Astudent prepareda calibration curve by plotting absorbance of the standards against the [FeSCN2+] molar concentration (M). The best fit line of the linear equation for the student's experiment was createdas y= 0.1827x-0.0513 (Spec 20). Determine the equilibrium concentration of iron in unknown solution if the absorbance for the unknown solution was measured to be 0.308.

Explanation:

Chemistry 1B Experiment 7

1-3 5.0 1.5 3.5

Part 2: Determining the equilibrium constant.

Label 5 medium-sized test tubes. Table 7.2 shows the amounts of 2.00 × 10–3

M

Fe(NO3)3 (in 1 M HNO3) solution, 2.00 × 10–3

M KSCN solution, and purified water

that should be added to each tube. Pipet the approximate amount of each solution into

each tube. (Record the exact amount of each solution that you actually add. You will

need to use these actual amounts in your calculations.)

Obtain five separate small pieces of parafilm. Close the top of each test tube with

the parafilm. Mix each solution thoroughly by inverting the test tube several times.

Record your observations.

Measure and record the absorbance of each solution at the 447 nm.

Table 7.2 Composition of solutions for determining the equilibrium constant.

Test Tube

Volume of

2.00 × 10–3

M Fe(NO3)3

in 1 M HNO3 (mL)

Volume of

2.00 × 10–3

M KSCN

(mL)

Volume of

purified water

(mL)

2-1 5.0 1.0 4.0

2-2 5.0 2.0 3.0

2-3 5.0 3.0 2.0

2-4 5.0 4.0 1.0

2-5 5.0 5.0 none

Calculations

Part 1. Graphing the relationship between absorbance and [FeSCN2+].

Assuming that “all” of the SCN–

ions have been converted to FeSCN2+ ions,

calculate [FeSCN2+] in each of the solutions in Part 1. For example, in test tube 1-2, 1.0

mL of a 2.00 × 10–3

M KSCN solution was diluted to 10.0 mL. The concentration of

SCN–

that results from this dilution is the one to use for determining [FeSCN2+].

Because of the 1:1 stoichiometry, that initial concentration of SCN– is equal to

[FeSCN2+].

Plot a full-page graph of the absorbance against the concentration of FeSCN2+ in

all standard solutions. Use a ruler to draw the best straight line that comes closest to each

of your five data points. Your line should pass through (0 M, 0). (Why?) This graph is

your calibration curve. When you measure the absorbance of a solution that contains an

unknown concentration of FeSCN2+ ions, you can use this calibration curve to determine

the unknown concentration

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.