CHAPTER 16: QUANTITATIVE ANALYSIS OF SALTS

Question 1

law of conservation of mass

Answer 1

the total mass of all products is equal to the total mass of all reactants in a chemical reaction

Question 2

water of hydration

Answer 2

x= the number of H2O molecules per formula unit of CuSO4
* number of H2O molecules (x) can be determined by heating crystal and using gravimetric analysis

Question 3

gravimetric analysis to determine water of hydration in salt crystals

Answer 3

1. Calculate mass of water removed: m(hydrated salt crystal) – m(dehydrated salt crystal)
2. Calculate number of moles of water in salt crystal: n(H2O) = mass of water (g)
   molar mass of water (g/mol)
3. Calculate number of moles of hydrated salt crystal: n(hydrated salt crystal) = mass of hydrated salt crystal (g)
   molar mass of hydrated salt crystal(g/mol)
4. Use stoichiometry to work out value of x: coefficient of H2O (x)
   coefficient of CuSO=
   n(H2O)
   n(CuSO4
   )

Question 4

spectroscopy

Answer 4

• Spectroscopy is the analysis of the interaction between matter (solutions) and any portion of the electromagnetic spectrum.
• Some solutions containing metal ions absorb light in the visible and ultraviolet regions of the spectrum.

Question 5

visible light

Answer 5

● White light: Contains all the colours (wavelengths) within the rainbow (ROYGBIV)
● All matter can absorb light
● The colour/s a substance reflects appears as the colour to our eyes.

Question 6

complementary colours

Answer 6

When a sample absorbs light of a particular color, we perceive the object as the complementary color
I.e. the colour opposite the absorbed color on the colour wheel.

Question 7

colorimeters and UV/visible spectrophotometers

Answer 7

Instruments used to determine the concentration of solutions by measuring their absorbance of radiation in the UV and visible region of the spectrum

Question 8

use of colorimetry and UV/visible spectroscopy

Answer 8

● Used a lot in medical laboratories to determine concentrations of
- Lead in urine
- Glucose in blood
- Cholesterol in blood
- Haemoglobin in blood (and many more)

● Measurement of chromium levels in a workplace
● Measuring phosphate levels in waterways

Question 8

how do we select a wavelength

Answer 8

• A sample can be scanned across multiple wavelengths to determine which would be suitable.
• You’ll want a wavelength that absorb strongly by the test substance but not by other substances that may be in the sample.
• this reduces interference
• analysis is more effective if a wavelenght of HIGH ABSORBANCE is used

Question 9

determining concentration of unknown

Answer 9

• To determine the concentration of a substance we can construct a calibration curve
• This involves comparing the sample to ‘standards’ of varying concentrations

Question 10

constructing calibration curve

Answer 10

1. Standards of varying concentrations are made up
2. The amount of light absorbed by each standard is measured.
3. Absorbances are plotted on a graph to construct a calibration curve.
4. determine concentration of sample from calibration curve

IV (x axis) concentration
DV (y axis) absorbance

Question 11

can only coloured susbtances be measured using this method

Answer 11

● Some metal ions do not absorb very strongly in the ultraviolet or visible part of the spectrum.

● A reaction may be carried out prior to analysis to convert the analyte to a coloured complex.

Question 12

analysing amount of salt in soil sample

Answer 12

Soil matrix contains:
* Inorganic particles (minerals in the form of sands, silts and clays)
* Organic matter (decaying plant or animal)
* Living organisms (fungi, bacteria, protozoans, invertebrates)
1. accurately weigh dried soil
2. add liquid chemical to extract salt
3. remove solid material
4. dilute extract to accurate volume - volumetric flast
5. analyse using colorimetry or UV-visible spectroscopy
6. use calibration curve to determine salt concentration