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A bracelet, originally made of pure silver, became tarnished over time with black silver sulfide (Ag2S) forming on the surface - Leaving Cert Chemistry - Question c - 2012
Question c
A bracelet, originally made of pure silver, became tarnished over time with black silver sulfide (Ag2S) forming on the surface. The bracelet was cleaned by convertin... show full transcript
Worked Solution & Example Answer:A bracelet, originally made of pure silver, became tarnished over time with black silver sulfide (Ag2S) forming on the surface - Leaving Cert Chemistry - Question c - 2012
Step 1
What substance was oxidised in this cleaning process?
Answer
The substance that was oxidised in this cleaning process is aluminum (Al). During the reaction, aluminum donates electrons, which causes the reduction of silver sulfide (Ag2S) back to metallic silver (Ag). In this process, aluminum is oxidised.
Step 2
How many moles of sulfur (S) were removed from the bracelet when the silver sulfide (Ag2S) was converted to aluminum sulfide (Al2S3)?
Answer
To find the moles of sulfur removed, we first need the amount of silver sulfide formed. Given that every mole of Ag2S contains 1 mole of sulfur, we can calculate it as follows:
From the decrease in mass: 0.0096 g of Ag2S contains:
- Molar mass of Ag2S = 2(107.87) + 32.07 = 247.81 g/mol
- Moles of Ag2S = ( \frac{0.0096 ext{ g}}{247.81 ext{ g/mol}} \approx 0.00003873 \text{ mol} )
As each mole of Ag2S contains 1 mole of sulfur, moles of sulfur removed = 0.00003873 mol. However, since we are converting it to Al2S3:
0.00003873 mol Ag2S converts to 0.00003873 mol S.
Step 3
What mass of aluminium was used in the reaction?
Answer
From the reaction, 3 moles of Ag2S react with 2 moles of Al. Therefore, the number of moles of aluminum used can be calculated from the moles of Ag2S:
Using the previously calculated moles of Ag2S (0.00003873 mol):
- Moles of aluminum (Al) = ( 0.00003873 imes \frac{2}{3} \approx 0.00002582 \text{ mol} )
Now, converting this to mass:
- Molar mass of Al = 26.98 g/mol
- Mass of Al = ( 0.00002582 ext{ mol} \times 26.98 ext{ g/mol} \approx 0.000694 ext{ g} )
Step 4
What would the loss in mass of the tarnished bracelet have been if it had been cleaned by the alternative method of removing all of the silver sulfide by polishing?
Answer
If we were to clean the bracelet by polishing, we would remove all the silver sulfide (Ag2S). The loss of mass can be calculated as follows:
Using the same moles of sulfur calculated previously:
- Total mass loss = ( 0.0003 ext{ mol} imes 248 ext{ g/mol} = 0.0744 ext{ g} )
Thus, the total loss in mass would be 0.0744 g.