PERCENTAGE CONCENTRATION:
You have to be very careful with percentages because
they can mean several things:
1 - percent by weight (weight/weight percent):
number of grams of solute per 100g of solution.
For example, a 0.9% solution of NaCl means that there
is a ratio of 0.9g of NaCl per 100g of solution.
2- percent by volume (volume/volume percent):
number of volumes of the solute in 100 volume of solution.
It can be used when the solute is another liquid.
For example,
a 20% solution of alcohol means that there is
20 ml of alcohol in 100 ml of solution.
3- weight/volume percent:
number of grams of solute in 100 ml of solution.
This is an hybrid expression because their units weight/volume don't cancel
(ordinary percentage have no units because units in the fraction cancel each other).
This percentage is used a lot in Physiology because
it is a very practical way to write recipes of solutions.
For example in this case, a 0.9% solution of NaCl means that there
is a ratio of 0.9g of NaCl per 100 ml of solution.