0.635g brass dissolved in acid and treated with excess KI, the liberated iodine consumes 60 ml of 0.2N sodiumthiosulphate solution. The w% of copper in brass is?

To determine the weight percentage of copper in the brass sample, we need to analyze the chemical reactions involved and the stoichiometry of the process. Brass is primarily an alloy of copper and zinc, but for this calculation, we'll focus on the copper content since iodine is liberated from the copper when brass is treated with acid and potassium iodide (KI).

Understanding the Reaction

When brass is dissolved in acid, the copper reacts with the iodide ions from potassium iodide to form copper iodide (CuI) and liberate iodine (I₂). The liberated iodine can then be titrated with sodium thiosulfate (Na₂S₂O₃), which reduces iodine back to iodide ions.

Key Reactions

• Cu + 2KI → CuI + 2K
• I₂ + 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆

Calculating Moles of Sodium Thiosulfate

We know that 60 mL of 0.2N sodium thiosulfate solution was used. First, we need to calculate the number of equivalents of thiosulfate used in the reaction:

Equivalents of Na₂S₂O₃:

Volume (L) = 60 mL = 0.060 L

Normality (N) = 0.2 N

Equivalents = Volume (L) × Normality (N) = 0.060 L × 0.2 N = 0.012 equivalents

Relating Iodine to Copper

From the stoichiometry of the reactions, we see that 1 mole of iodine (I₂) reacts with 2 equivalents of sodium thiosulfate. Therefore, the moles of iodine liberated can be calculated as follows:

Moles of I₂:

Equivalents of I₂ = 0.012 equivalents / 2 = 0.006 moles

Calculating Moles of Copper

Since 1 mole of copper produces 1 mole of iodine, the moles of copper that reacted is also 0.006 moles.

Finding the Mass of Copper

The molar mass of copper (Cu) is approximately 63.55 g/mol. Therefore, the mass of copper in the sample can be calculated as follows:

Mass of Copper:

Mass = Moles × Molar Mass = 0.006 moles × 63.55 g/mol = 0.4023 g

Calculating the Weight Percentage of Copper in Brass

Now, we can find the weight percentage of copper in the brass sample. The total mass of the brass sample is 0.635 g.

Weight Percentage of Copper:

Weight % of Cu = (Mass of Cu / Total Mass of Brass) × 100

Weight % of Cu = (0.4023 g / 0.635 g) × 100 ≈ 63.3%

Final Result

The weight percentage of copper in the brass sample is approximately 63.3%. This calculation illustrates how stoichiometry and chemical reactions can be used to analyze the composition of metal alloys effectively.