Principles of Chemistry
8 marks · NormalA student is given a mixture of table salt (sodium chloride) and sand. She wants to separate the components and measure the solubility of sodium chloride at 20°C.
Describe a method the student could use to separate the sand from the salt in the mixture. Include the main steps and the name of each process.
After separation, the student makes a saturated solution of sodium chloride in 50 g of water at 20°C. She filters the solution to remove any excess solid, then evaporates the water and obtains 18.6 g of dry sodium chloride crystals.
Use these results to calculate the solubility of sodium chloride in water (in g per 100 g water) at 20°C.
In a different experiment, the student runs a paper chromatography investigation using three coloured inks from different pens. The chromatogram develops as shown below (all spots and the solvent front are marked by distance from the pencil line):
| Pen | Colour | Distance moved by spot (cm) | Distance moved by solvent front (cm) |
|---|---|---|---|
| A | blue | 2.4 | 7.0 |
| B | red | 6.1 | 7.0 |
| C | green | 4.2 | 7.0 |
Calculate the R_f value for the green dye (Pen C).
Explain how a comparison of R_f values can help identify whether two pens use the same dye.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) — · must show Dissolving salt in water, Filtration to remove sand, Evaporation or crystallisation to separate salt
(b) Solubility = 37.2 g per 100 g water · must show Correct calculation using solubility formula
(c) 0.60 · must show Rf = 4.2 / 7.0
(d) — · must show Rf values should be similar under same conditions to suggest same dye, Differences in Rf indicate different dyes
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Method: describes dissolving salt in water and filtration to separate sand.
M2 (a) · 1 mark · Names the separation techniques used (filtration and crystallisation/evaporation).
M3 (b) · 1 mark · Calculates mass of sodium chloride and mass of water used.
A1 (b) · 2 marks · Correctly applies solubility formula and gives final answer to correct unit and significant figures.
A2 (c) · 1 mark · Accurately calculates the R_f value to two decimal places.
M4 (d) · 1 mark · Mentions that comparing R_f values helps in identifying dyes.
A3 (d) · 1 mark · Explains that matching R_f values (in same conditions) suggests the same dye, and different values mean different dyes.
Worked solution
a. The student should add the mixture to water and stir. The sodium chloride will dissolve, but the sand will not. Filter the mixture to remove the sand (residue); the filtrate contains dissolved salt. The water can then be evaporated or the solution crystallised to recover the sodium chloride.
b. Solubility (g per 100 g of water) = (mass of sodium chloride / mass of water) × 100 = (18.6 / 50) × 100 = 37.2 g per 100 g water.
c. R_f for the green dye = (distance moved by spot) / (distance moved by solvent front) = 4.2 / 7.0 = 0.6.
d. If two spots from different inks have the same R_f value when run under exactly the same conditions, it suggests that they are the same dye. However, if the R_f values are different, this indicates the dyes are different.