Acid Base Pwrpt

8/2/2019 Acid Base Pwrpt.

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Science Olympiad

Coach’s Clinic

CHEMISTRY LAB



Pt. 1ACID-BASE STATIONS

• Students will be expected to answerquestions and/or interpret data related toacids and bases; and their properties and

chemistry.



Sample Questions

1. If the following titration curve resulted fromtitrating acid X with a strong base, what mightbe 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 pointshould 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 weakmonoprotic acid) required 32.50 mL ofstandard 0.150M NaOH to reach the

endpoint, what is the molarity of theoriginal 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 HXsolution 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 originalHX 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 Terminology

Standard (Acid or Base)A solution of known concentration used to identify the unknown concentration ofanother acid or base.

Indicator

A weak acid or weak base that changes color at a specific pH or acidconcentration.

EndpointThe point in the titration when the color of the indicator changes and the additionof the titrant should stop. (Ideally, this will be when the moles of base addedequals 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 acidoriginally present based on the chemical equation and stoichiometry.



Color Change Ranges For SomeCommon 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 andtwo bases. We think of them in pairs, calledconjugate pairs .

• When the acid, HA, loses a proton it forms a

base, A-. When the base, A-, accepts a protonback 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 withwater.

• Each group is provided with 100mL of standardized0.175M NaOH and 100mL of HCl of unknown Molarity.

• Each group is provided Phenolphthalein indicator and apH probe.

• Each group is required to complete 2 titrations todetermine the Molarity of the HCl and the Standarddeviation.

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



Standard Deviation

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Acid Base Pwrpt