1.1.5 Moles-Mass Problems

• Since atoms are so small, any sensible laboratory quantity of substance must contain a huge number of atoms
• Such numbers are not convenient to work with, so using moles is a better unit to deal with the sort of quantities of substance normally being measured
• When we need to know the number of particles of a substance, we usually count the number of moles
• The number of moles or particles can be calculated easily using a formula triangle

The moles and particles formula triangle – cover with your finger the one you want to find out and follow the directions in the triangle

Answer:

• • There are 4 H atoms in 1 molecule of CH₃CHO
  • So, there are 0.040 moles of H atoms in 0.010 moles of CH₃CHO
  • The number of H atoms is the amount in moles x L
  • This comes to 0.040 x (6.02 x 10²³) = 2.4 x 10²² atoms

Answer:

• • In 3.612 x 10²³ molecules of H₂O₂  there are 2 x (3.612 x 10²³) atoms of H
  • So, there are 7.224 x 10²³ atoms of H
  • The number of moles of H atoms is the number of particles ÷ L
  • This comes to 7.224 x 10²³ ÷ (6.02 x 10²³) = 1.20 moles of H atoms

Moles and Mass

• We count in moles by weighing the mass of substances
• The number of moles can be calculated by using a formula triangle

The moles and mass formula triangle – cover with your finger the one you want to find out and follow the directions in the triangle

Answer:

• • From the periodic table the relative atomic mass of Zn is 65.38
  • So, the molar mass is 65.38 g mol^-1
  • The mass is calculated by moles x molar mass
  • This comes to 0.250 mol x 65.38 g mol^-1 = 16.3 g

Answer:

• • The molar mass of sucrose is 342.3 g mol^-1
  • The number of moles is found by mass ÷ molar mass
  • This comes to  2.64 g ÷ 342.3 g mol^-1 = 7.71 x 10^-3 mol