Lab session 3-2-2014

LAB SESSION 3-2-2014
PRECISION MEASUREMENTS MASS VS. VOLUME

Materials:
- Pipette
- Measuring Cylinder
- Test Tubes
- Test tube holder
- Scale
- Beaker
- Spatula
- NaCl
- Water
- Cyclohexane

PROCEDURE:

What happened? Why?
A) 10,0 mL of H2O have a mass of 10,0 grams.
B) 2,5 grams of NaCl have a volume of 1,0 - 1,5 mL.
C) 2,5 grams of NaCl in 10 mL of water shows a volume of 11,0 mL.

A) EXPERIMENT 1: 10,0 mL of H2O have a mass of 10,0 grams.

1) First, we measured the wieght of the empty measuring cylinder, which was 76,3
2) Then, we pour 10 mL of water with the pipette in the mesauring cylinder.
3) After that, we weight the measuring cylinder with the water in it.
4) Finally, we calculated the difference between the mass of the measuring cylinder with water and the mass of the empty measuring cylinder.

In our case, first we did it but the difference was 6,9 grams. The mass of the measuring cylinder with water in it was 83,2 grams and the measuring cylinder without water weight 76,3 grams. So the difference, that as firstly said was 6,9 grams was not correct because logically it needed to be 10 grams. So we had to do it again and on the second time, the mass without water was of 76,5 grams. The mass of the measuring cylinder with water was of 86,4 grams. This time, the difference was of 9,9 grams.

First time : 83, 2 - 76, 3 = 6, 9
Second time : 86, 4 - 76, 5 = 6, 9

EXPERIMENT 2:

Then, we did the same experiment but instead of water we used salt (NaCl)
1) This time our measuring cylinder weight 70, 8 grams.
2) Then we put 2,5 grams of NaCl in the measuring cylinder.
3) After that, we measured the mass of the measuring cylinder with the salt (NaCl) and weighted 73,4 grams.
4) Finally we calculated the difference (73,4 - 2,5) that was 2,6 grams, so that meant that the experiment was well done because the difference was very approximated to the grams of salt that we put in the measuring cylinder.

73, 4 - 70, 8 = 2, 6

B) 2,5 grams of NaCl have a volume of 1,0 - 1,5 mL.
In this experiment we used NaCl mixed with cylohexane (C6H12)
1) First, we took 2,5 grams of NaCl (measuring it first on the balance) and pour it on the measuring cylinder that weight 70,8 grams
2) Then, we pour 3,0 mL of cyclohexane (C6H12), measuring it first with the measuring pipette.
3) Finally, we calculated the final volume that was 3,0 mL.

In this experiment, the cyclohexane and the salt couldn't dissolve.

C) 2,5 grams of NaCl in 10 mL of water shows a volume of 11,0 mL.
In this third experiment also used NaCl mixed with water.
1) We put 2,5 grams of NaCl in the measuring cylinder.
2) Pour 10,0 mL of H2O.
3) The final volume was of 12,5 mL
4) The total mass was of 75, 6 grams.
In this experiment, the water and the salt could dissolve.


CONCLUSIONS:
In our results, we could prove that water could dissolve with sodium chloride (NaCl) but cyclohexane couldn't dissolve with NaCl. Water could dissolve with the salt (NaCl) because they have a positive and negative, so they are both polar substances, so they are ionic compounds. Polar substances attract each other, so in this case, they are easier to dissolve. On the other side, we could prove that hexane could not dissolve with NaCl because they are not polar substances, so they are not negative and positive charges. NaCl and cyclohexane are covalent bonds so its hard to dissolve them.
In this experiment, we have seen different changes in the volume; sometimes the volume stayed the same, however we had difficulties to prove that it had to be the same, for example in water. In the second experiment, the volume of the cyclohexane is 3,0 mL. When we mixed water with NaCl the final volume was of 12,5 because the amount of water, that first was 10,0 mL had dissolved with NaCl that wa 2,5 grams (the same as 2,5 mL) so the total volume was of 12,5. In the other case, cyclohexane and NaCl could not dissolve so it just rested the amount of cyclohexane, that was of 3,0 mL.

In this experiment, the measurements are not very accurate so this is because we could have had some problems during the experiment, such as not calculate well the volumes, or not do it very precisely so some measures were not very correct, as we had proved in the first experiment. However, they are realistic values and we proved them. To improve it we could do it more precisely with more time, concentration and ability. We could also impriove it by taking more pictures, that can complete our lab report.