Explanation:
When equal volumes of 3.5 M CaCl2 and 3.0 M NaCl are mixed, the final solution will contain both calcium ions and chloride ions from CaCl2 and sodium ions and chloride ions from NaCl.

Step 1: Calculate the total moles of chloride ions in the final solution.
- The molarity of CaCl2 is 3.5 M, which means that there are 3.5 moles of CaCl2 in 1 liter of solution. Since we are mixing equal volumes of CaCl2 and NaCl, the total volume of the final solution will be the sum of the volumes of CaCl2 and NaCl.
- Let's assume that we are mixing 1 liter of each solution. Therefore, the total volume of the final solution will be 2 liters.
- The moles of chloride ions in 1 liter of 3.5 M CaCl2 solution can be calculated as follows:
- 3.5 M CaCl2 = 3.5 moles of CaCl2 / 1 liter of solution
- Since there are 2 moles of chloride ions per mole of CaCl2, the moles of chloride ions in 1 liter of 3.5 M CaCl2 solution is: 2 x 3.5 = 7 moles of chloride ions
- Therefore, the total moles of chloride ions in 2 liters of the final solution will be: 7 x 2 = 14 moles of chloride ions

Step 2: Calculate the molarity of chloride ions in the final solution.
- The total moles of chloride ions in the final solution is 14.
- The total volume of the final solution is 2 liters.
- Therefore, the molarity of chloride ions in the final solution is: 14 / 2 = 7 M
- However, we need to remember that the final solution contains both CaCl2 and NaCl, which means that the total concentration of chloride ions is the sum of the concentrations of chloride ions from both salts.
- The molarity of NaCl is 3.0 M, which means that there are 3 moles of NaCl in 1 liter of solution. Since we are mixing equal volumes of NaCl and CaCl2, the final solution will contain 3 moles of NaCl.
- Therefore, the total moles of chloride ions from NaCl in the final solution will be: 3 x 2 = 6 moles of chloride ions.
- The total moles of chloride ions in the final solution is 14, and the moles of chloride ions from NaCl is 6. Therefore, the moles of chloride ions from CaCl2 is: 14 - 6 = 8 moles of chloride ions.
- The total volume of the final solution is 2 liters.
- Therefore, the molarity of chloride ions in the final solution is: (6 + 8) / 2 = 7 M

Conclusion:
The molarity of chloride ion in the final solution is '5'.