Sodium Hydroxide: How to make to 0.5 M strength

A. Hi Dr. Paul. The statement is incorrect.

We've now added a 'Tutorial' sidebar at the top of the page explaining the concept and offering the bottom line answer that a 1.0 M solution of sodium hydroxide contains 40 grams of sodium hydroxide per liter (1000 mL) of water, and the rest is just arithmetic.

Luck & Regards,

A. Hi Connie. We explain on this page how/why a 1.0 molar solution is 40 g of NaOH per 1000 mL, and that a 0.5 molar solution would be 20 grams per 1000 mL. If that doesn't do it for you, what step in the arithmetic are you having trouble with?

But remember that "How [much] water must be added ... to NaOH" is a dangerous way to pose the question! You add the NaOH to the water, you never add water to NaOH.

Luck & Regards,

Much appreciated! Thank you!

A. Hi Dozer. That would depend on how much of it you need to prepare, wouldn't it?

When you understand what a "1 mol" solution actually means, and what a "0.5 mol" solution actually means, the answer will be very obvious, and we are happy to take as much time trying to explain this as you need. But it would be a disservice for us to tell students whether to divide by two or to multiply by two so they can answer their homework questions without them putting in any effort to understand the question. Let's put in equal parts here.

Luck & Regards,

A. Hi cousin Angel. A "half molar" solution is exactly the same as a "0.5 M" solution, so this was already answered by 4 helpers. If you remain confused, please explain in what way these answers are proving unsatisfactory. Thanks.

Regards,

A. To make 0.5 M of NaOH, measure 20 g of sodium hydroxide and dissolve it in 1000 ml of water; we have used 20 g because the RMM [relative molar mass] of NaOH is 40 and we are making 0.5 M solution.

Jelo, I know that you know that to make half as much quantity of anything, you just use half as much of the ingredients.
Maybe these students are just pulling our leg now, rather than being unable to divide by two. Thank god I didn't go into teaching; I wouldn't last the first day :-)

Regards,

A. Hi Aijazullah. It's obviously either experimental error or a jar which contained reactable aluminum. Good luck.

Regards,

A. Hi Tammy. As you can see from the 5 explanations above, your 1N solution is 40 g / 1000 g or 4% by weight. You could heat it and evaporate the water until it was 20% NaOH by weight, but I doubt that evaporating the water from a dilute caustic solution to make a more concentrated and dangerous one would be approved by your university these days. Good luck.

Regards,

A. Hi cousin Umahi. The calculation for this has been explained 6 times on this page now. Please take the time to read it then tell us what it is that you are not understanding. I'd be worried about the sufficiency of my haz-mat training if I were you because this is pretty basic stuff you should know before you start working with hazardous chemicals. Good luck.

Regards,

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A. The best way is to make a 50% w/w solution of NaOH in DI water, allow it to cool, let any solids settle out (Na2CO3) and then weigh out 8.0 g of this prep into a volumetric flask. Make up to the mark with DI, with lots of swirling, let cool, then make up to the mark again.

For maximum accuracy, a standardization vs. KH pthalate would be advisable. Check "Standard Methods" for this procedure.

This is really kind of basic Chem 101 stuff, if I may say.

A. Hi Stan. I guess you're saying you have a solution with 50 grams of NaOH for every 1000 mls? And you would like to dilute it to the point where you have only 15 grams of NaOH for every 1000 mls?

So for every 15 mls of solution of 5% solution you wish to dilute, add 35 mls of water to end up with 50 mls of 1.5% solution.

Regards,