Illustrated Guide to Home Chemistry Experiments

Chapter 11 Laboratory: Acid-Base Chemistry 205

FIGURE 11-4:

Methyl orange (left) and phenolphthalein at their equivalence points

CUTIOA nS

Hydrochloric acid is corrosive. Wear splash goggles, gloves,

and protective clothing at all times. (Phenolphthalein,

formerly widely used as a laxative, was withdrawn from

the market because of concerns about possible links with

cancer, but the small amount used in an indicator solution

is not ingested and is no cause for concern.)

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SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute any suitable containers of similar
  size for the 150 mL beakers.


• You may substitute a 100 mL graduated cylinder for
  the 100 mL volumetric flask and/or a 10 mL graduated
  cylinder for the 10 mL pipette, with some loss in
  accuracy.


• If you do not have a burette, ring stand, and burette
  clamp, you may substitute a 100 mL graduated
  cylinder, with some loss in accuracy. To do so, fill the
  graduated cylinder with titrant and record the starting
  level. Transfer titrant to the reaction beaker by pouring
  carefully until the endpoint is near (evidenced by
  a nonpersistent color change in the indicator that
  disappears when you swirl the beaker). Then use a
  dropper or beral pipette to transfer titrant dropwise
  until the end point is reached. Transfer any remaining
  titrant from the dropper or beral pipette back into
  the graduated cylinder, record the ending level, and
  subtract to determine how much titrant was required
  to reach the endpoint.


• If you do not have anhydrous sodium carbonate,
  you may substitute equivalent weights of the
  monohydrate, heptahydrate, or decahydrate.


• You may substitute cresol red, thymol blue, or a similar
  indicator for the phenolphthalein indicator.

One way to minimize the scale of errors is to use a relatively large
amount of solute to make a starting solution and then use serial
dilution to make a dilute standard solution to use as the titrant.
Serial dilution simply means repeatedly diluting small aliquots of
known volume. For example, we might start with a 1.5 M solution
of a chemical. We use a pipette to take a 10.00 mL aliquot of that
solution and dilute it to 100.0 mL in a volumetric flask to make
a 0.15 M solution. We then take a 10.00 mL aliquot of the 0.15 M
solution and dilute it again to 100.0 mL, yielding a 0.015 M solution.

For example, a reference book tells us that the formula weight of
anhydrous sodium carbonate is 105.99 g/mol and its solubility
at 20°C is about 200 g/L. A saturated solution of sodium
carbonate is therefore about 1.9 M, because (200 g/L)/(105.99
g/mol) = 1.88+ M. In this lab, we’ll make up a 1.5 M solution of
sodium carbonate and then use serial dilution to produce a
0.15 M solution to use as our titrant. We’ll titrate our unknown
HCl solution once using phenolphthalein as the indicator, and
a second time using methyl orange as the indicator. Why two
passes with two separate indicators?

As it happens, the neutralization of sodium carbonate by

hydrochloric acid is a two-step process. In the first step, one mole

of sodium carbonate reacts with one mole of hydrochloric acid

to produce one mole of sodium hydrogen carbonate (sodium

bicarbonate) and one mole of sodium chloride:

Na 2 Co 3 (aq) + HCl(aq) → NaHCo 3 (aq) + NaCl(aq)

In the second step, a second mole of hydrochloric acid reacts with

the sodium hydrogen carbonate formed in the first step:

NaHCo 3 (aq) + HCl(aq) → NaCl(aq) + Co 2 (g) + H 2 o(l)

Each of these reactions has an equivalence point. The first occurs

when the first mole of HCl has reacted with the sodium carbonate

to form one mole each of sodium bicarbonate and sodium

chloride. The pH at the equivalence point of this reaction happens

to correspond very closely to the pH range where phenolphthalein

changes color, but is well above the pH range of methyl orange.

The second equivalence point occurs when the second mole of