Question

Calculate the number of oxygen molecules and the number of oxygen atoms in 30.5 g of O₂. Enter your answers in scientific notation.
Calculate the mass of 9.31 x 10¹⁴ SO₃ molecules.  Enter your answers in scientific notation

Answer 1

1)

a)

Molar mass of O2 = 32 g/mol

mass(O2)= 30.5 g

use:

number of mol of O2,

n = mass of O2/molar mass of O2

=(30.5 g)/(32 g/mol)

= 0.9531 mol

use:

number of molecules = number of mol * Avogadro’s number

number of molecules = 0.9531 * 6.022*10^23 molecules

number of molecules = 5.74*10^23 molecules

Answer: Number of oxygen molecules = 5.74*10^23 molecules

b)

1 molecule has 2 O atoms.

Number of atoms = 2*number of molecules

= 2*5.74*10^23 atoms

= 1.15*10^24 atoms

Answer: Number of oxygen atoms = 1.15*10^24 atoms

2)

use:

number of mol = number of molecules / Avogadro’s number

number of mol = 9.31*10^14 / 6.022*10^23 mol

number of mol = 1.546*10^-9 mol

Molar mass of SO3,

MM = 1*MM(S) + 3*MM(O)

= 1*32.07 + 3*16.0

= 80.07 g/mol

use:

mass of SO3,

m = number of mol * molar mass

= 1.546*10^-9 mol * 80.07 g/mol

= 1.238*10^-7 g

Answer: 1.24*10^-7 g