Question

How to prepare a percent solution by mass and also by volume from a stock (in molarity and in percent.

I need to prepare a 5% NaOH solution from a stock solution which is 10M NaOH. I also have a stock solution which is 10% NaOH. Which stock solution should I used and can you guide throughout the calculations. Thanks!

Answer 1

Its very easy to prepare %solutions as it does not involve any conversion and calculations relating to molar mass, valency etc.

If you want to prepare x% of given compounds solurion just take x gram of that compound and dissolve in 100 ml of solvent.

If you want to prepare 5% solutions as a fresh you should take 5 gm of NaCl and dissolve in solvent (when solvent not mentioned, it is considered that water is solvent)

Now you already have two different solutions so you dont need to prepare fresh solution.

The one is 10% and another is 10M.

Now 10% solution has 10 gram of NaCl per 100 ml water (it is not necessary to have 100 ml each time) you can also prepare any of desired volume. Just the % should remain same.

Now suppose you have 10% solution and you want to coonvert it to 5%. First decide what is your required amount of this 5% solution. If you require x ml of 5%, withdraw x/2 ml of 10% solutions and dilute it to x ml. Dilution doubles and concentration becomes half. (10% becomes 5%)

You can also prepare 5% from 10 Molar solution but first you need to count how many grams are there in 10Molar solution of NaCl.

1 mole has 40 gram in 1000 ml

So 10 mole means 400 gram in 1000 ml

Means 10 mole means 40 gram in 100 ml

So it is 40% solution. Now you want to convert it to 5%

It is clear that you have to dilute it 8 times. So withdraw amount requre and dilute to 8 times.

You will see that % to % dilution is much east than molar to % conversion.

Also remember that you cannot prepare higher % or higher molar solution feom lower % or lower molar solution respectively.