YEAR 12 HOMEWORK

SHEET 3 - Instrumental ANALYSIS

Read pp 49 - 60 of Chemistry 2 and run the program "The Bohr atom" on the Scott Visual Chemistry Program for background on absorption and emission spectra.

For these questions you may need to remember n=m/M, n=cV and pV=nRT

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.

Sample A gave an absorbance of 0.08 while sample B gave an absorbance of 0.16

1. Construct a flow chart of the procedure
2. Plot a calibration curve from the above data

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

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

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

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

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

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

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

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

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.

The values from the standard solutions are:

(a) Construct a calibration curve for sodium

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

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

(d) Is the manufacturer's claim accurate?

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

(f) Draw a flowchart for the procedure of using AAS

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

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.

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?

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

(c) Sketch the chromatogram of the liquid.

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

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.