cool down too quickly. Wire gauze or a clay square or ceramic tile will dissipate heat more slowly than the counter top and prevent the container from breaking.

12. Weigh the cooled dish, record its mass.

Part II: Extraction of Salt from Sand
1. Add 25 mL of deionized water to the contents of the smaller evaporating dish.

2. Stir the mixture gently with a stirring rod for five to ten minutes so that all of the salt dissolves.

3. Obtain a large evaporating dish from the lab cart.

4. Weigh the large evaporating dish and a watch glass together, record the mass in Part III of the lab report.

5. Carefully decant the liquid into the empty large evaporating dish. DO NOT TRANSFER ANY SOLID MATERIAL!

6. Wash the sand by repeating steps 1 – 2 this time using 5 mL portions of deionized water. Do this two or three times to extract all of the NaCl away from the sand.

7. Save the salt solution in the large evaporating dish for Part III.

8. The small evaporating dish should now contain only wet sand. Gently heat to evaporate the water left after decantation. Continue heating until the sand is dry. If the mixture gets too hot so that the contents spatter take it off the burner. Continue this cycle until the sand is dry. It may take a while.

9. Allow the dry evaporating dish and sand to cool to room temperature. While this is cooling, move to Part III.

10. When cool, weigh the evaporating dish and sand, record the mass.

Part III: Recovery of NaCl
1. Cover the large evaporating dish with the watch glass.

2. Gently heat the large evaporating dish with the watch glass cover in place to vaporize the water. Avoid spattering, see lab technique tip below. Carefully watch the salt for bubbling, popping or spattering.

Lab Technique Tip: Covering the evaporating dish with a watch glass allows air and steam to escape, but prevents the salt solution from spattering out when the solution boils. It also prevents the solid from popping out as the solution becomes completely dry.

3. Heat gently until dryness.

4. Once all water has evaporated, cool to room temperature and weigh, record the mass.

WASTE DISPOSAL: The sand can be disposed in the regular trash. The salt can be washed down the sink.

Calculations:
· mass of ammonium chloride = (mass of small evaporating dish with original sample) – (mass of small evaporating dish with sample after removing ammonium chloride)

· mass of sand = (mass of small evaporating dish with sample of dry sand) – (mass of empty small evaporating dish)

· mass of NaCl = (mass of large evaporating dish with watch glass and dry NaCl) – (mass of large evaporating dish with watch glass)

·

Sample Problem:

When a 2.00 g mixture of SiO2, NH4Cl, and NaCl was analyzed, the respective masses of SiO2, NH4Cl, and NaCl were 1.30 g, 0.50 g, and 0.16 g.

a. Calculate the percentage of each component.

b. Calculate the combined percentage (i.e. percent recovery)

Part a: Percentage of each component

Note : Always be mindful of the correct number of sig figs with calculations.

Part b: Percent Recovery

Total % = 65.0% + 25% + 8.0% = 98% of the mixture was recovered

Lab Technique Tip: The purpose of showing calculations in science is to record your method, not just your results. When showing your work for chemistry calculations, record the formula that you used and the units and chemical formulas in addition to the numbers and the math.

Separation of a Mixture Lab Report Sheet
Name

Date

Quiz

/ 20

Report

/ 80

Total

/ 100

Part I: Separation of Ammonium Chloride
Mass of small evaporating dish

41.621 g

Mass of small evaporating dish with sample

43.642 g

Mass of original sample

g

Mass of small evaporating dish after removing ammonium chloride

43.437 g

Mass of ammonium chloride

g

Percent of ammonium chloride (show calculations)

%

Part II: Extraction of Salt From Sand
Mass of small evaporating dish

41.621 g

Mass of small evaporating dish with dry sand

42.928 g

Mass of sand

g

Percent of sand in sample (show calculations)

%

Part III: Recovery of NaCl
Mass of large evaporating dish with watch glass

149.018 g

Mass of large evaporating dish with watch glass and salt after heating

149.436 g

Mass of NaCl

g

Percent of NaCl (show calculations)

%

Results:
Add all of the percents together to determine the percent recovery (show your work). Explain any variation from a total of 100%.

Conclusion:

Questions:

1. Fill in the following blanks.

· Two methods of separating an undissolved solid from a liquid are ________________ and ________________

· The method for separating a dissolved solid from the water in an aqueous solution is _______________

· Using a solvent to dissolve only one substance in a mixture is called ______________.

2. A mixture can consist of two or more pure substances. Does that mean a mixture can only consist of elements?

3. Explain your answer to question number 2.

4. Oil and water mixed together form a heterogenous or homogenous mixture?

5. 2.000 g of the unknown mixture was placed in an evaporating dish. The empty evaporating dish weighs 38.135 g. After subliming the ammonium chloride from the mixture, the evaporating dish and the remaining mixture weighs 39.775 g. (Put your answer in 3 sig figs and show your work)

· What is the weight of the ammonium chloride?

· What is the percent of ammonium chloride in the sample?To continue with the answer check on mycoursewriter.com/

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