# mass conservation

Essay by Nivel May 2013

The conservation of mass

# AIM

The aim of this experiment is to show that mass is conserved during a chemical reaction.

The Law of Conservation of Mass, established in 1789 by French Chemist Antoine Lavoisier, states that mass is neither created nor destroyed in any ordinary chemical reaction. Or more simply, the mass of products by a chemical reaction is always equal to the mass of the reactants.

The reaction between potassium iodide and lead (II) nitrate under room temperature and atmospheric pressure results with a yellow precipitate. This reaction can be expressed by the equation: 2KI(aq) + Pb(NO3)2(aq) ( PbI2(s) + 2KNO3(aq)

# HYPOTHESIS

The mass before and after a chemical reaction will be the same as mass is conserved.

INDEPENDENT VARIABLE:

The mass before the reaction

(By measuring the mass of the reactants before reaction this independent variable can be controlled)

DEPENDENT VARIABLE:

The mass after the reaction

(By controlling the independent variable this dependent variable can be controlled)

CONTROLLED VARIABLES:

 VARIABLE Digital balance Proportion of chemicals Beakers Reaction time

# MATERIALS

 NAME SIZE/CONC. AMOUNT PRECISION Beaker 100 mL 2 Irrelevant Measuring cylinder 25.0 mL 2 Irrelevant Access to a digital balance ----------- 1 ÃÂ±0.001g Potassium iodide solution 0.5 mol/dm3 100 mL Irrelevant Lead (II) nitrate solution 0.5 mol/dm3 100 mL Irrelevant Glass rod 20 cm 1 Irrelevant

# METHOD

Wear safety glasses, apron and covered shoes before setting up any equipment.

Measure out 20 mL of potassium iodide with a measuring cylinder and pour into a 100 mL beaker.

Measure out 20 mL of lead nitrate with another measuring cylinder and pour into another 100 mL beaker.

Zero the balance and put both beakers together on the balance (balance is big enough for two beakers). Weigh their total mass accurately.

Record the mass obtained including uncertainties.

Carefully pour the lead (II) nitrate solution into the 100 mL beaker containing the potassium iodide.

Record the observation during the reaction.

Use a glass rod to carefully stir the mixture in order to make sure the reaction is completed. Re-zero the balance. Place both beakers back on balance and weigh their total mass.

Record the mass obtained including uncertainties.

When finished, pour the contents of the beakers into the laboratory sink.

Rinse the beakers and the measuring cylinders under tap water.

Dry the beakers and the measuring cylinders.

Repeat steps 2-12 another four times to maintain accurate results.

Result

 Trials Mass before weighing(ÃÂ±0.001g) Weight after weighing(ÃÂ±0.001g) 1 138.805g 138.785g 2 139.007g 138.997g 3 138.794g 138.776g 4 138.638g 138.609g 5 138.798g 138.770g

Qualitative data:

Yellow precipitate was produced.

There were some spillages of the product.

Processed data

 Trials Mass difference ÃÂ± 0.002g Mass difference express as percentage Random Error 1 0.020 0.014% 0.0014% 2 0.010 0.0072% 0.0014% 3 0.018 0.0013% 0.0014% 4 0.029 0.021% 0.0014% 5 0.028 0.020% 0.0014% Average 0.021 0.013% 0.0014%

Conclusion

The mass of both the beakers and their contents before the reaction was not the same as the mass after the reaction had taken place. The processed data table reveals that the mass difference before weighing and after weighing is 0.021g which is greater than the acceptable error range, ÃÂ±0.002g, indicating that the mass is not conserved and hypothesis is not supported. It is suggested that this experiment relatively inefficient in terms of investigating the conservation of mass in a chemical reaction. The calculated random error was 0.0014%, the mass of the products from this reaction is not within the limits of random experimental error. The percentage discrepancy shown by the result is 0.013%. Since mass is conserved in a chemical reaction, this indicates the discrepancies in respect to systematic errors or mistakes.

Evaluation

Humidity (that was not identified at the beginning) has an effect on the experiment. However during the experiment humidity, pressure and temperature are assumed to be constant.

From the processed data table it can be seen that trial 1, 3, 4 and 5 have similar results hence they are reliable. On the other hand, trial 2 has a relatively low mass difference (0.010g) which could be an unreliable datum.

# Reference list

Ed Durnford . 1999. The Law of Conservation of Mass. [ONLINE] Available at: http://www.mi.mun.ca/users/edurnfor/1100/atomic%20structure/tsld004.htm. [Accessed 10 April 2013].

2013. quantitative chemistry: heat and chemical reactions. [ONLINE] Available at: http://www.iun.edu/~cpanhd/C101webnotes/quantchem/rxnheat.html. [Accessed 11 April 2013].

2013. Conservation of Mass Lab. [ONLINE] Available at: http://raytedder.tripod.com/chemistrymaterialsfromraytedder/id27.html. [Accessed 11 April 2013].

2013. report format: experiment #3 conservation of matter . [ONLINE] Available at: http://www.google.com/url?sa=t&rct=j&q=mass+conservation+lab+report&source=web&cd=34&ved=0CDsQFjADOB4&url=https%3A%2F%2Fhwchemistry.wikispaces.com%2Ffile%2Fview%2FREPORT%2BFORMAT%2BLAB%2B%25233.doc&ei=wz-IUcPYPPCSiQeLvoDACg&usg=AFQjCNHJq-4qPwtSlRZbFZUtEfrD98TDyQ&cad=rja. [Accessed 11 April 2013].

2013. Lavoisier and the Law of Conservation of Mass. [ONLINE] Available at: http://www.uta.edu/faculty/sawasthi/Lecture%20Notes%20Chem1451/Law%20of%20Conservation%20of%20Mass.htm. [Accessed 11 April 2013].

mh99320. (2010). High School Chemistry Lab - Law of Conservation of Mass. [Online Video]. 01 June. Available from: http://www.youtube.com/watch?v=WxsR2MOuIG0.[Accessed: 11 April 2013].