NOTES FROM THE LAB:
mass and volume
electronic scale: number of figures after the coma
volume
Materials: volumetric flask, beaker, pipette, measuring cylinder
Volume of water: 10mL= 9.62g
C: Salt dissolves in water because of its chemical bonds.
PROCEDURE AND RESULTS
We took 10 mL of cyclohexane and 2.5 grams of sodium chloride. We poured them in the same dry measuring cylinder. The total volume was of 11.5 mL
The variation when we poured the NaCl was of 1.5 mL.
As it does not dissolve, we can work out the volume of the NaCl by measuring the change in volume of the mixture.
They didn’t dissolve so the mass was the same, so the volume will be the sum of both of them. The final volume was 11.5 mL. Volume of NaCl: 1.5 mL.
We see that NaCl doesn’t dissolve in hexane, but we will see that it does in water. Why? One of them (NaCl) is a polar molecule and the other one (hexane) is a non-polar molecule. Bonds.
“Matter cannot be created or destroyed, so mass is always conserved”. Our data agrees with this statement as the mass of hexane plus the mass of salt was the same as the mass of the solution:
Mass of hexane:
Mass of salt: 2.5 grams We sum them: ___
Final mass of the solution: ____
2. Is mass conserved when 2.5 g of salt is dissolved in water?
Weight a clean, dry 25mL measuring cylinder. 73. 68 grams
Take 10 mL of water with a pipette and pour it in the cylinder. Weigh it again, now with the water.
What is the mass of the water? 10 mL of H2O weight 9.62 grams.
What should the mass of water be per gram? It should be 1 mL per gram but as it is not pure water we can observe that the mass is different.
Weigh 2.50 g of sodium chloride. Add it to the water and dissolve it.
Weigh the whole apparatus: 85.72 grams
Does the total mass equal the masses of the different parts?
Total mass: 85.72- 73.68= 12. 04 grams→ mass of the solution
mass of water: 9.62 grams, mass of salt: 2.5 grams. 9.62+2.5= 12.12
Is mass conserved? As we can see mass is conserved mostly, the 0.04 grams that varied may be due to human errors.
So mass is always conserved, but as we will see something different happens with volume.
What s the final volume of the solution?
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?
The volume of the mixture was smaller than the sum of both separately.
The answer is no, the volume of the water sums up to the volume of salt or sums up to the hexane is not the same as the total and final volume of the solution as in the final volume, salt is dissolved. This may have been because as the salt bonded with the water molecules, less space was occupied.
Demonstration:
What was the initial volume of water in part 2? 10 mL
What is the actual final volume of your sodium chloride solution? 11 mL. The final volume is different to the one we predicted, is less for the reason explained before. If 1 gram of NaCl is almost 1 mL of water (0.87 mL), the volume must be much more high if it was the sum of solute and solvent. The volume of salt should be 2.1 mL and the difference was of just 1 mL.
(11 mL , Vsolution, - 10mL ,Vwater, = 1 mL)
