What formula can I use to calculate the salinity percent in a sample of water
that I am adding salt to? Is is salinity = salt mass / total mass?
There are several common ways chemists calculate concentrations:
This is the concentration in moles per liter. A certain number of moles is
placed in a flask (either by weight or by volume from a solution of known
concentration), and the sample is diluted to a certain volume. The number of
moles divided by the number of liters is called the molarity.
This is the most commonly-used type of concentration specification in
- weight/volume percentage
- A sample of known mass is diluted to a particular volume. 100% is defined as
1 gram of solute per milliliter of solvent, so (for example) 5g diluted to 100
mL gives a 5% solution.
Precise workers will call this sort of solution a w/v solution.
- This is much less commonly used, but is a handy unit in some situations
(typically when we don't have volumetric glassware of the proper size!). In this
type of solution, a certain number of moles of solute is dissolved in a certain
number of kilograms of solvent, so (for example), 0.01 mol of NaCl dissolved in
10g of water is a (0.01 mol/0.01 kg) = 1 molal solution.
- weight/weight percentage
- This is analogous to molality, and is usually used when preparing mixtures
of two solutes in a solvent. w/w percentage is defined as grams of
substance A divided by grams of substance B, times 100%. For example, a
photopolymerization experiment I ran the other day had the monomer and the
initiator both dissolved in water; the initiator was present as 1.5% w/w with
respect to the monomer: I used 0.015 g initiator for every gram of monomer.
You can also prepare solutions w/w with respect to solute and
solvent, but that's much less useful than w/v.
Here's how it works, bottom line: most solutions are prepared so that a certain
volume will deliver a certain amount of solute. That way they're the most
convenient to work with. Therefore we typically prepare solutions either by
molarity or by w/v percentage.
To get back to your question, for chemists salinity is typically quoted as grams
of salt per 100 mL of water: that's the way w/v percentage is defined.
BUT... them medical types just have to be different.
They insist on quoting salinity as a w/w percentage,
grams of salt per grams of water. See