Lab 2

Question 1

What is the definition of visible light

Answer 1

Part of the electromagnetic spectrum that can be seen by the human eye.

Question 2

What is the range for wavelength of visible light

Answer 2

400-700nm

Question 3

Which color in the visible light spectrum has the highest wavelength and what is its range?

Answer 3

red. 640-700nm

Question 4

Name the 7 parts of the electromagnetic spectrum in order of decreasing energy

Answer 4

gamma rays<xrays<ultraviolet<visible light<infrared<microwaves<radio

Question 5

what is the definition of lambda max

Answer 5

wavelength at which a substance has its strongest absorption

Question 6

What is beer-lambert’s law and what does each symbol stand for

Answer 6

A= Ebc

A=absorption (no units)
E= molar absorptivity (Lcm-1mol-1)
b= path length (cm)
c= molar concentration

Question 7

What color is bromothymol blue when it’s not ionized and when it is ionized?

Answer 7

not ionized: yellow
ionized: blue

Question 8

What color will bromothymol blue be when added to a solution with a pH of 6.1

Answer 8

yellow

Question 9

What color will bromothymol blue be when added to a solution with a pH of 7.1

Answer 9

green

Question 10

What color will bromothymol blue be when added to a solution with a pH of 8.1?

Answer 10

blue

Question 11

The lower the pH, the _____ bromothymol blue will ionize

Answer 11

less

Question 12

give the formula for calculating pH of a weak acid

Answer 12

pH=pka + log[A-]/[HA]

Question 13

give the formula for calculating % ionization of a weak acid

Answer 13

%-ionization = log[A-]/[HA]+[A-]
all * 100%

Question 14

What is transmittance?

Answer 14

amount of light that passes through the sample (and reaches the detector)

Question 15

List and explain the function of the 6 parts of a spectrophotometer

Answer 15

1. Light source - provides the wavelength of light
2. Monochromator - splits light into individual wavelengths
3. Wavelength selector - select specific wavelength for analysis
4. Cuvette holder - holds the cuvette containing the sample solution
5. Photoelectric detector - Detects the light that is not absorbed by the sample
6. Digital display - to display the detected absorbance

Question 16

Bromothymol blue is a weak acid with pKa of 7.1. Calculate the percentage ionization of Bromothymol blue when it is present in the buffer of pH 8.1.

Answer 16

pH=pka+log[base]/[acid]

8.1=7.1+log[base]/[acid]

1=log[A-]/[HA]+[A-]
= 10^1 = [10]/[1]

10/11 = 90.9%ionization

Question 17

A 25 mM solution of compound A has an absorbance of 1.5 when measured in a 1 cm cuvette under λmax. What is the molar extinction coefficient (ε) of compound A? Must write unit.

Answer 17

A=ebc

25millimolar = 0.025Molar

1.5=0.025x

x=60 L Mol^-1*cm^-1

Question 18

The stock solution has pH= 7.2, pKa=6.2, calculate the molar ratio ([Base]/[Acid]) for the stock solution.

Answer 18

7.2-6.2 = 1
10^1 = 10/1

Question 19

Bromothymol is a weak acid with pKa 7.1. First test tube (H1) contains bromothymol blue with 0.2 M HCl, and second test tube (N1) contains bromothymol blue with 0.2 M NaOH. Explain which test tube contains ionized and non-ionized bromothymol blue and why?

Answer 19

ionized bromothymol blue in N1 because bromothymol blue is a weak acid and will readily donate its protons to a strong base like NaOH.

H1 contains nonionized bromothymol blue because HCl and bromothymol blue are both acids so no acid-base reaction can occur

Question 20

Explain Beer-Lambert law

Answer 20

Beer-Lambert Law is the formula to find absorption (no units) using path length, molar extinction coefficient (constant for certain molecules), and molar concentration. Since absorption has no units, the molar extinction coefficent has the unit Lcm^-1mol^-1 to cancel the units of molar concentration and path length.

Relates the light absorption properties of the material. Beer’s law gives the relationship between absorbance and concentration while Lambert’s law gives relationship between absorbance and distance traveled