Solution Preparation and pH

Lab 7

Purpose

The purpose of this lab is to provide students with the opportunity to engage in solution preparation. Students will

also calibrate a pH probe and make pH measurements of solutions to demonstrate the pH differences between acids,

bases and neutral solutions of equimolar proportions.

pH

• pH is an indication of the abundance of free H+ ions in solution – measured with a pH probe.

• Definition: pH = – log [H+]• Measured on a scale that ranges from 0 to 14• Acidic – pH less than 7• Neutral – pH 7• Basic – pH more than 7

Acids

• Strong acids completely dissociate in solution.Examples include H2SO4, HCl, and HNO3.

• Weak acids dissociate partially in solution.Examples include CH3COOH and HCN.

(Dissociation is the process by which a chemical combination breaks up into simpler constituents.)

Acids

• Because strong acids donate more hydrogen ions to the solution than weak acids, they have a lower pH at equimolar concentration.

HCl H+(aq) + Cl-

(aq)

• For every 100 HCl compounds that go into solution, 100 H+ and 100 Cl- result.

CH3COOH CH3COO-(aq) + H+

(aq)

• For every 100 CH3COOH molecules that go into solution, maybe 1 H+ and 1 CH3COO- result. 99 molecules remain intact as CH3COOH.

• Which one is the strong acid?

Degree of Dissociation

• The actual degree of dissociation depends on the compound and is described by the Ka (pKa) of each compound.

• We won’t be covering pKa in this lab.

Bases

• Strong bases are metal hydroxide salts that completely dissociate in solution.

Examples include NaOH and KOH

• Weak bases react with water to produce hydroxide ions to a small extent.

Examples include NH4OH and C5H5N (pyridine)

(Dissociation is the process by which a chemical combination breaks up into simpler constituents.)

Bases

• Because strong bases result in more hydroxide ions in solution than weak bases, they have a higher pH at equimolar concentration.

NaOH(s) Na+(aq) + OH-

(aq)

• For every 100 NaOH compounds that go into solution, 100 Na+ and 100 OH- result.

NH4OH NH4+

(aq) + OH-(aq)

• For every 100 NH4OH compounds that go into solution, maybe 1 NH4+ and 1 OH-

result. 99 compounds remain intact as NH4OH.

• Which one is the weak base?

Degree of Dissociation

• The actual degree of dissociation depends on the compound and is described by the Kb (pKb) of each compound.

• We won’t be covering pKb in this lab.

Neutral Solutions

• Under perfect conditions, these solutions have a pH of 7.

• Why won’t we see this exact pH for our “neutral” solutions today?

pH, pOH, [H+], [OH-] comparison

Solution Preparation

•When you are required to make a solution of accurate concentration, a volumetric flask is used.

•We never make solutions of accurate concentration in:•Beakers•Graduated cylinders•Erlenmeyer flasks

Solution Preparation from Solids

• Determine the mass of the solid needed. You will need the following:• Molar mass of the solid• Total volume desired• Final concentration desired

• Calculation:• Mass, g = concentration, mol/L x molar mass, g/mol x volume, L• Remember the precision of your glassware!

Solution Preparation from Solids

• Make the solution:• Weigh out the appropriate mass of solid.• Rinse the appropriate size volumetric flask a couple of times

with distilled water. Discard the water rinses as waste.• Place a small volume of distilled water in the volumetric flask.• Add the solid to the volumetric flask.• Add some more distilled water to the flask, stopper, and invert

several times.• Add distilled water to the calibration line (fill to volume) using

a medicine dropper, stopper, and invert several times.

Solution Preparation from Liquids

• Determine the volume of stock solution needed.You will need the following:• Concentration of stock solution (M1)• Desired concentration of diluted solution (M2)• Desired volume of diluted solution (V2)

• Calculation:• M1V1 = M2V2

• Remember the precision of your glassware!

Solution Preparation from Liquids

• Make the solution:• Obtain the appropriate volume of stock solution using a graduated

cylinder. (Always add a few mL extra.)• Rinse the appropriate size volumetric flask a couple of times with

distilled water. Discard the water rinses as waste.• Place a small volume of distilled water in a volumetric flask.• Use the appropriate glassware (usually but not always the pipet) to

transfer the correct volume of stock solution from the graduated cylinder to the volumetric flask. (Prep the transfer glassware as appropriate.)

• Add some more distilled water to the flask, stopper, and invert several times.

• Add distilled water to the calibration line (fill to volume) using a medicine dropper, stopper, and invert several times.

In today’s experiment you will…

• Make your solutions• Transfer your solutions to the 20 mL vials.• Calibrate your pH probe• Measure the pH of your known and unknown solutions.• Record your results and answer the questions in your manual.

When calibrating your probe…

• Be careful! These probes are fragile. • An internal pH calibration is needed first to obtain a

calibration curve that the computer can use to interpolate the correct pH.

• When you report your pH measurements, remember to use the correct digits of precision. (The number of digits depends on your calibration values.)

• Remember to rinse and dry the probe before each transfer. This will prevent cross-contamination.

Points of Interest

• Our pH probes respond slowly to “neutral” solutions. Allow additional stabilization time.

• On the data sheet, identify one strong acid, one weak acid, one strong base, one weak base, and one “neutral” solution. Match the dH2O and unknowns as indicated.

Safety Concerns

• Reagents:• HNO3 (1 N) / HCl (0.2 N)• NaOH / KOH• NH4OH• KHP• Na2SO4

• Eye Contact:• Irritating, burns, permanent eye damage, tearing, redness, pain, impaired vision,

blindness• Skin Contact:• Irritating, redness, pain, soreness, severe skin burns, ulcers, stains, destruction, scaling,

and blistering• Inhalation:• Breathing difficulties, pneumonia, pulmonary edema, coughing, choking, irritation of

nose, throat, and respiratory tract, burns, and death• Ingestion:• Pain and burns of the mouth, throat, esophagus and gastrointestinal tract. Corrosive to

mucous membranes, perforation of the esophagus and stomach, abdominal pain, nausea, vomiting, abnormal sensations of hands and feet, and general gastro-intestinal upset.

Waste

• All excess solid can go in the regular garbage.

• Dispose waste solutions in the appropriate waste receptacles.• Acidic and basic solutions / waste need to be disposed in the

acid/base waste container in the fume hood.• Solutions with a pH between 6 and 8 can be disposed down

the drain.

Next week: Skill Evaluations

• Carefully study pages 291 – 313.

• Remember to bring your goggles and lab manual!

• Submit your Lab 7 Report at the start of the next lab.

• Study for the quiz.