EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记：1.5.3 Moles, Mass & RFM

The Mole

• Chemical amounts are measured in moles
• The symbol for the unit mole is mol
• One mole of a substance contains the same number of the stated particles, atoms, molecules, or ions as one mole of any other substance
• The number of atoms, molecules or ions in a mole (1 mol) of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 10²³ per mole

For example:

• One mole of sodium (Na) contains  6.02 x 10²³  atoms of sodium
• One mole of hydrogen (H₂) contains  6.02 x 10²³  molecules of hydrogen
• One mole of sodium chloride (NaCl) contains  6.02 x 10²³ formula units of sodium chloride

Linking the Mole and the Atomic mass

• One mole of any element is equal to the relative atomic mass of that element in grams or for a compound the relative formula mass in grams
• This is called the molar mass
• If you had 6.02 x 10²³ atoms of carbon in your hand, that number of carbon atoms would have a mass of 12 g (because the A_r of carbon is 12)
• So one mole of helium atoms would have a mass of 4 g (A_r of He is 4), one mole of lithium would have a mass of 7 g (A_r of Li is 7) and so on
• To find the mass of one mole of a compound, we add up the relative atomic masses
  • So one mole of water would have a mass of (2 x 1) + 16 = 18 g
  • So one carbon atom has the same mass as 12 hydrogen atoms

   

• Although elements and chemicals react with each other in molar ratios, in the laboratory we use digital balances and grams to measure quantities of chemicals as it is impractical to try and measure out moles
• Therefore we have to be able to convert between moles and grams
• We can use the following formula to convert between moles, mass in grams and the molar mass:

Formula triangle for moles, mass and molar mass

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