Help on saturated solutions for washing....

[Mercury]

In MM's benzo wacker and Al/Hg, the procedures call for washing the product extract with saturated solutions, specifically, 500ml saturated sodium bicarbonate for benzo and 500ml saturated sodium chloride for Al/Hg.

What are the amounts required for each solution? I understand that 'saturated' means the maximum amount of solid that will dissolve in the water. Is it as simple as 250g of NaCl and 250ml water to make 500ml saturated NaCl?

She is curious since it would be time-consuming to determine when the solution is saturated from trial and error.

[terbium]

To prepare a saturated solution you simply add more solute (sodium chloride, sodium bicarbonate etc.) than will dissolve. You will know that you have added more solute than will dissolve when, even after shaking, there is still undissolved solute in the solution bottle. You then just leave the excess solute in the solution bottle, allowing the solute to settle to the bottom, and pour off saturated solution as needed.

[Mercury]

If a procedure calls for 500ml of saturated solution, then would one start with 500ml of water and add solute to the water until no more can dissolve? I'm just confused about the amount of water to use because to make, say, 100ml 5% NaOH solution, you would need 5g of NaOH and 95ml of water.

[Rhodium]

You take a 1L bottle and add 350g NaCl, then fill the rest of the bottle with water. Shake now and then for 10-15 minutes. Now the solution is saturated with salt, and there is still some undissolved salt at the bottom. Then use 500mL of the solution in your procedure, and you will also have some left for use later. Refill with salt and water as necessary.

It's too much work to prepare a precise amount each time you need to use it. I have bottles with 10, 25 and 50% NaOH prepared beforehand too for use in whatever procedure.

[Flippie]

I'm just confused about the amount of water to use because to make, say, 100ml 5% NaOH solution, you would need 5g of NaOH and 95ml of water.

No need to get confused. To make a 5% solution you take 5g solute (e.g. NaOH) and add solvent (e.g. water) till a final volume of 100 ml. Solutions are usually made weight per volume instead of weight percent for reasons stated below.


Weight percent = weight of solute (g) x 100
                         weight of solution (g)

Weight/Volume percent = weight of solute (g) x 100
                                    volume of solution (mL)


Weight/volume percent is used as a quick and easy concentration unit because volumes are easier to measure than weights and because the density of dilute solutions is generally close to 1 g/mL. Thus, the volume of a solution in mL is very nearly numerically equal to the mass of the solution in grams.

She is curious since it would be time-consuming to determine when the solution is saturated from trial and error.

Of course being a good chemist she takes notes in her lab book and she remembers next time how much it takes to make a certain saturated solution. Of course in the catalog of a big chem supplier you can often find the maximum amount of a certain solute in mol you can dissolve in water at 25°C.

Conclusion:Concentrations of solutions are given weight/volume percent unless stated otherwise and are made by weighing the solute and adding solvent till the final volume has been reached.

If I remember well a bit less than 6 mol of NaCl can be dissolved in 1 liter of water at 25°C. Since the molecular weight of NaCl=58.44 g/mol one can dissolve about 350 g/l.

[Mercury]

That sounds cool. Thanks Rhode.