How many atoms are there in each of the following substances?
a) 5.14 g p
b) 2.16 g hg
c) 1.80 g bi
d) 8.8×10−2 g sr

To know the number of atoms, it will be necessary to use the formula of avogadro. We calculate the number of moles then we can calculate the number of avogadro.

The molar mass of:

Phosphorus = 31g / mol

Mercury = 200 g / mol

Bismuth = 209 g / mol

Strontium = 87 g / mol.

The number of avogadro (N) is 6.023 10 ^ 23

n = m (mass) / M  (molar mass)

number of atoms = n. N = m / M  . N

number of phosphorus atoms = 5.14/31 . 6.023 10^23 = 9.98 10 ^ 22 atoms

Number of atoms of mercury = 2.16/200 . 6.023 10^23 = 6.5 10 ^ 21 atoms

Number of bismuth atoms = 1.8/209 . 6.023 10^23 = 5.18 10 ^ 21 atoms

Number of strontium atoms = 8.8 x 10-2 /87 . 6.023 10^23 = 6.09 10 ^ 20 atoms

there are two significant figures.