Thread: I have a little problem, can you help?

I know this is going to sound daft but.................I have forgot how to do wieghts and measures!
I have run out of a commercially made mix of Sodium Hydroxide and water at 20%. Other wise knows as 20% Sodium Hydroxide.................How many grams Sodium Hydroxide to ml's of water would I need to make up a batch of 250 ml's?

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20% by weight, or volume?*

You've mixed both. You'd need to know the relative weights of each constituent to work out exactly 250ml at 20% by weight.

if it was 20% by volume, that's be easy = 50ml of one, and 200 ml of the other.

if it was 20% by weight, it would be 1 gram Sodium Hydroxide to 4 grams water. As 4 grams water is 4ml, you'd need 50 grams of Sodium Hydroxide to 200 grams of water. Now depending on the relative density of Sodium Hydroxide as compared to water, this may be + or - 250ml, but the concentration would be as you described. (But probably fairly close to 250ml.)

I think I need a lie down now.


*(For the curious - any two constituents are likely to have different densities, to a greater or lesser extent - eg a kg of lead and a kg of feathers are going to occupy different volumes, similarly a litre of lead and a litre of feathers are going to have different weights. On a similar vein, be cynical of anything claiming to be '10% fat free'.)

It actually doesn't state whether volume or weight. But, as I am supposed to mix a dry amount with a wet amount to get the mix I would presume 50 grams Caustic Soda to 200grams of water. The weight of the caustic Soda doesn't come in to the equation

Only mentioned because that way you'd get 250g of solution, but that may not exactly amount to 250ml. (Don't know, never mixed it. Just giving a complete answer and stating my assumptions - have taken too many exams over the past 7 years. )

But if 250g will do and is close enough, then best of luck.