Science Olympiad

Coach’s Clinic

CHEMISTRY LAB

Pt. 1 ACID-BASE STATIONS

• Students will be expected to answer questions and/or interpret data related to acids and bases; and their properties and chemistry.

Sample Questions

1. If the following titration curve resulted from titrating acid X with a strong base, what might be the identity of acid X?

a) HCl b) H2CO3 c) HC2H3O2 d H2SO4

• 2. If a solution of ammonia is titrated with nitric acid, the pH at the equivalence point should be

• a) less than 7

• b) equal to 7

• c) greater than 7

• d) more information needed

• 3. If 20.00 mL of acid HX (a weak monoprotic acid) required 32.50 mL of standard 0.150M NaOH to reach the endpoint, what is the molarity of the original acid solution?

solution

• HX + Na+ + OH- Na+ + X- + H2O

• .03250L x .1500M OH- = .004875 mol OH-

• .004875mol OH- x 1mol HX / 1mol OH-

= .004875mol HX

• .004875mol HX/.02000L = .2438M HX

• 4. What is the pH of the original HX solution if the Ka is 1.81x 10-5 ?

solution

• HX + H2O X- + H3O+

• Init. Δ Equil.• HX .2438M -x .2438-x• H3O+ 0 +x x• X- 0 +x x• Ka = [ X-] [H3O+] [HX]• 1.81 x 10-5 = x2

.2438

x = .00210 = [H3O+] pH = 2.68

• 5. What is the pOH, and Kb of the original HX solution?

solution

• pH + pOH = 14• pOH = 14 – 2.68• pOH = 11.32

• Ka x Kb = 1.0 x 10-14

• Kb = 1.0 x 10-14

1.81x 10-5

Kb = 5.52 x 10-10

Titration TerminologyStandard (Acid or Base)

A solution of known concentration used to identify the unknown concentration of another acid or base.

IndicatorA weak acid or weak base that changes color at a specific pH or acid concentration.

EndpointThe point in the titration when the color of the indicator changes and the addition of the titrant should stop. (Ideally, this will be when the moles of base added equals the moles of acid originally present in the solution.

Equivalence PointThe point in the titration when the moles of base added equals the moles of acid originally present based on the chemical equation and stoichiometry.

Color Change Ranges For Some Common Indicators

Typical Titration Curves

Strong Base into Strong Acid

Strong Base into Weak Acid

Weak Base into Strong Acid

Strong Base into Diprotic Acid

Strong Acid into Strong Base

Acid/ Base Conjugate Pairs• The reversible reaction contains two acids and

two bases. We think of them in pairs, called conjugate pairs.

• When the acid, HA, loses a proton it forms a base, A-. When the base, A-, accepts a proton back again, it obviously refoms the acid, HA. These two are a conjugate pair.

• Members of a conjugate pair differ from each other by the presence or absence of the transferable hydrogen ion.

Another Conjugate Pair

• Ammonia in Water:

Part 2 Titration Race

• Each group is provided with 2 burets, each filled with water.

• Each group is provided with 100mL of standardized 0.175M NaOH and 100mL of HCl of unknown Molarity.

• Each group is provided Phenolphthalein indicator and a pH probe.

• Each group is required to complete 2 titrations to determine the Molarity of the HCl and the Standard deviation.

• Scoring will be ranked by accuracy/precision and ties will be broken by time.

Standard Deviation