George wishes to increase the percent of acid in 50ml of a 15% acid solution in water to 25% acid, how much pure acid must you add?

25% - 15% = 10%

10% of 50 is 5.

You would need to add 5 more ml of pure acid.

I'm not 100% on this answer, but I tried.

Amount times concentration=quantity

50 (0.15) + x (1) = (50+x) (0.25)

x=amount of acid to add

so

7.5+x=12.5+0.25x

minus 0.25x both sides

7.5+0.75x=12.5

minus 7.5 both sides

0.75x=5

divide both sides by 0.75

x=6.666666666

converts to 6 and 2/3

you have to add 6 and 2/3ml of pure acid