Question

Describe how you would prepare 250.0 mL of a 0.1500 M NaCl solution given that the NaCl is supplied as a solid.
Describe how you would prepare 1.25 L of a 0.025 M sucrose solution from a stock solution with concentration of 5.00 M.
What is the benefit of adding salt to water when cooking rice or pasta? Explain your answer by discussing changes to vapour pressure.

Answer 1

1) Molecular weight of NaCl=58.44

So 58.44g of NaCl in 1000ml gives 1M solution.

Quantitiy of soicum chlroide required to prepare 1000ml of 0.1500M = 0.1500/58.44/1=8.766g

Quantity of sodium chloride required to prepare 250ml of 0.1500M NaCl solution= 8.766/4=2.19g

2) Formular for volume calculation: V1M1=V2M2

V1= volume of sucrose solution to prepared, M1= molar concentration of sucrose solution to be prepared,

V2 is the volume requaired for preparing above solution and N2 is molar concentration of the stock solution.

So V2= V1/M1/M2= 1.25X0.1500/5=0.00625L=6.25ml.

Take 6.25ml of 5M stock solution and dilute upto 1.25L.

3) By adding salt to water, vapour pressure of water decreases. This is due to the higher electronegetivity of chloride ions present in the solution. This will not allow water molecules vapourise easily. Since the vapour decreases boling point of water increases, thereby increasing the cooking temperature. This will result in faster cokking.