During this experiment we are going to investigate the uses of the knowledge we already acquired about the properties of substances, how to calculate the molarity and molality with the given data, and also we are going to understand the concepts of the conservation of mass, and additive/non-additive volumes. For the performance of these experiments we will be using different types of materials we already know, and others we aren't used to yet.
OBJECTIVES
1. To improve practical skills, such as the use of different scales, test tubes and other materials in a precised way.
2. To learn how to calculate the molarity, molality and the number of moles of a substance.
3. To investigate how mass is conserverd all over the experiment.
4. To investigate how volume is not conserverd all over the experiment.
5. To evaluate our results, through different questions and have the ability to create a new experiment.
SOME INTRODUCTORY QUESTIONS;
Is mass always conserved?
Due to the Law of Lavoisier mas cannot be created or destroyed, so mass is always conserved during an experiment. As we can observe in part 2, where although 2,5g of sodium chloride are dissolved in water, the mass is conserved as when we weighted the whole apparatus it weoghted 2,5 g more than before, which means that even though the sodium chloride has been dissolved, the 2,5g stay dissolved in the solution.
Is volume always conserved?
Volume is not conserved in a chemical reaction. Mass is one of the only chemical and physical properties that is conserved during a chemical reaction. As it can be observed in part 3, Even though we add 2.5g of solute to the 10mL solution, the total solution is not 12.5mL, it is 11mL as volume is not conserved during the solution, and 1.5mL have been "lost". This is due to that the solvent or in this experiment water, "mixes" with the solute, and finally the volume is not conserved, and that it is why, the volume decreases in 1.5mL.
What are molality and molarity?
Molarity and molality are both measures for the concentration of the solutions. Molarity is the number of moles per volume of the solution, while molality is the number of moles per kilogram of the solvent. As it can be observed in part 4, where we calculated the molarity and molality of the final solution, applying the needed formula and components of a solution.
Due to the Law of Lavoisier mas cannot be created or destroyed, so mass is always conserved during an experiment. As we can observe in part 2, where although 2,5g of sodium chloride are dissolved in water, the mass is conserved as when we weighted the whole apparatus it weoghted 2,5 g more than before, which means that even though the sodium chloride has been dissolved, the 2,5g stay dissolved in the solution.
Is volume always conserved?
Volume is not conserved in a chemical reaction. Mass is one of the only chemical and physical properties that is conserved during a chemical reaction. As it can be observed in part 3, Even though we add 2.5g of solute to the 10mL solution, the total solution is not 12.5mL, it is 11mL as volume is not conserved during the solution, and 1.5mL have been "lost". This is due to that the solvent or in this experiment water, "mixes" with the solute, and finally the volume is not conserved, and that it is why, the volume decreases in 1.5mL.
What are molality and molarity?
Molarity and molality are both measures for the concentration of the solutions. Molarity is the number of moles per volume of the solution, while molality is the number of moles per kilogram of the solvent. As it can be observed in part 4, where we calculated the molarity and molality of the final solution, applying the needed formula and components of a solution.
1. WORKING OUT THE VOLUME OF 2.5 G
SODIUM CHLORIDE USING CYCLOHEXANE.
a. Measure 3 mL of cyclohexane with a pipette and pour it into a dry measuring cylinder.
Weigh the cylinder with the cyclohexane: ___73.50g__.
Weigh 2.50 g of sodium chloride and place it in the cylinder as well.
Weigh the whole apparatus: ___76.00g_.
Does the total mass equal the masses of the different parts? ___YES_____.
A French scientist named Lavoisier stated that “matter cannot be created or destroyed, so mass is always conserved”. Does your data agree (approximately) with this statement? __YES, as it can be observed 73,50+2,50= 76g which is the total weigh of the whole apparatus_____.
b. Why does sodium chloride not dissolve in cyclohexane (Hint: which kind of substance are they – ionic, covalent (organic) or metallic)?
Sodium chloride doesn't disslove in cyclohexane, as in sodium chloride there is an ionic bonding, while in cyclohexane there is a covalent bonding. Sodium chloride is formed by a cation and a anion, so to dissolve in a substance, it also has to have a cation and a anion, to tear apart the atoms forming each molecule, and the ionic bond can be broken. Cyclohexane is formed by two non-metals, so there will not be a negative or positive part, so when sodium chloride is introduced in cyclohexane, it won't dissolve as there isn't a positive or a negative part to tear apart the atoms forming it.
As it does not dissolve, we can work out the volume of the salt by measuring the change in volume of the mixture:
What was the initial volume of cyclohexane? __3ml______.
What is the final volume (after adding the salt)? __4.5mL_.
What is the volume of the sodium chloride? ___1,5ml_____.
2. IS MASS CONSERVED WHEN 2.5 G OF
SALT IS DISSOLVED IN WATER?
Weigh a clean, dry 25 mL measuring cylinder: __70.00g_.
Take 10 mL of water with a pipette and pour it in the cylinder.
Weigh it again, now with the water: __80.00g_.
What is the mass of the water? ___10.00g_____.
What should the mass of water be per gram? (use the internet) _1liter= 1Kilogram 1g=1ml.
Weigh 2.50 g of sodium chloride. Add it to the water and dissolve it.
Weigh the whole apparatus: __82.50g_.
Does the total mass equal the masses of the different parts? __YES______.
Is mass conserved? ____YES____.
What is the final volume of the solution? __11.0mL__.
3. IS VOLUME ´ADDITIVE´ (CAN WE JUST
ADD THE INDIVIDUAL VOLUMES TO GET THE FINAL VOLUME) WHEN 2.5 G SODIUM CHLORIDE
IS DISSOLVED IN WATER?
What was the initial volume of water in part 2? __10mL_.
What volume should be taken up by the salt solution? __2,5mL______.
What is the actual final volume of your sodium chloride solution? 11mL.
Is there a difference between your answer and what you predicted? Explain why there is or might be:
Yes, there final volume should have been 12,5mL, but due to that volume is not conserved in a solution, the actual final volume is 11mL, so as it can be observed the volume is not conserved during the solution, and this time the final volume has decreased 1.5mL.
4. WORK OUT THE MOLARITY AND MOLALITY
OF THE SODIUM CHLORIDE SOLUTION:
Molarity, M (mol/L) = number of moles of solute ÷ volume of solution (L)
Calculate the molarity of your sodium chloride solution (in water):
Solute= sodium chloride = 2.5g ---------) 58.5 grams/mol
2,5/58,5= 0,0427moles.
M = 0,0427/0,011= 3,88mol/L.
Molality, m (mol/kg) = number of moles ÷ mass of solvent (kg)
Calculate the molality of your sodium chloride solution (in water):
m= 0,0427/0,01=4,27 mol/Kg.
BONUS QUESTIONS
Why is it suggested to use the same set of scales for each measurement?
In order for us to perform in an easier way the calculation and trying to avoid any calculation error or misinterpretation, as usually when you use different scales, the weight may change some miligrams. Due to this, it is preferible to use the same scale to avoid those more or less miligrams, that can influence a lot in our experiment.
Briefly design a different experiment that could be used to investigate additive volumes.
1. To determine if mass is conserved when 2.5 g of Magnesium chloride are dissolved in water?
a)Weigh a clean, dry 25 mL measuring cylinder:
b)Take 10 mL of water with a pipette and pour it in the cylinder.
c)Weigh it again, now with the water:
d)Weigh 2.50 g of sodium chloride. Add it to the water and dissolve it.
e)Weigh the whole apparatus.
2. To determine if volume is conserved when 2.5 g of Magnesium chloride are dissolved in water?
3. To determine the molarity and molality of the performed solution;
What are “colligative properties”?
Colligative properties are those properties of solutions that depend on the number of dissolved particles in the solution, but not in the solute, that depend upon the concentration of the solute molecules but not upon the identity of the solute. Colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.
REFERENCES
Helmenstine, T. (2014). What is the difference between molarity and molality? - molarity vs molality. [online] Retrieved from: http://chemistry.about.com/od/chemistryterminology/a/What-Is-The-Difference-Between-Molarity-And-Molality.htm [Accessed: 9 Mar 2014].
Hyperphysics.phy-astr.gsu.edu. (2014). Colligative properties of solutions. [online] Retrieved from: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/collig.html [Accessed: 9 Mar 2014].
Wikipedia. (2014). Conservation of mass. [online] Retrieved from: http://en.wikipedia.org/wiki/Conservation_of_mass [Accessed: 9 Mar 2014].
Please include an introduction and objectives.
ResponderEliminarYou have a mistake in the calculation of molarity (the number of moles is missing a 0.
For the 3 introductory questions, I would like you to use the data from the experimental part to support your answers. Give specific examples.
Formative B - 3 D - 4 E - 5
Summative:
ResponderEliminarB - 5 Again, your information is well communicated but you could explain in a little more detail about the conservation of mass/volume.
E - 6 All data has been processed correctly.