Question

How many atoms are in a sample of 72.8 grams calcium (Ca)?

Answer 1

Atomic mass Ca = 40.078 a.m.u

40.078 g -------------------- 6.02x10²³ atoms
72.8 g ---------------------- ??

72.8 x ( 6.02x10²³) / 40.078 =

4.38x10²⁵ / 40.078 = 1.093x10²⁴ atoms

hope this helps!

Answer 2

First we need to convert grams in moles.
1 mole of calcium has 40 grams of calcium.
72.8 grams are:
72.8/40=1.82 moles

Now we use Avogadro's number to determine number of atoms in 1.82 moles

Number of moles N is:
N=1.82 * 6.02*10^23 = 1.0956 * 10^24 atoms.