1. Interpret ppm in dilute aqueous solution

For aqueous solutions, $1\,\text{ppm} \approx 1\,\text{mg L}^{-1}$

So, a concentration of $48\,\text{ppm}$ of Mg means: $48\,\text{mg of Mg per litre of solution}$

2. Find mass of Mg in 2 L solution

Given volume = $2\,\text{L}$

Hence mass of Mg present is $48\,\text{mg L}^{-1} \times 2\,\text{L} = 96\,\text{mg}$

Convert into grams: $96\,\text{mg} = 0.096\,\text{g}$

3. Calculate moles of Mg

Molar mass of Mg = $24\,\text{g mol}^{-1}$

So, number of moles is $n = \frac{0.096}{24} = 0.004\,\text{mol} = 4 \times 10^{-3}\,\text{mol}$

4. Calculate number of Mg atoms

Using Avogadro number, $N = nN_A = 4\times 10^{-3} \times 6.02\times 10^{23}$

$N = 24.08\times 10^{20} \text{ atoms}$

Thus, $N = x \times 10^{20}$ so $x = 24.08$

Nearest integer: $x = 24$

5. Comparison with stored answer

Stored correct answer = $24$

Our derived answer also gives $24$.