Year 9 Chemistry, Worksheet 19
5 Quantitative aspects of chemical reactions
5.1 Avogadro’s number and the mole
To be able to measure the huge quantities of atoms or molecules in manageable numbers, chemists quantify them using the Avogadro constant or Avogadro’s number.
Scientifically, the Avogadro constant is defined as ___________________________
in 12 g of 12C carbon.
The symbol for the Avogadro constant is ______ it’s value is _________________.
The term mole (dt. Mol) is directly linked to the Avogadro constant to mean exactly this amount of atoms or molecules.
Examples: There are roughly 10.000.000.000.000.000 ants on earth (10 x 1015).
This means there are ____________________ moles of ants on earth.
If a dish contains 2 moles of water molecules, then it contains ________________
molecules
Atoms or molecules are counted in multiples of moles. This count is referred to as _____________________________________ (dt. Stoffmenge). Its symbol is __________, its unit mol.
Examples (continued): n(ants) = n(water) =
12 g of 12C carbon contain 1 mole of atoms. This can be turned around to define the molar mass.
The mass of one mole of any atom, ion or molecule is referred to as its molar mass (dt.
molare Masse). The symbol for the molar mass is ___________, its unit is _________.
The value for the molar mass of an atom is identical to its ___________________ and can be found in ________________________.
The molar mass of a molecule can be calculated by _______________ the molar masses of all atoms in the molecule. When doing calculations with molar masses, it‘s enough to only use the first decimal!
Examples: The molar mass of water (H2O) is M (H2O) = 2 x M(H) + M(O)
= 2 x 1 g
mol + 16 g mol
= 18 g mol
Determine the molar mass M for oxygen (O2), methane (CH4) and chloroform (CHCl3).
Year 9 Chemistry, Worksheet 19
The _______________________ (dt. Stoffmengenkonzentration) of a solution tells us how many moles of a substance are dissolved in _______________ of a solution.
Sometimes the term molarity is used for the same property.
The symbol is ___________, its unit ________.
Examples: The molar mass of NaOH is 40 g mol
If 40 g of NaOH are dissolved in 1 litre of solution, the concentration of the resulting solution is c = 1 mol
l
Determine the concentration of 1 litre of solution containing 117 g of sodium chloride (NaCl).
Determine the mass of sodium hydroxide (NaOH) in 500 ml of a solution with c = 4 mol l .
Tasks
a) Determine the molar mass of ethanoic acid (CH3COOH) and sulfuric acid (H2SO4)
b) Determine the molar concentration for the following solutions:
i. 12 g of ethanoic acid are dissolved in 1 litre of solution
ii. You find a litre of an ethanoic acid solution with a molar concentration of c = 2,5 mol
l . Determine the mass of ethanoic acid dissolved in the solution.
c) Mass (m), molar mass (M), substance amount (n), volume (V) and molar concentration (c), as so many other physical quantities, are linked to each other mathematically.
i. Determine the mathematical relation of n, m and M
Year 9 Chemistry, Worksheet 19
ii. Determine the mathematical relation of V, c and n
