MC Question (1 Viewer)

[J-Wang]

How do you do question 20?

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf

 

[someth1ng]

Concentration is PROPORTIONAL of absorbance.

This information should allow you to do the question (they assuming the above as knowledge).

 

[habitres]

Is the answer A? Just working it out then and I got A. (simple maths calculations)

EDIT: I forgot to molecular weight of Copper Carbonate. The correct answer should be B.
Calculations:
10ppm for 0.400 absorbance
?ppm for 0.500 absorbance

Therefore 10/0.4 = 25
25*0.5 = 12.5

As it is in parts per million divide by 1,000,000
= 1.25*10^-5

Times by 100 as it is 100mL
=1.25*10^-3

Now we have the mass of copper in the solution. As it is reacted/precipitated with sodium carbonate to form copper carbonate (CuCO3) we need to get mass of that

since n=m/M
for copper
n =1.25*10^-3/(63.55) - atomic weight of copper
= 1.9669*10^-5

now we need to times that by the atomic weight of copper carbonate

Therefore 1.9669*10^-5 * (63.55+12.01+16*3) = 2.43*10^-3

Correct answer is B.

 

[J-Wang]

Is the answer A? Just working it out then and I got A. (simple maths calculations)

nope. it's B. so how did you do it?

 

[deswa1]

Is the answer A? Just working it out then and I got A. (simple maths calculations)

Nah iirc the answer is B. OP do you want a worked solution?

 

[habitres]

nope. it's B. so how did you do it?

see my worked solution in my above posted (the edit)

 

[J-Wang]

see my worked solution in my above posted (the edit)

thanking you