Amount of Substance - Chemical Ideas 1.1
Basics
Avogadro constant- this is the value used to measure the number of particles in one mole of a substance. Its value is 6.02 x 1023. (NA)
Relative atomic mass- This is the number of times an atom of an element is heavier than one-twelfth of an atom of 12C. Relative atomic mass does not have a unit. (AR)
Relative formula mass- the total mass for each atom in the formula. It is also called relative formula mass (MR)
A mole- the weight of the AR or Mr of a substance
Empirical formula- the ratio of atoms of each element in a compound
Molecular formula- the exact number of atoms of each element in a molecule. This is the same as or a whole number multiple of the empirical formula
The formula for working out the number of moles in a substance is- moles = mass (g)
AR
The formula for working out the number of moles in a molecule or compound is- mass (g)
MR
Example 1
2.43g of magnesium reacts with 0.2g of hydrogen. What is the formula of magnesium hydride?
Step 1 2.43g of Mg = 2.43 = 0.1 moles
24.3
Step 2 0.2g of H = 0.2 = 0.2 moles
1
Step 3 0.1 is the smallest so divide both results it this gives 1 Mg : 2 H. The empirical formula is MgH2
Example 2
Calculate the percentage by mass of hydrogen in ammonium sulphate, (NH4)2SO4.
Step 1 Mr (NH4) 2SO4 = 2 x (14 + 1+ 1 + 1 +1) + 32.1 + (4 x 16) = 132.1
Step 2 Mass of nitrogen in 1 mole of (NH4)2SO4 = 14 +14 = 28
Step 3 % by mass = 28.0 x 100 = 21.2%
132.1
Avogadro constant- this is the value used to measure the number of particles in one mole of a substance. Its value is 6.02 x 1023. (NA)
Relative atomic mass- This is the number of times an atom of an element is heavier than one-twelfth of an atom of 12C. Relative atomic mass does not have a unit. (AR)
Relative formula mass- the total mass for each atom in the formula. It is also called relative formula mass (MR)
A mole- the weight of the AR or Mr of a substance
Empirical formula- the ratio of atoms of each element in a compound
Molecular formula- the exact number of atoms of each element in a molecule. This is the same as or a whole number multiple of the empirical formula
The formula for working out the number of moles in a substance is- moles = mass (g)
AR
The formula for working out the number of moles in a molecule or compound is- mass (g)
MR
Example 1
2.43g of magnesium reacts with 0.2g of hydrogen. What is the formula of magnesium hydride?
Step 1 2.43g of Mg = 2.43 = 0.1 moles
24.3
Step 2 0.2g of H = 0.2 = 0.2 moles
1
Step 3 0.1 is the smallest so divide both results it this gives 1 Mg : 2 H. The empirical formula is MgH2
Example 2
Calculate the percentage by mass of hydrogen in ammonium sulphate, (NH4)2SO4.
Step 1 Mr (NH4) 2SO4 = 2 x (14 + 1+ 1 + 1 +1) + 32.1 + (4 x 16) = 132.1
Step 2 Mass of nitrogen in 1 mole of (NH4)2SO4 = 14 +14 = 28
Step 3 % by mass = 28.0 x 100 = 21.2%
132.1