40 g.
To do this, use the molar mass of aluminum, which is approximately 26.98 g/mol.
\[
\text{Moles of Al} = \frac{\text{mass of Al}}{\text{molar mass of Al}} = \frac{3.40 \, \text{g}}{26.98 \, \text{g/mol}} \approx 0.126 \, \text{mol}
\]
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