The technique illustrated is used to analyse chemical substances in a sample.
What is the technique shown?
- Flame test
- Mass spectrometry
- Atomic absorption spectroscopy
- Ultraviolet-visible spectrophotometry
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The technique illustrated is used to analyse chemical substances in a sample.
What is the technique shown?
\(C\)
By elimination:
→ Although a flame test uses a flame to analyse chemical samples, it does not require a detector, prism, lamp or lens. Hence it is not the analytical method being demonstrated in the above diagram (eliminate A).
→ Mass Spectroscopy is used for organic compounds and requires an electromagnet which is not present in the above diagram (eliminate B).
→ Atomic Absorption Spectroscopy (used for inorganic compounds) is based on the idea that atoms can absorb light at a specific unique wavelength. The above image demonstrates this.
→ Ultraviolet-visible Spectrophotometry: although this uses the same principle as AAS to detect sample concentration, Ultraviolet-visible Spectrophotometry uses a different wavelength of light and requires a different apparatus to the one in the diagram above (eliminate D).
\(\Rightarrow C\)
Limestone \(\ce{(CaCO_3)}\) contributes to the hardness of water by releasing \(\ce{Ca^2^+}\) ions. The following chemical equation represents this reaction.
\(\ce{CaCO3($s$) + H_2O($l$) + CO_2($g$) \rightleftharpoons Ca^2^+($aq$) + 2HCO3^-($aq$)}\) \((\Delta H<0)\)
It has been suggested that heating water reduces its hardness.
Explain how this suggestion can be tested accurately, validly and reliably. (9 marks)
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→ Atomic absorption spectroscopy (AAS) can be used to test if heating reduces water hardness.
→ It does this by calculating the concentrations of metal ions in solutions. AAS can calculate the concentration of \(\ce{Ca^{2+}}\) in heated and non-heated samples of water and any difference in the relative concentrations of \(\ce{Ca^{2+}}\) can be used to verify the suggestion.
→ It should be noted that a reduced concentration of \(\ce{Ca^{2+}}\) indicates that the water hardness is reduced.
Methodology of testing
→ Prepare standard solutions with known concentrations of \(\ce{Ca^{2+}}\) and measure their absorbance. Plot the concentrations against the absorbance of the standard solutions and draw a calibration curve (i.e. a line of best fit).
→ Measure the absorbance of a water sample before heating and another after heating. Using the absorbance and the calibration curve, calculate the concentration of \(\ce{Ca^{2+}}\) in each sample and compare the concentrations between the heated and unheated samples.
→ The AAS should be calibrated, at which point the concentration of calcium ions can be calculated to an accuracy in the parts per million (ppm). To ensure accurate calibration of the AAS, the standard solutions need to be prepared precisely which will involve the accurate weighing of solids and the use of a pipette or a similar instrument to measure solution volumes.
→ Water used in the experiment should be de-ionised (normal drinking water has an abundance of \(\ce{Na+}\) and \(\ce{Ca^{2+}}\)).
→ The margin of experimental error decreases when sufficient calibration samples are used and the measurement of absorbance of these samples is repeated and averaged.
→ The reliability of results increases when many samples of heated and non-heated water are used to confirm that the concentrations of \(\ce{Ca^{2+}}\) in the heated water samples are consistently lower than the concentrations of \(\ce{Ca^{2+}}\) in the unheated water samples.
→ AAS can also be used to test the validity of the results. A hollow cathode lamp for calcium can direct light through the solution. This light has a specific wavelength that will only be absorbed by calcium ions. In this way, accurate measurements are made which can then be compared against the results and provide evidence of the validity of the original suggestion.
→ Atomic absorption spectroscopy (AAS) can be used to test if heating reduces water hardness.
→ It does this by calculating the concentrations of metal ions in solutions. AAS can calculate the concentration of \(\ce{Ca^{2+}}\) in heated and non-heated samples of water and any difference in the relative concentrations of \(\ce{Ca^{2+}}\) can be used to verify the suggestion.
→ It should be noted that a reduced concentration of \(\ce{Ca^{2+}}\) indicates that the water hardness is reduced.
Methodology of testing
→ Prepare standard solutions with known concentrations of \(\ce{Ca^{2+}}\) and measure their absorbance. Plot the concentrations against the absorbance of the standard solutions and draw a calibration curve (i.e. a line of best fit).
→ Measure the absorbance of a water sample before heating and another after heating. Using the absorbance and the calibration curve, calculate the concentration of \(\ce{Ca^{2+}}\) in each sample and compare the concentrations between the heated and unheated samples.
→ The AAS should be calibrated, at which point the concentration of calcium ions can be calculated to an accuracy in the parts per million (ppm). To ensure accurate calibration of the AAS, the standard solutions need to be prepared precisely which will involve the accurate weighing of solids and the use of a pipette or a similar instrument to measure solution volumes.
→ Water used in the experiment should be de-ionised (normal drinking water has an abundance of \(\ce{Na+}\) and \(\ce{Ca^{2+}}\)).
→ The margin of experimental error decreases when sufficient calibration samples are used and the measurement of absorbance of these samples is repeated and averaged.
→ The reliability of results increases when many samples of heated and non-heated water are used to confirm that the concentrations of \(\ce{Ca^{2+}}\) in the heated water samples are consistently lower than the concentrations of \(\ce{Ca^{2+}}\) in the unheated water samples.
→ AAS can also be used to test the validity of the results. A hollow cathode lamp for calcium can direct light through the solution. This light has a specific wavelength that will only be absorbed by calcium ions. In this way, accurate measurements are made which can then be compared against the results and provide evidence of the validity of the original suggestion.
Which of the following greatly enhanced scientific understanding of the effects of trace elements?
`B`
→ AAS allows trace elements to be detected at much lower concentrations than previous techniques.
`=>B`
Atomic absorption spectroscopy was used to determine the concentration of zinc in a water sample. The absorbance of a series of standard solutions of known concentration of zinc was measured. The results are shown in the table.
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Stormwater from a mine site has been found to be contaminated with copper\(\text{(II)}\) and lead\(\text{(II)}\) ions. The required discharge limit is 1.0 mg L¯1 for each metal ion. Treatment of the stormwater with \(\ce{Ca(OH)2}\) solid to remove the metal ions is recommended.
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a. Recommended Treatment:
→ Calcium hydroxide is a slightly soluble compound, while copper\(\text{(II)}\) hydroxide and lead\(\text{(II)}\) hydroxide are very insoluble in water.
→ When these compounds are added to water, the metal ions tend to precipitate out of solution.
→ For example, the addition of solid calcium hydroxide to water produces calcium ions \(\ce{Ca^2+}\) and hydroxide ions \(\ce{OH-}\), which can then react with lead\(\text{(II)}\) ions (\(\ce{Pb^2+})\) and copper\(\text{(II)}\) ions \(\ce{Cu^2+}\) to form precipitates of lead\(\text{(II}\) hydroxide and copper\(\text{(II)}\) hydroxide, respectively.
→ These reactions are represented by the equations:
\(\ce{Pb^2+ + 2OH- -> Pb(OH)2, \ \ Cu^2+ + 2OH- -> Cu(OH)2}\)
b. Atomic absorption spectroscopy (AAS):
→ Can be used for determining the concentration of metal ions in a sample by measuring the absorbance of light at specific wavelengths that are characteristic of each metal.
→ AAS uses light wavelengths that correspond to atomic absorption by the element of interest, and since each element has unique wavelengths that are absorbed, the concentration of that element can be selectively measured in the presence of other species.
→ As a result, AAS can be used to independently measure the concentrations of different metal ions, such as lead\(\text{(II)}\) ions and copper\(\text{(II)}\) ions in a sample containing both types.
c. Concentrations of ions:
\begin{array} {|l|c|c|c|}
\hline \text{Sample }& \ce{Cu^2+ \times 10^{-5} mol L^{-1}} & \ce{Pb^2+ \times 10^{-5} mol L^{-1}} \\
\hline \text{Water (pre-treatment)} & 5.95 & 4.75 \\
\hline \text{Water (post-treatment)} & 0.25 & 0.85 \\
\hline \end{array}
→ Concentrations of copper and lead have been significantly reduced.
→ Convert concentrations to compare with standard:
\begin{array} {ccc}
\ce{Cu^2+}: & 5.95 \times 10^{-5} \times 63.55 \times 1000 = 3.78\ \text{mg L}^{-1} \\
& 0.25 \times 10^{-5} \times 63.55 \times 1000 = 0.16\ \text{mg L}^{-1} \\
& \\
\ce{Pb^2+}: & 4.75 \times 10^{-5} \times 207.2 \times 1000 = 9.84\ \text{mg L}^{-1} \\
& 0.85 \times 10^{-5} \times 207.2 \times 1000 = 1.76\ \text{mg L}^{-1} \end{array}
Conclusion:
→ The concentration of copper ions has been reduced to a level that is lower than the discharge limit (0.16 < 1.0) but the lead ion concentration has not (1.76 > 1.0).
→ The treatment has only been partially successful.
a. Recommended Treatment:
→ Calcium hydroxide is a slightly soluble compound, while copper\(\text{(II)}\) hydroxide and lead\(\text{(II)}\) hydroxide are very insoluble in water.
→ When these compounds are added to water, the metal ions tend to precipitate out of solution.
→ For example, the addition of solid calcium hydroxide to water produces calcium ions \(\ce{Ca^2+}\) and hydroxide ions \(\ce{OH-}\), which can then react with lead\(\text{(II)}\) ions (\(\ce{Pb^2+})\) and copper\(\text{(II)}\) ions \(\ce{Cu^2+}\) to form precipitates of lead\(\text{(II}\) hydroxide and copper\(\text{(II)}\) hydroxide, respectively.
→ These reactions are represented by the equations:
\(\ce{Pb^2+ + 2OH- -> Pb(OH)2, \ \ Cu^2+ + 2OH- -> Cu(OH)2}\)
b. Atomic absorption spectroscopy (AAS):
→ Can be used for determining the concentration of metal ions in a sample by measuring the absorbance of light at specific wavelengths that are characteristic of each metal.
→ AAS uses light wavelengths that correspond to atomic absorption by the element of interest, and since each element has unique wavelengths that are absorbed, the concentration of that element can be selectively measured in the presence of other species.
→ As a result, AAS can be used to independently measure the concentrations of different metal ions, such as lead\(\text{(II)}\) ions and copper\(\text{(II)}\) ions in a sample containing both types.
c. Concentrations of ions:
\begin{array} {|l|c|c|c|}
\hline \text{Sample }& \ce{Cu^2+ \times 10^{-5} mol L^{-1}} & \ce{Pb^2+ \times 10^{-5} mol L^{-1}} \\
\hline \text{Water (pre-treatment)} & 5.95 & 4.75 \\
\hline \text{Water (post-treatment)} & 0.25 & 0.85 \\
\hline \end{array}
→ Concentrations of copper and lead have been significantly reduced.
→ Convert concentrations to compare with standard:
\begin{array} {ccc}
\ce{Cu^2+}: & 5.95 \times 10^{-5} \times 63.55 \times 1000 = 3.78\ \text{mg L}^{-1} \\
& 0.25 \times 10^{-5} \times 63.55 \times 1000 = 0.16\ \text{mg L}^{-1} \\
& \\
\ce{Pb^2+}: & 4.75 \times 10^{-5} \times 207.2 \times 1000 = 9.84\ \text{mg L}^{-1} \\
& 0.85 \times 10^{-5} \times 207.2 \times 1000 = 1.76\ \text{mg L}^{-1} \end{array}
Conclusion:
→ The concentration of copper ions has been reduced to a level that is lower than the discharge limit (0.16 < 1.0) but the lead ion concentration has not (1.76 > 1.0).
→ The treatment has only been partially successful.
The manganese content in a 12.0 gram sample of steel was determined by measuring the absorbance of permanganate \(\ce{(MnO4^-)} \) using the following process.
The steel sample was dissolved in nitric acid and the \(\ce{Mn^2+(aq)}\) ions produced were oxidised to \(\ce{MnO4^-(aq)}\) by periodate ions, \(\ce{IO4^-(aq)}\) , according to the following equation.
\(\ce{2Mn^2+(aq) + 5IO4^-(aq) + 3H2O(l) → 2MnO4^-(aq) + 5IO3^-(aq) + 6H+(aq)} \)
The resulting solution was made up to a volume of 1.00 L, then 20.0 mL of this solution was diluted to 100.0 mL. The absorbance at 525 nm of the resulting solution was 0.50.
A calibration curve for \(\ce{MnO4^-(aq)}\) was constructed and is shown below.
What was the percentage by mass of manganese in the steel sample?
`C`
From graph, 0.5 absorbance corresponds to 0.25 mg L ¯1.
\(\ce{[MnO4^-]_{dilute}}\) | `=(25 xx 10^(-3))/(54.94+16 xx 4)` | |
`=2.1019 xx 10^(-4)\ text{mol L}^(-1)` | ||
\(\ce{[MnO4^-]_{conc}}\) | `=2.1019 xx 10^(-4) xx 1/2` | |
`=1.05095 xx 10^(-3)\ text{mol L}^(-1)` |
\(\ce{n(MnO4^-) = c \times V = 1.05095 \times 10^{-3} \times 1 = 1.05095 \times 10^{-3} mol}\)
\(\ce{n(Mn^2) = n(MnO4^-) = 1.05095 \times 10^{-3} mol}\)
\(\ce{m(Mn^2+)}\) | `=\ text{n} xx text{MM}` | |
`=1.05095 xx 10^(-3) xx 54.94` | ||
`=5.774 xx 10^(-3)\ text{g}` |
`:.\ text{Mn}^(2+)text{(%)} = (5.774 xx 10^(-3))/12.0 xx 100text(%) = 0.48text(%)`
`=>C`
A blue solution of copper(`text{II}`) sulfate was investigated using colourimetry. Orange light (λ = 630 nm) was used and the pathlength was 1.00 cm.
Which change would result in a higher absorbance value?
`D`
→ Consider Beer-Lambert law, `A = epsilonlc`.
→ As the pathlength (`l`) increases, the absorbance (`A`) increases.
`=> D`
A UV-visible spectrometer was used to obtain the spectra of solutions of substances `P` and `Q`. The absorbance spectra are shown.
Which wavelength would be appropriate to determine the concentration of `Q` in a mixture of the two solutions?
`D`
→ The most appropriate wavelength would be a wavelength that is mostly absorbed by `Q` but minimally interfered with by `P`.
→ Thus, the best wavelength is 630 nm because is a significant absorbance by `Q`, and an insignificant absorbance by `P`.
`=> D`
An analytical chemist was using atomic absorption spectroscopy (AAS) to determine the manganese concentration in a sample.
The following diagram shows the absorbance lines of manganese.
The diagrams below show the emission spectra of four AAS lamps.
Which lamp should be used to determine the manganese concentration in the sample?
`C`
Consider Option C:
→ The emission spectrum of this lamp emits wavelengths of light that correspond to the absorption spectrum of `text{Mn}`.
→ This lamp would be best used to determine the `text{Mn}` concentration because it will emit wavelengths of light that are only absorbed by `text{Mn}`.
`=> C`
A sample of nickel was dissolved in nitric acid to produce a solution with a volume of 50.00 mL. 10.00 mL of this solution was then diluted to 250.0 mL. This solution was subjected to colorimetric analysis. A calibration curve for this analysis is given.
The solution gave an absorbance value of 0.30.
What was the mass of the sample of nickel?
`D`
Absorbance = 0.30
From the calibration curve, an absorbance of 0.30 corresponds to a \(\ce{[Ni2+]}\) of \(\pu{0.0021 mol L-1}\).
\( \ce{n[(Ni^2+)]} = \pu{0.0021 mol L-1} \)
\(\ce{n(Ni^2+)}\ \text{diluted}\) | `=\ text{c × V}` | |
`= 0.0021 xx 0.250` | ||
`= 0.000525\ text{mol}` |
This is the moles of \(\ce{Ni^2+}\) in the diluted sample, which contained only 10.0 mL of the initial 50.0 mL of the original solution.
Thus, in order to find the number of moles in the original sample, multiply the number of moles by 5.
\(\ce{n(Ni^2+)}\ \text{undiluted} = \pu{5 \times 0.000525 = 0.002625 mol}\)
\(\ce{n(Ni^2+)}\) | `= text{m} / text{MM}` | |
\(\ce{m(Ni^2+)}\) | `=\ text{n × MM}` | |
`= 0.002625 xx 58.69` | ||
`= 0.15406125… ` | ||
`= 0.15\ text{g (2 d.p.)}` |
`=> D`