Lab 2 Flashcards
(20 cards)
What is the definition of visible light
Part of the electromagnetic spectrum that can be seen by the human eye.
What is the range for wavelength of visible light
400-700nm
Which color in the visible light spectrum has the highest wavelength and what is its range?
red. 640-700nm
Name the 7 parts of the electromagnetic spectrum in order of decreasing energy
gamma rays<xrays<ultraviolet<visible light<infrared<microwaves<radio
what is the definition of lambda max
wavelength at which a substance has its strongest absorption
What is beer-lambert’s law and what does each symbol stand for
A= Ebc
A=absorption (no units)
E= molar absorptivity (Lcm-1mol-1)
b= path length (cm)
c= molar concentration
What color is bromothymol blue when it’s not ionized and when it is ionized?
not ionized: yellow
ionized: blue
What color will bromothymol blue be when added to a solution with a pH of 6.1
yellow
What color will bromothymol blue be when added to a solution with a pH of 7.1
green
What color will bromothymol blue be when added to a solution with a pH of 8.1?
blue
The lower the pH, the _____ bromothymol blue will ionize
less
give the formula for calculating pH of a weak acid
pH=pka + log[A-]/[HA]
give the formula for calculating % ionization of a weak acid
%-ionization = log[A-]/[HA]+[A-]
all * 100%
What is transmittance?
amount of light that passes through the sample (and reaches the detector)
List and explain the function of the 6 parts of a spectrophotometer
- Light source - provides the wavelength of light
- Monochromator - splits light into individual wavelengths
- Wavelength selector - select specific wavelength for analysis
- Cuvette holder - holds the cuvette containing the sample solution
- Photoelectric detector - Detects the light that is not absorbed by the sample
- Digital display - to display the detected absorbance
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.
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
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.
A=ebc
25millimolar = 0.025Molar
1.5=0.025x
x=60 L Mol^-1*cm^-1
The stock solution has pH= 7.2, pKa=6.2, calculate the molar ratio ([Base]/[Acid]) for the stock solution.
7.2-6.2 = 1
10^1 = 10/1
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?
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
Explain Beer-Lambert law
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
