Mole & Chemical Composition Honors chemistry – Semester 1

Converting between grams, moles and number of particles

Avogadro's Number

• The number of particles present in 1 mol of substance Avogadro’s Number (NA)

• 1 mol of pure substance = 6.022 x 1023 particles

• Avogadro’s number can be used to count ANY kind of particle, including atoms, molecules, ions, photons …

Sample Problem – converting moles to number of particles

Find the number of molecules in 2.5 mol of sulfur dioxide.

Solution:

• Conversion path: • mol multiply by Avogadro’s number # of particles

• 2.5 mol SO2 x 6.022 x 1023 molecules SO

2

1 mol SO2

= 1.5 x 1024 molecules SO2

Practice Problems

• How many ions are there in 0.187 mol of Na+ ions?

• 1.13 x 1023 ions of Na+

• How many atoms are there in 1.45 x 10-17 mol of arsenic?

• 8.73 x 106 atoms As

• How many molecules are there in 4.224 mol of acetic acid, CH3COOH?

• 2.544 x 1024 molecules CH3COOH

Sample Problem – converting number of particles to moles

A sample contains 3.01 x 1023 molecules of SO2. Determine the amount in moles.

Solution:

• Conversion Path:

• number of particles divide by Avogadro’s Number mole

• 3.01 x 1023 molecules SO2 x 1 𝑚𝑜𝑙 𝑆𝑂

2

6.022 𝑥 1023 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑆𝑂2

= 0.500 mol SO2

Practice Problems

• How many moles of xenon do 5.66 x 1023 atoms equal?

• 0.940 mol Xe

• How many moles of silver nitrate do 2.888 x 1015 formula units equal?

• 4.796 x 10-9 mol AgNO3

• A biologist estimates that there are 2.7 x 1017 termites on Earth. How many moles of termites is this?

• 4.5 x 10-7 mol termites

Converting between Moles and Grams • Use molar mass of a substance

• Molar mass = the mass in grams of 1 mol of a substance (unit: g/mol)

• Atomic mass (amu) same number as molar mass (grams)

• Example: atomic mass of copper is 63.5 amu molar mass of copper is 63.5 g/mol

Sample Problem – converting number of particles to mass Find the mass in grams of 2.44 x 1024 atoms of carbon,

whose molar mass is 12.0 g/mol.

Solution:

• Conversion path: • Divide number of particles by Avogadros’ Number moles

• Multiply moles by molar mass grams

• 2.44 x 1024 atoms C x 1 mol C

6.022 x 1023 atoms C x

12.0 g C1 mol C

= 48.6 g C

Practice

Find the mass in grams of each of the following:

• 2.11 x 1024 atoms of copper

• 222 g Cu

• 3.01 x 1023 formula units of NaCl

• 29.2 g NaCl

• 3.990 x 1025 molecules of CH4

• 1060 g CH4

Sample Problem – Converting Mass to Number of Particles Find the number of molecules present in 47.5 g of glycerol, C3H8O3.

The molar mass is 92.1 g/mol Solution: • Conversion path:

• Divide grams by molar mass moles • Multiply moles by Avogadro’s number number of particles

• 47.5 g glyc. x 1 mol glyc.92.1 g glyc.

x 6.022 x 1023 molecules glyc.

1 mol glyc. =

3.11 x 1023 molecules glycerol

Practice

• Find the number of atoms in 237 g Cu

• 2.25 x 1024 atoms Cu

• Find the number of ions in 20.0 g Ca2+

• 3.00 x 1023 ions Ca+

• Find the number of atoms in 155 mol of arsenic.

• 9.33 x 1025 atoms As