Molarity and Molality (lab report) Maria M, Kristina P, Victor S and Alvaro P

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? 

Mass is always conserved in chemical reactions. This just means that stuff can’t be created or destroyed just because the atoms reorganised themselves. So if you weigh your magnesium ribbon, then burn it, then weigh it again, it will have got heavier, and the amount it has gained is the mass of oxygen you used from the air in the room in the burning. But Einstein’s famous equation E=mc2 us how much energy we will get if we do manage to convert some mass into energy. In a nuclear reactor or in the sun a tiny amount of mass disappears and is converted into energy. If you multiply the amount of mass destroyed by the speed of light squared, you get the answer for how much energy you’ve made. Since the speed of light is a really big number, the amount of energy is enormous.

·                Is volume always conserved?

No, volume is not conserved in chemical reactions. Volume isn’t conserved during dissolving because the water (or solvent) molecules ‘’make room’’ for the substance that is dissolving (the solute) which means the volume can decrease, increase or stay the same depending on the substances. There is no conservation of volume.

·                What are molality and molarity?

 Molarity is the number of moles of a solute dissolved in a liter of solution. The molar unit is probably the most commonly used chemical unit of measurement.

Molality is the number of moles dissolved in one kilogram of solvent. The molal unit is not used nearly as frequently as the molar unit.

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)?

This is an ionic bond. The water is made up of polar molecules, so they overcome the forces between the ions in this tyoe of bond. Cyclohexane, however is an organic solent, which is a non-polar molecule. They aren’t able to separate polar substances, so they cant dissolve ionic substances.

-       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 / 1g 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

-Is mass conserved? If we use this experiment to answer the question, yes, the mass is conserved. We can relate this to Lavoisiers law. (No Matter cant be created or destroyed, so mass is always conserved).

-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?                              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?     12.5 ml

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

No, volume cant increase or decrease if the experiment has been done properly, because you cant lose it for no reason. Therefore, the volume will be the one we predicted.

- Explain why there is or might be:

There are different factors that might change the volume of the solution, but these factors will only take place if the experiment has been done incorrectly or not taking care of the material and substances we use. Examples of the reasons of the change in volume could be:

* The weigh scale wasn’t put to 0 before using

* The materials haven’t been cleaned

* Spilling the solution, of damaging any of the substances that compose it.

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 cannot be created or destroyed (Lavoisiers Law), but it can change its state (solid to liquid, liquid to gas…).

·       The volume is also constant, but in this experiment, due to the evaporation of some of the elements, we cant measure liquids’ and gases volume together.

·       The molarity and molality measure concentration. This is very important in reactions, because the concentration of each substance can affect the rate of reaction.