Sodium is an essential element in our diets - VCE - SSCE Chemistry - Question 1 - 2003 - Paper 1
Question 1
Sodium is an essential element in our diets. However, the amount of sodium present in some foods is often much higher than levels recommended for health. A sauce was... show full transcript
Worked Solution & Example Answer:Sodium is an essential element in our diets - VCE - SSCE Chemistry - Question 1 - 2003 - Paper 1
Step 1
a. Use the above data for the Na⁺(aq) standards to plot a calibration line on the graph below.
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Answer
To plot the calibration line, take the absorbance values from the standard solutions and plot them against their corresponding sodium concentrations. Ensure to draw a best fit line through the plotted points.
Step 2
b. Use your calibration graph to determine the sodium ion concentration in the diluted sample of the sauce and in the original sauce.
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Using the calibration graph, find the absorbance value of the diluted sauce (0.185) on the y-axis, and draw a horizontal line to intersect the calibration curve. Read the corresponding concentration from the x-axis. For the original sauce, consider the dilution factor (1:40), thus multiply the value obtained from the graph by 40.
Step 3
c. i. What important assumption must you make in order to calculate the NaCl content of the sauce from the Na⁺ concentration?
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You must assume that all the sodium present in the sample comes exclusively from NaCl (sodium chloride), and there are no other sodium sources present.
Step 4
c. ii. Calculate the concentration of NaCl in the original (undiluted) sauce in g L⁻¹.
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First, determine the concentration of Na⁺ in the original sauce. If the concentration of Na⁺ is calculated to be 372 mg L⁻¹, then convert this to NaCl concentration:
m(NaCl)=m(Na+)×MW(Na)MW(NaCl)m(NaCl)=372 mg L−1×22.9958.44≈1.36 g L−1
Step 5
c. iii. What percentage of a maximum daily recommended intake would be consumed by a person who eats 10 mL of the original (undiluted) sauce?
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Calculate the amount of NaCl consumed:
m(NaCl) consumed=10 mL×1.36 g L−1=0.0136 g
The percentage of the maximum daily intake:
Percentage=(2.5 g0.0136 g)×100≈0.54%
Step 6
d. Why is it that atomic absorption spectroscopy will measure only the sodium ion concentration in your sample and not the concentration of some other substance or substances as well?
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Atomic absorption spectroscopy specifically measures the absorption of light by free ions in a gaseous state. Each ion has a specific wavelength that it absorbs light at, allowing it to be distinguished from other substances present in the sample.
