Why would the concentration be less?

I thought if NaOH absorbs water it's mass would increase so the moles of the solution would increase as mol = m x Mr so the concentration will increase since conc = mol/volume so why?

When you are weighing it out you are actually weighing out less moles of NaOH per gram mass weighed.

Less moles? How?

Some of the mass that you have measured out is water, not NaOH.

Say for example that you have a sample of dehydrated NaOH that weighs 100g and a sample of hydrated NaOH that is 75% NaOH, 25% water (by mass), that also weighs 100g. The second sample actually contains only 75g of NaOH. If you dissolve both of the samples in water, the dehydrated one will be more concentrated because it contains more NaOH.

So then why is the concentration of NaOH even affected?
Suppose we have 2 samples of NaOH both 100g. One in a sealed container with vacuum/no moisture and the other in the open air or in water why would the concentration of the second NaOH sample decrease?

So then why is the concentration of NaOH even affected?
Suppose we have 2 samples of NaOH both 100g. One in a sealed container with vacuum/no moisture and the other in the open air or in water why would the concentration of the second NaOH sample decrease?

Concentration = moles/volume

Moles = mass/Mr

Concentration = mass / (volume*Mr)

In a pure sample, you'll use X amount of NaOH. It will contain X amount of NaOH. So you have an accurate concentration.

In an impure sample, you'll use the same amount, X of NaOH. But Some of it will be water. Let's say Y is water. X-Y is the mass of NaOH. So the concentration of the solution has decreased.

I think I kinda get it but I'm not sure. I mean overall mass of X-Y > X so total conc of X-Y also > X but you're saying conc of X in X-Y < X? How?

Where did you get X-Y > X? By definition, X-Y < X

The mass of NaOH is X-Y, which is < X. The overall mass is still X.

X is the amount of NaOH you need
Y is amount of water.

Concentration can only be found of a specific component of a solution. So you can not count Y in the concentration of NaOH. Again X-Y < X

You have to remember, you have no idea how much water is in your sample. It could be 2% water, 5%, 10%... literally anything. So you can't adjust your amount to take into account water mass. You make and use the assumption, which is untrue, that it is 100% NaOH.

I know I'm being such a pain right now but just 2 small qs. Thank you so so much 😊
You said X-Y < X. How? Why?

Imagine X is just a number. X, minus a small amount Y, is X-Y. The result of X-Y, must be smaller than X right? It's just subtraction.

So X is the amount of NaOH that you need. You take this from your impure sample/ bottle. Imagine this is 50g.
Y is the water in your sample. Imagine this is 5g.
The actual amount of NaOH in your sample, is X-Y, 50-5 = 45g.
45 < 50, therefore X-Y < X right?

Does this make more sense?

Oh ohhh you were saying X MINUS Y! I'm such an idiot. I was taking it as X dash Y. I thought X-Y was the new NaOH which absorbed the water. It makes perfect sense now. God bless yer amazing soul! Thanks sooo much