Solutions containing copper ions were analysed by AAS. A standard solution of 10 ppm copper had an AAS absorbance of 0.400. A second solution of unknown concentration was found to have an absorbance of 0.500. 100 mL of this second solution was reacted with excess sodium carbonate solution. The precipitate was then dried and weighed. What mass of precipitate was formed? (A) 1.25 x 10^-3 g (B) 2.43 x 10^-3 g (C) 1.54 g (D) 2.43 g

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Solutions containing copper ions were analysed by AAS - HSC - SSCE Chemistry - Question 20 - 2010 - Paper 1

Question 20

Solutions containing copper ions were analysed by AAS. A standard solution of 10 ppm copper had an AAS absorbance of 0.400. A second solution of unknown concentratio... show full transcript

Worked Solution & Example Answer:Solutions containing copper ions were analysed by AAS - HSC - SSCE Chemistry - Question 20 - 2010 - Paper 1

Step 1

Calculate the concentration of the unknown solution

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Answer

The absorbance of the unknown solution is 0.500. Using Beer's Law:

$A = k \cdot c$ where $A$ is the absorbance, $k$ is the constant related to the standard solution, and $c$ is the concentration. Rearranging gives:

$c = \frac{A}{k} = \frac{0.500}{0.400} \cdot 10 ppm = 12.5 ppm$

Thus, the concentration of the unknown solution is 12.5 ppm.

Step 2

Calculate the mass of copper in the unknown solution

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Answer

To find the mass of copper in the 100 mL solution, we can use the formula:

$mass = concentration \times volume$

Here, the volume is in litters (100 mL = 0.1 L):

$mass = 12.5 ppm \times 0.1 L = 12.5 mg$

Converting to grams:

$12.5 mg = 0.0125 g$

Step 3

Determine the mass of the precipitate formed

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Answer

Copper(II) carbonate (CuCO₃) is precipitated from the reaction. From stoichiometry, we know:

$Cu^{2+} + CO_3^{2-} \rightarrow CuCO_3$

The molar mass of CuCO₃ is approximately 123.55 g/mol. Thus, the mass of CuCO₃ formed from 0.0125 g of Cu:

Find moles of Cu:

$moles_{Cu} = \frac{0.0125 g}{63.55 g/mol} \approx 1.97 \times 10^{-4} mol$

Since 1 mole of Cu produces 1 mole of CuCO₃, moles of CuCO₃ formed is the same:

$moles_{CuCO₃} = 1.97 \times 10^{-4} mol$

Now calculate mass:

$mass_{CuCO₃} = moles_{CuCO₃} \times molar mass_{CuCO₃} = 1.97 \times 10^{-4} mol \times 123.55 g/mol \approx 0.0243 g = 2.43 \times 10^{-2} g$

Solutions containing copper ions were analysed by AAS - HSC - SSCE Chemistry - Question 20 - 2010 - Paper 1

Worked Solution & Example Answer:Solutions containing copper ions were analysed by AAS - HSC - SSCE Chemistry - Question 20 - 2010 - Paper 1

Calculate the concentration of the unknown solution

Calculate the mass of copper in the unknown solution

Determine the mass of the precipitate formed

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