How many formula units are in a 3.5g sample of aluminum hydroxide

Use stoichiometry.

3.5g-> moles

22.99+16+1.0079=39.99

Now 3.5/39.99 = .088 moles of NaOH

Now you can use avagadros number 6.022x10^23

.088 moles -> formula units

.088/6.022x10^23

You should end up with

1.46x10-25 formula units.

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.