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A student was given a bucket of sea water for analysis - Leaving Cert Chemistry - Question 3 - 2006
Question 3
A student was given a bucket of sea water for analysis. The student was asked to find out the concentrations of suspended and dissolved solids in the sea water. The ... show full transcript
Worked Solution & Example Answer:A student was given a bucket of sea water for analysis - Leaving Cert Chemistry - Question 3 - 2006
Step 1
Why did the student wash the filter paper with distilled water after filtering the sea water?
Answer
The student washed the filter paper with distilled water to remove any dissolved solids or salts that may have adhered to the filter paper during the filtration process. This ensures that the mass measured after drying is solely due to the suspended solids collected from the sea water, thus providing an accurate measurement of the suspended solids concentration.
Step 2
Express the concentration of suspended solids in p.p.m.
Answer
To calculate the concentration of suspended solids in ppm (parts per million), use the formula:
ext{Concentration (ppm)} = rac{ ext{mass of suspended solids (g)}}{ ext{volume of solution (L)}} imes 10^6Given that the mass of the suspended solids is 0.44 g and the volume of the sea water filtered is 500 cm³ (which is equivalent to 0.5 L), the calculation is:
ext{Concentration (ppm)} = rac{0.44 ext{ g}}{0.5 ext{ L}} imes 10^6 = 880 ext{ ppm}Step 3
Describe how the student could have then measured the concentration of dissolved solids in the sea water.
Answer
To measure the concentration of dissolved solids in the sea water, the student would follow these steps:
- Weigh a clean beaker to obtain its mass.
- Evaporate water from the filtered sea water sample using a heat source until all water has evaporated, leaving only the dissolved solids behind.
- Reweigh the beaker with the solid residue to find the new mass.
- Calculate the increase in mass, which will provide the mass of dissolved solids that were present in the seawater sample. This mass can then be used to express the concentration of dissolved solids in parts per million (ppm) based on the original volume of sea water used.
Step 4
Describe how the student could have carried out a flame test to show that a sodium salt was present in the dissolved solids collected. What flame colour would indicate the presence of sodium ions?
Answer
To carry out a flame test for sodium ions:
- Obtain a sample of the dissolved solids, preferably in the form of a paste or solution.
- Use a clean sliver spatula to place a small amount of the sample onto a clean platinum wire or nichrome wire probe, ensuring it is free from contamination.
- Place the wire in the flame of a Bunsen burner, allowing it to heat up.
- Observe the color of the flame. If sodium ions are present, the flame will impart a bright yellow color. This characteristic color is due to the excitation of sodium atoms in the flame, which emit light at specific wavelengths.
Step 5
How could the student have tested the sea water to show that chloride ions were present?
Answer
To test for the presence of chloride ions (Cl⁻) in the sea water, the student could use the following procedure:
- Add a few drops of silver nitrate solution (AgNO₃) to a sample of the sea water in a clean test tube.
- Observe any changes. If chloride ions are present in the sea water, a white precipitate of silver chloride (AgCl) will form, indicating the presence of chloride ions.
- The formation of the white precipitate is a positive test for chloride ions in the sample.