A 228-mL sample of an aqueous solution contains 2.35% MgCl₂ by mass. Exactly one-half of the magnesium ions are Mg-28, a beta emitter with a half-life of 21 hours. What is the decay rate of Mg-28 in the solution after 4.00 days? (Assume a density of 1.02 g/mL for the solution.)

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1. Calculate the mass of the solution using its volume and density: \( \text{mass} = \text{volume} \times \text{density} \).
2. Determine the mass of MgCl2 in the solution using the percentage by mass: \( \text{mass of MgCl2} = \text{mass of solution} \times \frac{2.35}{100} \).
3. Calculate the moles of MgCl2 using its molar mass: \( \text{moles of MgCl2} = \frac{\text{mass of MgCl2}}{\text{molar mass of MgCl2}} \).
4. Since one mole of MgCl2 contains one mole of Mg ions, find the moles of Mg-28 by taking half of the moles of MgCl2.
5. Use the decay formula \( N(t) = N_0 \times \left(\frac{1}{2}\right)^{\frac{t}{t_{1/2}}} \) to find the remaining moles of Mg-28 after 4 days, and then calculate the decay rate using \( \text{decay rate} = \frac{\ln(2)}{t_{1/2}} \times N(t) \).