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Experiment 14 – Baking Soda Name __________________Lab Section __________________

Experiment 14 – Decomposition of Baking Soda

Introduction

The decomposition of any metal hydrogen carbonate will result in the formation of that metal carbonate, water vapor, and carbon dioxide. This lab will investigate the decomposition of sodium hydrogen carbonate, NaHCO3, commonly called baking soda. It is governed by the following chemical equation:

2 NaHC O3 ( s )❑→

Na2C O3 (s )+H2O ( g )+C O2(g)

In this lab, the NaHCO3 will be decomposed to form Na2CO3. The first half of the experiment will use a pure sample of NaHCO3, and the second part will use a mixture containing only some NaHCO3. Depending on how well the first half of the experiment is performed, the percent yield of this reaction should be 100%. The second half of the experiment should see something less than 100%, since it is not a pure sample.

Percent yield is defined as the actual yield divided by the theoretical yield. The actual yield is the amount of product one produces during the experiment. The theoretical yield is the amount of product one should have produced.

%Yield= actual yieldtheoretical yield

x100 %

Unless there are experimental errors, the % Yield should not be over 100%. In many cases, especially ones in which the reaction is difficult to control; the % Yield can be much lower than 100%.

In the case of impure samples, like in the second half of this lab, the % Yield can give the percent of active material in the sample. The concept is the same. The only trick is to keep track of which value is the actual amount, and which is the theoretical amount.

Refer to the sample calculations for more insight.

Equipment

100 or 250 mL beakerWatchglassBunsen burner

Wire gauzeIron ringNaHCO3

Unknown sample


Examples

Percent Yield from a Pure Sample

A 0.685 g sample of Ca(HCO3)2 is decomposed by heating to form CaCO3. When the sample is cooled it weighs 0.418 g. Calculate the theoretical yield of CaCO3.

Ca ¿

To find the theoretical yield, calculate the amount of CaCO3 one expects to obtain from the Ca(HCO3)2. Set up the mass-mole-mole-mass calculation is as follows:

0.685 gCa¿

¿0.422907 g CaCO3

in proper sig figs ¿0.423 g CaCO3 Theoretical Yield

The percent yield is actual yield divided by the theoretical yield. After decomposition, the actual yield of CaCO3 was 0.418 g. Divide this actual yield by the theoretical yield and convert to percentage.

%CaC O3=actual yield

theoretical yieldx100 %

%CaC O3=0.418 g0.423 g

x 100%

%CaC O3=98.8 %


Percent of Active Material in Unknown Mixture

A 1.425 g sample of powder contains KHCO3 and an unknown amount of unreactive salt. When it is decomposed it weighed 1.225 g. Calculate the %KHCO3 in the unknown powder.

2 KHC O3 (s )❑→

K 2 C O3 ( s )+H 2 C O3(g)

The only mass that is truly known in this problem is the mass that was lost, namely the H 2O and CO2 (grouped as H2CO3 (g) in the equation above).

The mass of gas that was produced is merely the mass before heating minus the mass after heating: 1.425 g – 1.225 g = 0.200 g gas released.

Using this mass, we can determine the actual amount of KHCO3 that produced this gas by the following mass-mole-mole-mass calculation:

0.200 g H 2C O3[ 1 mol H 2C O3

60.03 g H2 C O3 ] [2mol KHC O3

1 mol H2 C O3 ][ 100.02 g KHC O3

1mol KHC O3 ]¿0.666467 g KHC O3

In proper sig figs ¿0.666 g KHC O3 Actual amount of KHCO3 in sample

The original sample was 1.425 g. If all of the sampler were KHCO3, then the amount of gas released would be much more (0.428 g, using the same calculation as in the first example). In this case we see that our calculated value above is the actual amount of KHCO3 that made the gas, and our initial measured amount in the problem statement is the theoretical amount that should have made the gas.

The percent of active material is the amount of KHCO3 that decomposed (made the gas) divided by the total amount of material.

% KHC O3=amount decomposed

totalamount of materialx100 %

% KHC O3=0.666 g1.425 g

x100 %

% KHC O3=46.7 %


Prelaboratory Questions

A 0.685 g sample of NaHCO3 is decomposed by heating to form Na2CO3. When the sample is cooled it weighs 0.418 g. Calculate the theoretical yield of Na2CO3. Calculate the percent yield of Na2CO3.

2 NaHC O3 ( s )❑→

Na2C O3 (s )+H2C O3(g)

A 4.076 g sample of powder contains LiHCO3 and an unknown amount of unreactive salt. When it is decomposed it weighed 1.496 g. Calculate the %KHCO3 in the unknown powder.

2 LiHC O3 ( s)❑→

Li2 C O3 (s )+H 2C O3(g)


Procedure

Percent Yield of Na2CO3 from pure NaHCO3

Weigh out approximately 1 g of NaHCO3 into your beaker.

Place the watchglass on the beaker. This will act as the indicator of when the experiment is done.

Place the beaker on the wire gauze and heat the NaHCO3. A vigorous flame will be used in this experiment. Be sure that the tip of the cone of the flame is touching the wire gauze. Water droplets should begin to form on the watchglass within a few minutes. Note: the initial “fog” one sees upon heating is merely the water vapor that is naturally occurring on the glass being driven off by the heat.

Continue to heat the NaHCO3 until the water droplets on the watchglass have disappeared.

Allow the beaker to cool to room temperature and weigh it. This is the mass of the Na2CO3

resulting from the decomposition.

Calculate the theoretical yield of the Na2CO3 from the original mass of the NaHCO3.

Percent of NaHCO3 in an Unknown MixtureWeigh out approximately 1 to 2 g of the unknown sample into your beaker.

Repeat the steps of the previous procedure. This time, however, the resulting mass is the expected Na2CO3 and the unreactive component of the unknown.

Use the mass lost in the experiment, the H2CO3 gas, to determine the mass of NaHCO3 in your unknown sample.

Post Experiment Instructions

All waste can be placed in the trash or rinsed down the sink. Return unknown vial to its place of origin.


Data Table

Percent Yield of Na2CO3 from a pure sample of NaHCO3

Mass of NaHCO3 (before heating)

Mass of Na2CO3 (after heating)

Trial 1

__________

__________

Trial 2

__________

__________

Theoretical Yield of Na2CO3 – show calculations

Theoretical Yield of Na2CO3 __________ __________

Percent Yield of Na2CO3 – show calculations

Percent Yield of Na2CO3 __________ __________


Average percent yield of Na2CO3 __________


Percent of NaHCO3 in an Unknown Mixture Unknown Number ____

Mass of Unknown MixtureBefore Heating

Mass of Unknown Mixture After Heating

Mass of gas released, H2CO3

Trial 1

__________

__________

__________

Trial 2

__________

__________

__________

Mass of NaHCO3 in Unknown Mixture – show calculations

Mass of NaHCO3 in Unknown Mixture __________ __________

Percent of NaHCO3 in Unknown Mixture – show calculations

Percent of NaHCO3 in Unknown Mixture __________ __________


Average Percent of NaHCO3 in Unknown Mixture __________


Postlaboratory Questions

1) A 1.897 g sample of Mg(HCO3)2 was heated and decomposed. When the sample cooled, it weighed 1.071 g. What is the % Yield of this reaction?

2) An geologist brought in a sample of ore containing Fe(HCO3)3 and some other, unreactive material. If the sample weighed 1.394 g before it was decomposed, and 0.978 g after, what is the %Fe(HCO3)3 of the sample?


3) A 1.112 g strip of magnesium is dropped into a giant container of HCl (aq). What is the volume of the gas that is produced at 0°C and 1 atm?

4) Calcium metal is dropped into water, and 15.7 L of gas is produced at STP, what was the mass of calcium that produced the gas?

5) If 2.112 g of Fe2O3 reacts with 0.687 g of Al, how much pure Fe will be produced? Fe2 O3 ( s )+2 Al (s )❑

→Al2O3 (s )+2Fe (s)

experiment 14 - baking soda.docx word docs... · web viewexperiment 14 – decomposition of...

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