Worked Example

Question: A mixture of magnesium bromide and magnesium sulfate contains 3 moles of magnesium and 4 moles of bromide ions. How many moles of sulfate ions are present?

Step 1: Write Formulas

Write the formulas for both compounds:

• Magnesium bromide: Mg²⁺ + (Br⁻)₂
• Magnesium sulfate: Mg²⁺ + SO₄²⁻

Step 2: Determine Ion Ratios

Examine the ratio of ions in each compound and the given information:

• Magnesium bromide: Mg²⁺ + (Br⁻)₂
• Magnesium sulfate: Mg²⁺ + SO₄²⁻

Ratios:

• Mg²⁺ + (Br⁻)₂ has a ratio of 1:2 for Mg²⁺ to Br⁻
• Mg²⁺ + SO₄²⁻ has a ratio of 1:1 for Mg²⁺ to SO₄

Step 3: Calculate Moles of Ions

Use the ratio of ions and information in the question to determine the moles of ions:

• For Mg²⁺ + (Br⁻)₂, the given information states that there are 3 moles of Mg²⁺ and 4 moles of Br⁻. Since the ratio of Mg²⁺ to Br⁻ is 1:2, you have 3 moles of Mg²⁺ and (2 x 3) = 6 moles of Br⁻.
• For Mg²⁺+SO₄-Mg²⁺, the ratio of Mg²⁺ to SO₄²⁻ is 1:1. Since you have 3 moles of Mg²⁺, you also have 3 moles of SO₄²⁻.

Conclusion:

In the mixture, there are 3 moles of sulfate ions (SO₄²⁻). Therefore, the answer is option C.