Mass, volumes and solutions(ONLY by:Manuel Pascual & Javier Rojas)

Solutions lab session

During this session you will visit a number of areas of knowledge that you have used before – precision measurements, properties of substances, molarity, molality. You will also be introduced to ideas such as the conservation of mass and additive/non-additive volumes. As you will be measuring a number of different masses, you should try to use the same set of scales for each measurement.

Key questions to answer:

• Is mass always conserved?
• Is volume always conserved?
• What are molality and molarity?

1.    Working out the volume of 2.5 g sodium chloride using cyclohexane.

          - Measure 3 mL of cyclohexane with a pipette and pour it into a dry measuring cylinder.

               Weight the cylinder with the cyclohexane:                              73.50g

               Weight 2.50 g of sodium chloride and place it in the cylinder as well. 

               Weight the whole apparatus:                                                76.00g

- Does the total mass equal the masses of the different parts?    Yes, as we have seen the total             weights equal the sum of each of the components mass individually.

- 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 we have seen in this experiment mass is always conserved, due to matter existing.

- Why does sodium chloride not dissolve in cyclohexane (Hint: which kind of substance are they –           ionic, covalent (organic) or metallic)?

Because it is an ionic bonding, water consists of polar molecules; their polar nature allows             them to overcome the electrostatic forces between ions in NaCl, however cyclohexane as an         organic solvent is a non polar molecule, and non polar molecules aren't able to separate polar       substances, therefore the cannot dissolve ionic substances like NaCl. 

- 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?                             2.5ml

                                                                                                                                         
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)     1ml per each gram of H2O.

- 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 it remains equal
-Is mass conserved?                                                                 

According to our experiment mass is conserved from each material individually and as a whole they are a sum of their masses, according to Lavoisier´s Law (Matter cannot be created or destroyed, so mass is always conserved)
-What is the final volume of the solution?                                     11ml

                                                                                                                                    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?                              10 ml

           - What volume should be taken up by the salt solution?                     2.5 ml

           - What is the actual final volume of your sodium chloride solution?     11 ml

-  Is there a difference between your answer and what you predicted? 

No, because it’s impossible to lose a perceptible quantity of volume or mass in this simple reaction, so according to this, the volume will stay the same, just like it happened.

- Explain why there is or might be:

There could be some different results because of the human error: not exact measurement (we could use an digital measuring system), accuracy of materials ( and always new material properly cleaned), or not being precise adding the sodium chloride, NaCl, to the solution (human eye does not differentiate the meniscus properly). Also there might be some anomalies in the environment , which could affect the reaction, such as:
* a non constant temperature
* a non sterilised environment
* non properly cleaned materials 

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):

0.04 moles/0.0115=3.47M

Molality, m (mol/kg) = number of moles ÷ mass of solvent (kg)

Calculate the molality of your sodium chloride solution (in water):

0.04/0.01=4m

Conclusions:
- Mass in universe is always conserved due to Lavoisiers Law, you cannot destroy matter, but you could change its appearance or state (solid, liquid or gas) by altering the natural factors involving it, but the universe will always have the same mass.

- Volume as well is constant however for salt it does not stays in the same state due to the energy produced in a chemical reaction some of the compounds evaporate slightly, and during this experiment we weren't able to measure gases and liquids volume together. Volume is not always conserved because in all compound there is always a very low amount of particles with enough energy to change of state, that is the volatility described in Maxwell Boltzmann curve showing energy of molecules of a substance and which particles have the enough energy to evaporate without reaching the BP temperature.
 
-Molarity(M) and Molality(m) are units which measure the concentration. Molarity measures the moles of solute per litres of solution, while Molality measures the kilograms of solute per litres of solution. Both measuring units are important in reactions, in aqueous solution because affects reaction rates.

Bibliography:

*Chem.wisc.edu. (2014). [online] Retrieved from: http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/Text1/Tx14/tx14.html [Accessed: 25 Feb 2014].

*Canning, O. (2014). [online] Retrieved from: http://mrcanning10c.wikispaces.com/Topic+1a+-+Properties+of+substances [Accessed: 25 Feb 2014].

*Molality, M. (2014). Molarity - molality - chemistry video by brightstorm. [online] Retrieved from: http://www.brightstorm.com/science/chemistry/chemical-solutions/molarity-molality/ [Accessed: 25 Feb 2014].

*Conservationphysics.org. (2014). Conservation physics: designing cold storage for film. [online] Retrieved from: http://www.conservationphysics.org/coolfilm/bfi_lecture.php [Accessed: 13 Mar 2014].