Two Moles or Not Two Moles

Since Michael asked, on the right is a mole of silica and on the left is a mole of magnesium carbonate. Notice the size difference! But they have the same number of atoms: 6.03 x 1023 or 603,000,000,000,000,000,000,000. One is very light and fluffy and the other is heavy and dense.

John