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Chlorophyll from crushed green leaves was dissolved in water to form 2 L of solution in which the magnesium concentration is 48 ppm. The number of Mg atoms in this solution is x * 10²⁰. Find x (nearest integer). (Atomic mass of Mg = 24 g/mol, NA = 6.02 * 10²³ /mol.)

1. 24
2. 48
3. 12
4. 6

Correct answer: 24

Solution

48 ppm = 48 mg/L, so in 2 L there are 96 mg = 0.096 g Mg; moles = 0.096/24 = 0.004; atoms = 0.004 * 6.02e23 = 2.408e21 = 24.08 * 10²⁰, so x = 24.

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