Chemistry Reference
In-Depth Information
This means that 100 g of water at 90
o
C can usually hold about 40 g of
dissolved table salt. If you add less than 40 g of NaCl to the 100 g of H
2
O at
90
o
C, you get an unsaturated solution. An unsaturated solution is one that is
holding less solute than it normally can, at that temperature. If you were to
add more salt to this unsaturated solution, it would dissolve, up to the maxi-
mum solubility, when you would get a saturated solution. A saturated solution
is one that cannot dissolve any more of the given solute under the current
conditions. Do you remember ever drinking hot chocolate and finding the
“residue” on the bottom of the cup? This is evidence that your hot chocolate
represented a saturated solution. When you add additional solute to a satu-
rated solution, the additional solute will fall to the bottom of the vessel.
It is possible, under the right conditions, to produce a supersaturated
solution. A supersaturated solution is one that is holding more dissolved sol-
ute than it should be able to under the current conditions. For example, if
you create a solution that contains 60 g of NaCl/100 g of H
2
O at 90
o
C, it
would be holding 20 more grams of NaCl than it should be able to. Creat-
ing a supersaturated solution can be a little tricky, and the resultant solu-
tion is unstable, but the practical applications of such solutions include
producing rock candy.
Because the concentration of a solution is so variable, we need two
ways to indicate how much solute is in a particular solution. There are sev-
eral ways to measure the concentration of a solution, including molarity,
molality, and mole fraction. The type of measurements you use will often
depend upon the situation or on the calculations that you want to be able
to carry out.
Molarity is probably the most common measurement for the concen-
tration of a solution. Molarity is a measure of the number of moles of sol-
ute dissolved in every liter of solution. The formula for molarity is:
Molarity (M) =
moles of solute
liters of solution
Molarity is measured in moles/L, but we use the symbol “M” for short,
referring to the unit as “molar.” So, for example, if we dissolved 3 moles of
solute in 1 liter of solution we would write that we have a 3M solution, and
we would say that the solution is “3 molar.” Let's try a straightforward
example.
Example 1
Calculate the molarity of a 2.0 L solution made with 3.0 moles of
NaOH.
