Shalom, Guys! The molar mass of strontium (\ce{Sr}SrS, r) is \pu{87.62 g/mol}87.62 g/mol87, point, 62, space, g, slash, m, o, l. Calculate the number of atoms in a \pu{67.5 mg}67.5 mg67, point, 5, space, m, g sample of \ce{Sr}SrS, r. Write your answer in scientific notation using three significant figures. \text{atoms Sr} atoms Sr
Love, Piper Rockelle

4.30 × 10²⁰ atoms

Explanation:

You need to find out how many moles you have.  First, convert milligrams to grams.

62.5 mg × (1 g)/(1000 mg) = 0.0625 g

Next, use the molar mass to convert from grams to moles.

0.0625 g ÷ 87.62 g/mol = 7.13 × 10⁻⁴ mol

To convert from moles to atoms, you need to use Avogadro's number (6.022 × 10²³).  Avogadro's number is the number of atoms in a mole.

7.13 × 10⁻⁴ mol × 6.022 × 10²³ atoms/mol = 4.296 × 10²⁰ ≈ 4.30 × 10²⁰ atoms

You will have 4.30 × 10²⁰ atoms of strontium.