Illustrated Guide to Home Chemistry Experiments

Chapter 7 Laboratory: Solubility and Solutions 133

LABORATORY 7 .2:

mAkE Up A moLAL SoLUTIoN of A SoLId CHEmICAL

In this laboratory, we make up 100 mL of

a 0.5 molal stock solution of potassium

hexacyanoferrate(III) (potassium ferricyanide),

which is used in several of the other labs in

this book. Although we won’t standardize the

solution, we will make every effort to achieve an

accurate concentration by weighing the masses

of the solute and solvent carefully.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing papers

£ beaker, 150 mL

£ funnel

£ eyedropper

£ wash bottle (distilled or deionized water)

£ labeled storage bottle

£ potassium ferricyanide (16.63 g)

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute a foam cup or similar container
  for the 150 mL beaker. In fact, because the combined
  mass of the potassium ferricyanide and 100 g of
  water in a one molal solution is 116.63 g, using a glass
  beaker may exceed the maximum capacity of your
  balance. In that case, a lightweight foam cup is a
  better choice.

As always when making up a solution, the first thing to
determine is the concentration of a saturated solution of the
chemical to make sure the chemical is soluble enough to
make up a solution of the desired concentration. From that
information, we can calculate how much potassium ferricyanide
is required. Proceed as follows:

1. Looking up potassium ferricyanide in a reference book,
   we find that its room-temperature solubility is about
   330 g/L and its formula weight is 329.24 g/mol.


2. Dividing 330 g/L by 329.24 g/mol tells us that a
   saturated solution of potassium ferricyanide contains
   about 1.00 mol/L, which is to say 1.00 M. (In dilute
   solutions, molality and molarity are nearly equal, so this
   calculation is valid even though we’re making up the
   solution to a specified molality.)


3. To give us a safety margin against the solution
   crystallizing at lower temperatures, we decide to make
   our stock solution 0.50 M, or 0.50 mol/L. We could make
   up the solution to 0.60, 0.75, or some other molality
   (0.90 m would be pushing it), but round numbers are
   easier to handle when you’re calculating equivalents and
   measuring solutions for use in other tasks.


4. A one molal (1 m) solution is defined as one mole of
   solute dissolved in one kilogram of solvent. Because
   we’re making up a 0.50 molal solution with 100 g of
   water as the solvent, we need 0.05 moles of potassium
   ferricyanide.


5. The formula weight of potassium ferricyanide is 329.24
   g/mol, so we need 16.46 g of potassium ferricyanide for
   our solution.
   6. The assay on our bottle of reagent grade potassium
   ferricyanide lists the contents as 99% potassium
   ferricyanide, meaning that this substance may
   contain up to 1% of something other than potassium
   ferricyanide. Dividing 16.46 g by 0.99 (99%) tells us
   that we need to weigh out 16.63 g of the substance in
   the bottle to get 16.46 g (0.0500 mol) of potassium
   ferricyanide.

With the calculations complete, we’re ready to prepare

the solution.