1) How many sulfur atoms are there in 21.0 g of Al2S3? A) 8.42  1022 atoms D) 6.02  1023 atoms B) 2.53  1023 atoms E) 6.30  1026 atoms C) 2.14  1023 atoms Ans) B

In 21 grams of Al2S3 we have  2.53 *10^23 S atoms (optionB is correct)

Explanation:

Step 1: Data given

Mass of Al2S3 = 21.0 grams

Molar mass of Al2S3 = 150.158 g/mol

Number of Avogadro = 6.022 * 10^23 / moles

Step 2: Calculate moles of Al2S3

Moles Al2S3 = mass Al2S3 / molar mass Al2S3

Moles Al2S3 = 21.0 grams / 150.158 g/mol

Moles Al2S3 = 0.140 moles

Step 3: Calculate moles sulfur

For 1 mol Al2S3 we have 3 moles S

For 0.140 moles Al2S3 we have 3*0.140 = 0.420 moles S

Step 4: Calculate sulfur atoms

Number of sulfur atoms = moles * Number of Avogadro

Number of sulfur atoms = 0.420 moles * 6.022 * 10^23 / moles

Number of sulfur atoms = 2.53 *10^23 S atoms

In 21 grams of Al2S3 we have  2.53 *10^23 S atoms (optionB is correct)