ANSWERS TO WEEK 3 HOMEWORK

YEAR 12 HOMEWORK Answers

SHEET 3 ANSWERS - Instrumental ANALYSIS

1. Colorimetry was used to determine the amount of a yellow dye in a soft drink.

A series of standards were prepared and the amount of light received measured.

(a) Construct a flow chart of the procedure

Determine/select appropriate wavelengths by selecting appropriate coloured filter
Find the absorbance of a series of solutions of known dye concentration
Graph/construct the standard curve
Find the absorbance of the unknown sample
Read the concentration of the dye in the softdrink from the standard curve

(b) Plot a calibration curve from the above data

(Note that the line of best fit should be a straight line)

(c) Estimate the concentration (in mg/L) of dye in both samples

Reading from the curve: [Dye] in sample A = 3.3 mg/L

[Dye] in sample B = 6.8 mg/L

(d) Why do atoms absorb light of particular wavelengths?

The difference between electron energy levels (shells) in atoms is specific to that atom. An electron will absorb a photon of light that has the energy exactly equivalent to the amount of energy needed to promote the electron to an excited energy level. As the energy of light is related to the wavelength og the photon, only specific wavelengths of light are absorbed.

(e) Why would UV-visible spectroscopy give a more reliable result than simple colorimetry?

UV-visible spectroscopy uses a monochromater which selects specific wavelengths, while colorimetry uses a filter which allows a wider range of wavelengths to pass through the sample. Thus the UV-visible spectroscopy is much more specific for the chemical under investigation and much less prone to interference from other chemicals.

(f) What are the similarities and differences between UV-visible spectroscopy and colorimetry? What advantages does each method have?

Both methods use the principle that electrons are excited by photons of light of specific wavelengths.
Both methods use a light source, some method of filtering out undesired wavelengths of light and a detector.
Both methods use the principle that the absorbance of the solution is directly proportional to the concentration of sample in solution.
Both methods involve the construction of a standard curve from a series of solutions of known concentration, and reading the concentration of the unknown from the standard curve.

Calorimetry is cheap, simple and accurate to 1-2%

UV-Visible spectroscopy is reliable and very accurate

2. AAS is used to check the iron content of a vitamin syrup for children. 10.0 ml of the syrup was dissolved in water to make 1.0L (ie 1000ml) of solution. This gave an absorbance of 0.265. The AAS was calibrated with a series of standards.

(a) Construct a calibration curve from the above data

(b) From the curve, estimate the concentration in ppm (mg/L) of the diluted syrup

Reading from the graph, the [iron] was 4.3 ppm

(c) What was the concentration (in mg/L) of iron ion the undiluted syrup?

Using C₁V₁= C₂V₂

C₂ = 4.3 x 1000/10 ppm

= 430 ppm

(d) Could the syrup have been fed directly into the AAS? Why or why not?

Probably not as it needed to be diluted 100 x to keep the absorbance within a reasonable level

(e) The syrup also contains magnesium, copper and potassium ions as well. Why don't these interfere with the analysis?

The special iron lamp will only emit light with wavelengths that will excite electrons in iron atoms. In other words, only iron atoms will absorb the light and other atoms will not interfere with the measurements.

3. The salt levels in a hamburger are to be analysed. 1.00g of the burger was ground with water and then filtered (it was assumed that all the salt dissolved in the water). The filtered liquid was diluted to a volume of 1.00L and checked with AAS which gave a reading of 0.030. The manufacturer claims that the burger contains a maximum of 2.0% salt by mass.

(a) Construct a calibration curve for sodium

(b) What is the concentration of sodium (in mg/L) in the solution?

From the curve, the [Sodium] is 1.2 mg/L

(c) What is the concentration of salt (NaCl) by mass in the hamburger? (NB: M_Na=23, M_Cl=35.5)

As the salt from 1.00 g of the hamburger was dissolved in 1.00 L, this means that in 1.00 g of hamburger there was 1.2 mg of Sodium.

ie there is 1.2 x 10^-3/23 = 5.2 x 10^-5 mol of sodium

As n(NaCl) = n (Na) = 5.2 x 10^-5 mol

So m(NaCl) = 5.2 x 10^-5 x (23 + 35.5) = 3.1 x 10^-3g

% Salt by mass = 3.1 x 10^-3 / 1.00 x 100% = 0.31% w/w

(d) Is the manufacturer's claim accurate?

Yes it is well below the 2% mark!

(e) In what ways are AAS and colorimetry similar? How are they different?

Both methods use the principle that electrons are excited by photons of light of specific wavelengths.
Both methods use a light source and a detector.
Both methods use the principle that the absorbance of the solution is directly proportional to the concentration of sample in solution.
Both methods involve the construction of a standard curve from a series of solutions of known concentration, and reading the concentration of the unknown from the standard curve.

AAS uses a flame to atomise the sample, colorimetry can only be used for solutions.

AAS uses specific wavelength lamps to excite electrons, while colorimetry uses a filter to select wavelengths.

Draw a flowchart for the procedure of using AAS

Select appropriate AAS lamp
Set/adjust flame
Set recorder
Find the absorbance of a series of solutions of known concentration
Graph/construct the standard curve
Find the absorbance of the unknown sample
Read the concentration of the metal in the unknown sample from the standard curve

4. What are the similarities and differences between HPLC and GLC?

Similarities:

Both separate on the principles of differences and rates of adsorption and desorption of components
Both have an injection port, a separating column, a detector and recorder

Differences:

HPLC uses a liquid under high pressure as it's mobile phase, while GLC uses an inert carrier gas
HPLC uses a tightly packed silica column as it's stationary phase, while GLC uses a coiled heated tube to which the gas components adsorb and desorb to as they change phase from gas to liquid and back again.
GLC uses a flame detector, while HPLC uses a UV detector
GLC is more accurate, while HPLC can be used for temperature sensitive compounds

5. An oil spill was found in Bass Strait, the oil was discovered to be fuel oil which is a mixture of hydrocarbons. 3 ships were in the area at the time. How could instrumental analysis be used to identify the ship that was the source of the spill.

You could use HPLC to "fingerprint" the different hydrocarbons and their proportions in the oil spill. By comparing this to the HPLC chromatograms of samples taken from the 3 ships, the offending ship can be identified. This method is frequently used, and the evidence is good enough for conviction in courts of law.

6. A liquid contains 3 different alcohols: methanol (CH₃OH), ethanol (C₂H₅OH) and propanol (C₃H₇OH).

(a) Would you use HPLC or GLC to analyse the liquid. Why?

Probably HPLC as the high temperatures may damage the volatile organic components.

(b) Which of the alcohols would have the greatest retention time? Which would have the shortest? Why?

Methanol would have the shortest retention time, while propanol would have the largest. This is because methanol has a lower molecular weight (32), while propanol has the highest (60). Not only is propanol heavier, it is larger so it will be harder to push through the stationary phase.

(c) Sketch the chromatogram of the liquid.

Order: Methanol Ethanol Propanol

(d) What factors influence the absorption and desorption of a particular compound in chromatography?

Polarity

Size

Weight

Strength of attractions with stationary phase

Strength of attractions with solvent

7. A student claims that all chromatography can be summarised as involving 4 aspects:

(a) Stationary phase, (b) Mobile phase, (c) Separation process, (d) data collection

Summarise the 5 types of chromatography we have studied in terms of these 4 aspects.

Type	Stationary Phase	Mobile Phase	Separation Process	Data Collection
TLC	Silica bound to glass or plastic	Liquid Solvent such as water	Separation occurs as solvent moves up the TLC plate	Measurement of R_f by visual inspection
Paper Chromatography	Paper	Liquid Solvent such as water	Separation occurs as solvent moves up the paper	Measurement of R_f by visual inspection
Column Chromatography	Silica in a glass column	Liquid solvent such as water	Separation occurs as solvent moves down the column	Separation of the components into separate solutions
HPLC	Silica tightly packed into a steel column	Liquid under high pressure	Components are forced through the column	UV Detector
GLC	Coiled metal tube	Inert gas	Components liquify/evaporate onto the metal tube at different rates	Flame ionization detector