[tressol]

I'm returning to school after two years so I ned help catching up with formulas in math and chemistry

[Unknown008]

You are adding 25 g of powdered alcohol to 100 mL of water?

To be able to determine the percentage of alcohol, I need to know what alcohol you are using. I guess that you are referring to ethanol.

The relative molecular mass of ethanol is 46.

Thus, number of moles of ethanol in water = 25/46 = 0.543 mol

So 100 mL of water contains 0.543 mol of ethanol.
1000 mL of water will contain 5.43 mol of ethanol.

Hence, concentration = 5.43 mol/L

Were you looking for something more specific? :)

[DrBob1]

When a known mass or volume of solute is added to a known volume of solvent we lose all knowledge of the final volume of the mixture. Volumes do not add! In this case we would certainly expect more than 100 ml of solution -- those 25 grams of ethanol (let's assume that is what it is) have a volume of their own. If, for the sake of argument, we assume that 25 grams ethanol is exactly 30 ml, the mixture will not have a final volume of 130 ml. THE VOLUMES DO NOT ADD. This eliminates molarity (moles of solute per liter of solution) as a measure of concentration..

We do know the mass of the solute and solvent however, so we can do some calculations with certainty: there are 5.43 moles of ethanol per KILOGRAM of water, so the solution is 5.43 MOLAL. This is not particularly useful -- that's why we almost always use MOLARITY.
Additionally we can calculate that there are 25 grams of ethanol in 125 grams of solution, so it is 20% ethanol (w/w). This would probably be the best measure of concentration, but using weights of liquids is a pain!
Fortunately, masses do add!

[cleggy0151]

simply use the fact that 1ml of water weighs 1g.

So, we have 100g of water,

we can now use a simple % calculation. (25/100) * 100 = 25%