Zn rod weighing 25 g was kept in 100 mL of 1M copper sulphate solution. After
certain time interval, the molarity of Cu2+ was found to be 0.8 M. What is the
molarity of SO4 -2 in the resulting solution and what should be the mass of Zn rod after cleaning and drying ?

To solve this problem, we need to analyze the reaction between zinc (Zn) and copper sulfate (CuSO4) and determine the changes in molarity and mass that occur during the reaction. Let's break this down step by step.

The Reaction Between Zinc and Copper Sulfate

When a zinc rod is placed in a copper sulfate solution, a displacement reaction occurs. Zinc, being more reactive than copper, displaces copper ions from the solution. The balanced chemical equation for this reaction is:

• Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

This equation tells us that one mole of zinc reacts with one mole of copper sulfate to produce one mole of zinc sulfate and one mole of copper metal.

Initial Conditions

Initially, we have:

• Volume of CuSO4 solution = 100 mL = 0.1 L
• Molarity of CuSO4 = 1 M
• Initial moles of CuSO4 = Molarity × Volume = 1 mol/L × 0.1 L = 0.1 moles

Final Conditions

After some time, the molarity of Cu²⁺ ions is found to be 0.8 M. We can calculate the moles of Cu²⁺ remaining in the solution:

• Final moles of Cu²⁺ = Molarity × Volume = 0.8 mol/L × 0.1 L = 0.08 moles

Now, we can find out how many moles of Cu²⁺ were consumed:

• Moles of Cu²⁺ reacted = Initial moles - Final moles = 0.1 moles - 0.08 moles = 0.02 moles

Calculating Molarity of SO₄^2-

Since the reaction produces zinc sulfate (ZnSO4) in a 1:1 ratio with CuSO4, the moles of SO₄^2- produced will be equal to the moles of Cu²⁺ that reacted. Therefore, we have:

• Moles of SO₄^2- produced = 0.02 moles

To find the molarity of SO₄^2- in the resulting solution, we use the formula:

• Molarity = Moles/Volume

Substituting the values:

• Molarity of SO₄^2- = 0.02 moles / 0.1 L = 0.2 M

Determining the Mass of the Zinc Rod

Next, we need to calculate the mass of the zinc rod after the reaction. Initially, the rod weighed 25 g. The moles of zinc that reacted can be determined from the moles of Cu²⁺ that reacted, which is also 0.02 moles (from the stoichiometry of the reaction).

The molar mass of zinc (Zn) is approximately 65.38 g/mol. Therefore, the mass of zinc that reacted can be calculated as follows:

• Mass of Zn reacted = Moles × Molar mass = 0.02 moles × 65.38 g/mol = 1.3076 g

Now, we can find the final mass of the zinc rod:

• Final mass of Zn rod = Initial mass - Mass of Zn reacted = 25 g - 1.3076 g ≈ 23.69 g

Summary of Results

In conclusion, the molarity of SO₄^2- in the resulting solution is 0.2 M, and the mass of the zinc rod after cleaning and drying is approximately 23.69 g.