Illustrated Guide to Home Chemistry Experiments

126 DIY Science: Illustrated Guide to Home Chemistry Experiments

LABORATORY 7 .1:

mAkE Up A moLAR SoLUTIoN of A SoLId CHEmICAL

In this laboratory, we make up 100 mL of a

stock solution of copper (II) sulfate, which is

used in many 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 masses carefully

and measuring volumes carefully.

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute a 100 mL graduated cylinder
  for the 100 mL volumetric flask, with some loss in
  accuracy.


• You may substitute any convenient mixing container of
  the appropriate size for the 150 mL beaker.


• You may substitute 16.12 grams of copper sulfate
  anhydrate (the anhydrous form) for the 25.22 g of
  the pentahydrate salt. (Note that this substitution
  assumes 99.0% purity.)


• If you are using the copper sulfate pentahydrate that
  you recrystallized yourself (see Lab 6.3), I suggest that
  you assume 99% purity.


• If you are using technical-grade copper sulfate
  pentahydrate that has no assay listed, I suggest you
  assume 96% purity and recalculate the required
  mass accordingly. This is one of the hazards of using
  technical-grade compounds; you’re never certain of
  the actual purity.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing papers

£ volumetric flask, 100 mL

£ graduated cylinder, 100 mL

£ beaker, 150 mL

£ funnel

£ eye dropper

£ wash bottle (distilled or deionized water)

£ labeled storage bottle

£ copper sulphate pentahydrate (25.22 g)

The first question we need to answer is what the concentration of
the stock solution should be. Ideally, we want the concentration
to be as high as possible without quite being saturated. (We
don’t want copper sulfate crystallizing out of solution if one day
the lab happens to be a bit cooler than usual or if some of the
solvent evaporates.) So, the first thing we need to determine is
the molarity of a saturated solution of copper sulfate at room
temperature. From that, we can decide what the molarity of our
stock solution should be, and calculate how much copper sulfate
is required to make up 100 mL of stock solution at that molarity.
We proceed as follows:

1. Looking up copper sulfate pentahydrate in a reference
   book, we find that its its solubility at 20°C is 317 g/L and
   its formula weight is 249.7 g/mol.


2. Dividing 317 g/L by 249.7 g/mol tells us that a saturated
   solution of copper sulfate contains about 1.27 mol/L,
   which is 1.27 M.


3. To give us a safety margin against the solution
   crystallizing at lower temperatures, we decided to make
   our stock solution 1.00 M, or 1.00 mol/L.


4. Because we’re making up 100 mL (0.1 L) of solution, we
   need 0.1 moles of copper sulfate pentahydrate.


5. The formula weight of copper sulfate pentahydrate is
   249.7 g/mol, so we need 24.97 g of copper sulfate
   pentahydrate for our 100 mL of solution.

The assay on the bottle of copper sulfate pentahydrate lists
the contents as 99.0% copper sulfate pentahydrate, which
means that this substance contains 1.0% of something other
than copper sulfate pentahydrate (probably mostly metallic
copper, copper oxide, and other insoluble copper salts). Dividing
24.97 g by 0.99 (99%) tells us that we need to weigh out 25.22 g
of the substance in the bottle to get 24.97 g of copper sulfate
pentahydrate. With the calculations complete, it’s time to head
into the lab and actually make up the stock solution of copper
sulfate pentahydrate.