When a atom loses electrons it becomes a ion
MgCl2(aq) is an ionic compound which will have the releasing of 2 Cl⁻ ions ions in water for every molecule of MgCl2 that dissolves.
MgCl2(s) --> Mg+(aq) + 2 Cl⁻(aq)
[Cl⁻] = 0.73 mol MgCl2/1L × 2 mol Cl⁻ / 1 mol MgCl2 = 1.5 M
The answer to this question is [Cl⁻] = 1.5 M
Energy transferred per second in given area:
8000 x 3.9 / 10000
Energy transferred = 3.12 J
Using Planck's equation:
E = nhc/λ
n = (3.12 x 510 x 10⁻⁹) / (3 x 10⁸ x 6.63 x 10⁻³⁴)
n = 8 x 10¹⁸ photons