What is the Correct Way to Prepare a 20% NaOH Solution?

• Chemistry
• Rajini
In summary, the conversation revolves around preparing a 20% (w/w) NaOH solution and the correct method to do so. The questioner is unsure if dissolving 20 g of NaOH in 80 g of water or 20 g of NaOH in 100 ml of water would yield the correct concentration. They also ask for the exact definition of weight percentage and clarify that they are preparing the solution for a liquid detergent. The conversation also touches on the safety precautions while handling NaOH and the importance of using distilled water to keep the solution cool. In conclusion, the solution can be prepared by adding 20 g of NaOH to a large volume of water and using the density values for 20% w/w found
Rajini
please post this type of questions in the homework section, filling the template
Homework Statement
This is not a homework question, therefore I don't know what to write here!
Relevant Equations
20% of NaOH solution: The final solution should contain 20% NaOH. Let the final solution needed be 100 ml (=100 g). In that 20% should be caustic soda / NaOH. So 20 g of NaOH dissolved in 100 ml water will give 20% NaOH. 20 g/(100 g)=20% will be NaOH. Is it correct.
or
Adding 20 g NaOH and 80 g water will give 100 g solution. In this 20/(20+80)=20% will be NaOH. So is this the correct 20% NaOH solution?
thanks.
Rajini
Dear All,
To prepare 20% (w/w) NaOH solution is it okay to dissolve 20 g of NaOH in 80 g of water.
or
20 g of NaOH in 100 ml water?
Which is correct?
rajini.
PS: This is not a homework question, I am already 43 yrs!

Last edited:
Nickjohny
What is the definition of w/w%?

Bystander
Borek said:
What is the definition of w/w%?
It is weight percentage. Denoting the concentration by weight. Here the weight of NaOH dissolved in water, expressed in percentage. If it is confusing, please ignore it
Thanks

Rajini said:
It is weight percentage. Denoting the concentration by weight. Here the weight of NaOH dissolved in water, expressed in percentage. If it is confusing, please ignore it

You missed the point.

I asked you about the exact definition. Please list it and we will start answering the question from there.

Part of post from @Rajini
The final solution should contain 20% NaOH. Let the final solution needed be 100 ml (=100 g).
Not Right. Numerical volume and numerical mass for such a final solution will not be equal. You will need to make use of density (or maybe, specific gravity).

symbolipoint said:
Part of post from @Rajini

Not Right. Numerical volume and numerical mass for such a final solution will not be equal. You will need to make use of density (or maybe, specific gravity).
Hi, to prepare 20% NaOH solution, dissolving 20 g of NaOH in little water (~60 ml) and after cooling add further water so that final volume becomes 100 ml. Now this final solution will 20% NaOH solution (20g/100ml=20%w/v). Is this correct? I need to know this detail for liquid detergent preparation.
Thanks

You are moving the goalposts. w/v % is not the same as w/w%.

For most applications the difference is not large and in practice negligible, but as long as you won't try to understand what these numbers mean you will be not able to move ahead.

Bystander
Hi Borek,
Yes. 20 g NaOH in 80 g water gives 20% (w/w) NaOH solution, AND
dissolving 20 g NaOH in ~60 ml water and then again adding water so that the final solution will be 100 ml. This is 20% (w/v) NaOH solution.
Correct?
Thanks

Rajini, through post #7, your description contains some physical risk and likely danger. Your detergents employer better be sure to train you thoroughly for proper safety and handling practices.

Rajini said:
Homework Statement:: This is not a homework question, therefore I don't know what to write here!
Relevant Equations:: 20% of NaOH solution: The final solution should contain 20% NaOH. Let the final solution needed be 100 ml (=100 g). In that 20% should be caustic soda / NaOH. So 20 g of NaOH dissolved in 100 ml water will give 20% NaOH. 20 g/(100 g)=20% will be NaOH. Is it correct.
or
Adding 20 g NaOH and 80 g water will give 100 g solution. In this 20/(20+80)=20% will be NaOH. So is this the correct 20% NaOH solution?
thanks.
Rajini

Dear All,
To prepare 20% (w/w) NaOH solution is it okay to dissolve 20 g of NaOH in 80 g of water.
or
20 g of NaOH in 100 ml water?
Which is correct?
rajini.
PS: This is not a homework question, I am already 43 yrs!
Clarify for us. Is the concentration to be as weight per weight, or as weight per volume?
Would you be using a ~50 % solution of NaOH to start, or would you be using NaOH solid pellets to start?

What training and education for Chemistry have you finished up to now?

Hi,
I am prepare liquid detergent for my self, hobbyist! For the past 3 yrs I was preparing powder, which do not require NaOH. Now I am preparing liquid detergent, which is relatively easy than preparing powder. I am aware of the risks and danger in handling caustic soda and labsa. Already prepared 14 litres successfully. But not fully satisfied. So need to accurately added NaOH solution. So curious to know this calculation. Post 7 correct? Thanks

Rajini said:
Post 7 correct?
No, it is not. That is why I said what I did in post #10. We still wait.

symbolipoint said:
Clarify for us. Is the concentration to be as weight per weight, or as weight per volume?
Would you be using a ~50 % solution of NaOH to start, or would you be using NaOH solid pellets to start?

What training and education for Chemistry have you finished up to now?
Hi, I will prepare NaOH solution using NaOH flakes. PhD in physical chemistry. But my research work was related to synchrotron x-ray characterization of molecules (no chemistry lab work or synthesis). In one website "Add 72.31% of water in the mixing tank and start mixing. Add 5.04% of sodium hydroxide (50% solution) followed by 9.6% of DDBSA"
here I want to know what means NaOH 50% solution?

Rajini said:
PhD in physical chemistry.
My a**...

pinball1970
Hi,
Just want to know post 8 correct or not? I have done for w/w and also for w/v.
PS: in post 8, w/w calculation is correct, just verified.
But in w/v calculation I have doubts.

Last edited:
Bystander said:
My a**...
LOL. Thread closed for Moderation...

Oldman too
After a Mentor discussion, the thread is reopened for now. Thanks for your patience.

Oldman too
Best to add the solid to a large volume of water because a lot of heat is generated. You can use distilled water ice to keep things cool.

symbolipoint
tech99 said:
Best to add the solid to a large volume of water because a lot of heat is generated. You can use distilled water ice to keep things cool.
To add: this isn't one of those things where "it will be fine if you are sloppy". Adding pure NaOH pellets to water too fast will cause it to straight up boil and splash.

symbolipoint
To obtain a volume from a w/wt% you need the density. Then you can determine the mass of 100ml (or other volume)at an appropriate temperature. For 20% w/w the values can be found in many handbooks such as Perry's Handbook. For 20% w/w the values are:
Concentration (% Weight)Density (kg/L)
Temperature in degrees Centigrade (°C)
0°C15°C20°C40°C60°C80°C100°C
 20% 1.2296 1.22183 1.2191 1.2079 1.196 1.1833 1.17

1. How do I calculate the amount of NaOH needed for a 20% solution?

To prepare a 20% NaOH solution, you will need to calculate the amount of NaOH needed based on the desired volume of the solution. The formula for calculating the amount of NaOH is:
Amount of NaOH (in grams) = Desired volume (in liters) x Concentration (in g/L) x Molecular weight of NaOH (in g/mol)

2. What is the molecular weight of NaOH?

The molecular weight of NaOH (sodium hydroxide) is 40 g/mol. This value is important for calculating the amount of NaOH needed for a 20% solution.

3. How do I prepare a 20% NaOH solution from a solid form of NaOH?

To prepare a 20% NaOH solution from a solid form, you will need to first measure the appropriate amount of NaOH using a scale. Then, add the NaOH to the desired volume of water and stir until it fully dissolves. It is important to add the NaOH to the water and not the other way around to avoid a potentially dangerous reaction.

4. Can I use tap water to prepare a 20% NaOH solution?

Yes, you can use tap water to prepare a 20% NaOH solution. However, if your tap water is hard or has a high mineral content, it may affect the accuracy of your solution. In this case, it is recommended to use distilled water for more precise results.

5. How should I store a 20% NaOH solution?

A 20% NaOH solution should be stored in a tightly sealed container to prevent evaporation. It should also be stored in a cool, dry place away from direct sunlight. Remember to label the container with the concentration and date of preparation.

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