5 Steps to a 5 AP Chemistry

298  Step 4. Review the Knowledge You Need to Score High

A water-soluble sample of a solid containing some barium nitrate is to be analyzed for

barium. The barium is to be precipitated as barium sulfate. Answer the following questions

about this experiment.

a. List the apparatus needed for this experiment.

b. Outline the steps in this experiment.

c. Set up the calculations needed to determine the percent barium in the sample.

d. What changes would be required in the procedure if the original sample also contained lead?

Second Free-Response Question

Answer the following question. You have 15 minutes, and you may not use a calculator. You

may use the tables at the back of the book.

A sample of a solid, weak monoprotic acid, HA, is supplied, along with solid sodium

hydroxide, a phenolphthalein solution, and primary standard potassium hydrogen phtha-

late (KHP).

a. Describe how a standardized sodium hydroxide solution may be prepared for the titration.

b. Sketch a graph showing pH versus volume of base added for the titration.

c. Sketch the titration curve if the unknown acid was really a diprotic acid.

d. Describe the steps to determine Kafor HA.

e. What factor determines which indicator should be chosen for this titration?

 Answers and Explanations

First Free-Response Question

a. analytical balance beaker(s) crucible and cover

desiccator drying oven funnel

Meker burner support stand triangle crucible support

You get 1 point for the following set: analytical balance, crucible and cover, and funnel.

You get 1 point for any others you list. There is a maximum of 2 points.

You can only get the second point if you have the entire first set.

b. ∗Weigh a sample of the solid (into a beaker).

Heat the crucible and lid with the Meker burner repeatedly until constant weight is

achieved.

∗Weigh the crucible and lid.

Add sufficient deionized water to dissolve the sample.

(Warm the solution.)

∗Slowly add a solution containing the sulfate ion (usually sodium sulfate) to the solution.

Allow the precipitate to settle, and check for complete precipitation.

∗Filter the precipitate and place it in the crucible.

Heat the crucible and lid with the sample to constant weight.

∗Weigh the cooled crucible, lid, and sample.

You get 2 points for everything in order. Information in parentheses is optional.

The first two items may be in any order. You get 1 point if the order is wrong or if any

of the starred items is missing.

c. mass of precipitate (ppt.) =(crucible +lid +sample) −(crucible +lid)

mass of barium in sample g ppt

mol BaSO

233.3

4

=()

1

4 4g

mol Ba

1mol BaSO

gBa

4 1mol

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1 137 33.

BBa

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