3 questions for solutions lab

• Is mass always conserved?

As we know mass is always conserved because Lavoisier’s law. He says that “In a chemical reaction, matter is neither created nor destroyed”. So, he says that mass is always conserved.

Example of the experiment: We needed to add 2,5 g to 73,5 grams of cyclohexane and our final volume was 76 grams, so mass is conserved because if we sum both (73,5+2,5) we have a result of 76 grams. This is Lavoisier’s law.

Chemteam.info. (2014). Chemteam: lavoisier and his law. [online] Retrieved from: http://www.chemteam.info/Equations/Conserv-of-Mass.html

• Is volume always conserved?

No, volume is not always conserved in a chemical reaction. Mass is always conserved but not volume. There’s not a volume’s conservation law. For example, if we want to mix two different chemicals the final volume is not conserved.

When we did the experiment, we needed to mix 2,5 grams of sodium chloride (NaCl) with 10 mL of water. Our final volume 11 mL and not 12,5 mL. With this example, we can deduce that volume is not always conserved.

• What are molality and molarity?

We use molality and molarity for concentration of solutions.

- Molality (m): Moles of solute/ Mass of solvent (Kg).

We need to remember that the number of moles is: mass (g)/ molecular mass.

Example: Calculate the molality of your sodium chloride solution (which have a mass of 2,5g) in water (which is 10mL):

Firstly, we need to do do the number of moles and then we need to pass the volume into L:

m=0.0427/0.01=4.27 mol/kg

- Molarity (M): Moles of solute/ volume of solution (L).

The unit is mol/L

Example of the experiment:

Calculate the molality of your sodium chloride solution (which have a mass of 2,5g) in water (which is 10mL):

NaCl mm=58.44 g/mol

58.44/1=2.5/x à 2.5/58.44=0.0427 mol

M=0.0427/0.011=3.881͡8