Lab 2: Using Spectrophotometry (Colorimetry) to Determine the Formula of a Hydrate
Change the length and explain the effect of changing of the cuvette size and concentration of the solution on the absorbance PRE LAB QUESTIONS Solution: Copper (II) Sulfate Concentration 200 mM (0.200 M) l= 1 cm
-The absorbance increases as you change the length -The absorbance decreases as you lower the concentration
How does an increase in the wavelength affect the absorbance readings? PRE LAB QUESTIONS Solution: Copper (II) Sulfate Concentration 200 mM (0.200 M) l= 1 cm
-as the wavelength increases, the absorbance increases (but is zero at blue)
How does an increase in wavelength affect the transmittance readings? PRE LAB QUESTIONS Solution: Copper (II) Sulfate Concentration 200 mM (0.200 M) l= 1 cm
-as you increase the wavelength from 380 nm-530 nm, the transmittance increases. -After 530 nm the transmittance decreases
1.) What was the absorbance of the unknown that my specific lab had? 2.) What was the trendline equation from the excel graph? 3.) What was the molarity of Cu2+ from this information?
1.) 0.510 2.) y= 1.012x 3.) y is the absorbance & concentration is x so 0.510= 1.012x 0.510/1.012= .5040 concentration of Cu2+
what two solution in this lab are eye, respiratory and skin irritants, moderately toxic if inhaled or ingested?
1.) Copper (II) Sulfate Solution 2.) Copper (II) Nitrate Solution
What are the four LED light wavelength options for the Vernier Colorimeter?
1.) violet (430 nm) 2.) blue (470 nm) 3.) green (565 nm) 4.) red (635 nm)
Absorbance is calculated as a logarithmic function of T:
A = - log T = log(1/T), or A = 2 -log T[%] where T [%] is percent transmittance and 2 is log 100(%), the exact number
Suppose you plan to use colorimetry to measure the concentration of cobalt (II) chloride, which forms a red solution. What part of the procedure would you change? (hint: you may use pre-lab computer simulation to answer this question)
wavelength of light would need to be blue, because if the solution is red then the absorbance would be 0.
After completing this experiment, you will understand:
-basic lab procedures and calculations involved in preparing solutions -how to use Beer's Law in chemical analysis -the importance of calibration techniques in the determination of solution concentrations
what are the warnings provided for Nitric Acid?
-is an extremely corrosive acid capable of causing severe chemical burns very rapidly. - If nitric acid mists are inhaled, health risks include corrosion of mucous membranes, delayed pulmonary edema, and even death. -Contact with eyes can cause permanent cornea damage. - In addition, if ingested, nitric acid can produce severe, rapid corrosive burns of the mouth, throat and gastrointestinal tract.
What is the Beer-Lambert Law (also known as Beer's Law)
-states that there IS a linear relationship between the absorbance and the concentration of a sample -For this reason, Beer's Law is NOT APPLICABLE when solutions have a very low or very high concentration
The intensity of light (number of photons) that is transmitted through the sample container, called a cuvette, depends on what?
-the length or diameter of the cuvette (generally 1.000 cm) and the concentration of the sample. -the intensity of light that passes through the solution can be referred to as transmittance (T)
What was the unknown hydrate?
Copper (II) nitrate trihydrate Cu(NO3)2·3H2O
After completing this experiment, you will be able to:
✓ perform serial dilutions of the standard copper (II) nitrate hydrate solution ✓ set up and use the colorimeter and determine the absorbance of the solutions ✓ use Excel to generate a calibration graph. ✓ use the measured absorbance of an unknown solution and the calibration graph to determine the concentration of a solution of unknown molarity ✓ calculate the empirical formula of copper (II) nitrate hydrate used in an unknown solution.
Calculate the molar absorptivity of copper (II) solution PRE LAB QUESTIONS Solution: Copper (II) Sulfate Concentration 200 mM (0.200 M) l= 1 cm
-Absorbance of 635 nm/red for this solution was 0.07 -A = ε l c 0.07= (ε)(1)(0.200M) 0.07/0.200= .35
How would I find the concentration of Cu2+ ions, M if I have 8.00 mL of the stock solution and 2.00 mL of HNO3 when the concentration of CuSO4 is 0.600?
0.600/10.00 times 8.00= .480 M
How would I find the concentration of Cu2+ ions, M if I have 9.00 mL of the stock solution and 1.00 mL of HNO3 when the concentration of CuSO4 is 0.600?
0.600/10.00 times 9.00= .540 M
Solve the following example of calculations: A 3.026g sample of copper(II) chloride hydrate was dissolved in 50.00 mL of deionized water. The concentration of copper (II) ion determined spectrophotometrically using the equation from the calibration curve, was 0.355M (you may need to review Part 1: Simple Linear Plot of your Excel Lab to recall the calculation of molarity from the calibration graph). Determine the formula of the hydrate and then write out the name of the hydrate. Since the formula of hydrate is a mole ratio of the anhydrous ionic compound to water which belongs to the hydrate, you would need to find a number of moles of both compounds first. A. A number of moles of copper (II) nitrate can be calculated from the measured absorption. Because there is one copper (II) ion in the formula unit of copper (II) chloride, the molarity of copper in the solution is the same as the molarity of copper (II) nitrate: MCu2+ = MCuCl2 = 0.355M 1.) Determine the number of moles of CuCl2 in the solution from given molarity and volume of the solution: 2.Calculate the mass of CuCl2 in the solution. Molar mass of copper (II) chloride is 134.45g/mol B. A number of moles of water can be calculated from the difference between the mass of the sample of hydrate, Cu(NO3)2· XH2O, given and the mass of anhydrous CuCl2, calculated above: 3. Calculate the mass of water in hydrate: 4. Calculate a number of moles of water in hydrate: C. Now you can calculate the mole ratio H20/ CuCl2: What is the formula of the hydrate?
1.) nCuCl2= 0.355mol/L x 0.0500L = 0.0178 mol CuCl2 2.) mCuCl2= 0.0178 mol x 134.45g/mol = 2.38g CuCl2 3.) mH20= msample - mCuCl2=3.026g - 2.38g = 0.646g H20 (2 sig. figs) 4.) nH20=0.646g/18.02g/mol=0.0358mol H20 (2 sig. figs) Formula of the hydrate is: CuCl2x 2H20. The name is copper (II) chloride dihydrate or cupric chloride dihydrate.
What was the highest absorbance for: PRE LAB QUESTIONS Solution: Copper (II) Sulfate Concentration 200 mM (0.200 M) l= 1 cm
635 nm/red
which wavelength did we use in this lab and why?
635 nm/red because it had the highest absorbance
What is the Beer's Law Equation?
A = ε l c -where: A is the measure of absorbance (no units), ε is the molar extinction coefficient or molar absorptivity (or absorption coefficient), l is the path length generally 1.000 cm, and c is the molar concentration. -The molar extinction coefficient is given as a constant for a particular chemical in aqueous solution. Each chemical system has a different molar extinction coefficient. Since absorbance does not carry any units, the units for ε must cancel out the units of length and concentration. As a result, ε has the units: L mol-1 cm-1
What is the purpose of this lab?
A laboratory technician prepared 6 solutions of these hydrates and marked them as unknowns 1-6. The objective of this experiment is to determine the empirical formula of unknown copper (II) nitrate hydrate used to prepare the solution
What is the equation to find transmittance?
T = I t / I0 where I t is the light intensity after the beam of light passes through the cuvette and I0 is the light intensity before the beam of light passes through the cuvette.
What is transmittance (T)?
Transmittance is the percent of the light that is not absorbed, but rather passes through the sample.
does every chemical compound absorb, transmit, or reflect light (electromagnetic radiation) over a range of wavelengths?
YES
What is spectrophotometry?
a measurement of light absorbed or transmitted by a chemical solution.
What is a spectrophorometer?
an instrument that measures the amount of photons (the intensity of light) absorbed after it passes through a sample solution.
Why should the y-intercept of your plot equal 0?
because when concentration is zero, absorbance is zero
How will you determine the concentration of the unknown and molar absorptivity of the solution in this lab?
by using the equation of the trendline
what is the sample container for the spectrophotometer called?
cuvette (little rectangular plastic container)
how will you calculate the formula of the hydrate used?
from the measured concentration of Cu 2+ ion
what might happen if you leave nitric acid (HNO3) unwashed on your skin
nitric acid if left unwashed may leave bright yellow spots on your skin
Practice all calculations in lab 2 manual (mainly post lab questions)
see manual
What happens to the readings when the laser is a similar color to the solution in the cuvette? PRE LAB QUESTIONS Solution: Copper (II) Sulfate Concentration 200 mM (0.200 M) l= 1 cm
the absorbance becomes zero and the transmittance is at it's highest
What else can be determined by using a spectrophotometer?
the concentration (molarity) of a solution of unknown concentration can also be determined by measuring the intensity of light transmitted through the solution and detected by the instrument.
What is absorbance (A)?
the number of photons that are absorbed by the solution.
