Alright, let's start with an X% solution by volume.
- Say you want a 3% NaCl aqueous solution b/v(this stands for "by volume")
to make it you would choose a really convenient final volume...like 100mL:
3% of 100mL is 3 mL, so you would put 97 mL of water in a graduated cylinder(not what you'd really
use but it's convenient); then you would add NaCl until the meniscus read 100 mL, a change of 3 mL.
Now an X% solution by weight.
- If you instead wanted a 3% aqueous solution b/w(by weight), you would again choose a really
convenient final mass for the solution, like 100g. 3% of 100g is 3g. So you would weigh out 97g of
water into a beaker, then weigh out 3g of NaCl and add it to the water, giving you 100g of solution.
Think about the difference between the amount of NaCl you would need to add to each solution.
That should help clear things up.