I'm about to try to make my own soap for the first time. In order to create my recipe, I'm using this calculator. As I'm using a lye solution, I have to fill the box %w/w of the solution. The calculator says:

If you are using a ready made lye solution, check the box above and enter the enter the strength of your solution. The solution strength needs to be measured as a percentage of lye weight to water weight (% w/w).

I'm a bit confused here, I would have thought the w/w ratio was the ratio of the mass of NaOH in the solution to the total mass of the solution (and not the water mass used to disolve NaOH). So my first question is: what is really that w/w ratio?

My second question is how to interpret what I read on the lye solution (1L) I've bought. It's written:

Sodium hydroxyde - N°CAS: 1310-73-2: 30%

What does this 30% refer to? Is this a ratio of volumes? Of mass? Is this the w/w ratio I want to know?

Thanks for helping me clarify this!

  • 1
    $\begingroup$ CAS: 1310-73-2 is a chemical ID number for sodium hydroxide. The 30% would be 30% sodium hydroxide by weight of solution. So 100 grams of solution would contain 30 grams of sodium hydroxide. // No idea what the doofus lye calculator is doing. $\endgroup$ – MaxW Sep 30 '17 at 13:59

Whatever that lye calculator is doing it is either wrong by design or internally correct and using a very obscure definition that no other chemist would understand.

Percentage solutions are always (barring calculators programmed by laymen) defined as weight of the solute divided by weight of the solution times 100. So a $\pu{30\%}$ solution of $\ce{NaOH}$ means $\pu{30g}\ \ce{NaOH}/\pu{100g}\ \text{solution}$.


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