How many aluminum atoms are in 3.78 g of aluminum?

Molar mass of aluminum = 26.982 g/mol.

1 mol of Al = 26.9≈27g = 6.02*10²³ atoms [ avogadro's number]

meaning

1 g | Al = 6.02/27 * 10²³ atoms

so 3.78g | Al = 6.02*3.78]/27 * 10²³

= 0.8428 * 10²³ atoms

= 8.428 * 10²² atoms