This is the q29 of the 2019 Chemistry HSC question. (1 Viewer)

[jinglebells1464646]

Hi Everyone,
This is the q29 of the 2019 Chemistry HSC question. My answer is at the very bottom (method: find the equation of the lines of best fit and then plug in values to get output. It is wrong though. The answer was that the treatment was effective for Cu ions but not for Pb ions. Anyone can pinpoint my error in my working?


Q29
Stormwater from a mine site has been found to be contaminated with copper(II) and lead(II) ions. The required discharge limit is 1.0 mg L–1 for each metal ion. Treatment of the stormwater with Ca(OH)2 solid to remove the metal ions is recommended.

See attachments


My Answer. Perhaps I formulated the line equation wrong...? I suspect so.

 

[MaccaFacta2]

Beer-Lambert Law applies most accurately at low concentrations, so I'd use the 0.00001 mol / L absorbances as the basis for calculations.

This means that for Cu(II) the "after treatment" concentration is 0.040 / 0.140 x 0.00001 mol / L = 0.00000286 mol/L
and for Pb(II) the "after treatment" concentration is 0.080 / 0.090 x 0.00001 mol / L = 0.00000889 mol/L.

1 mg/L is the limit = 0.001 g/L.

For copper, 0.00000286 mol/L = 0.00000286 x 63.54 = 0.00018 grams per Litre, which is below 0.001 g/L.

For lead, 0.00000889 mol/L = 0.00000889 x 207.2 = 0.0018 grams per Litre, which is above 0.001 g/L.

 

[jinglebells1464646]

Should have observed that. Also I got my line formula wrong, should by y=mx not x=my. Thanks