Relative Masses

• What is the purpose of using relative masses instead of actual masses in chemistry?

  To make it easier to calculate and compare the masses of different atoms
• What does the relative atomic mass (A) represent in the periodic table?

  It is the average mass of an element's isotopes
• How is the relative atomic mass (A) of an element with one isotope determined?

  It is the same as its mass number
• How is the relative atomic mass (A) calculated for an element with multiple isotopes?

  It is the average of the mass numbers of all isotopes, weighted by their abundance
• What are the stable isotopes of chlorine mentioned in the study material?

  Chlorine-35 and chlorine-37
• What is the relative formula mass (M) of a compound?

  It is the sum of the relative atomic masses of all the atoms in its formula
• How do you calculate the relative formula mass of magnesium chloride (MgCl₂)?

  • Find the relative atomic mass of magnesium (Mg) = 24.3
  • Find the relative atomic mass of chlorine (Cl) = 35.5
  • Calculate:

  $M =$$24.3 +$$(2 \times 35.5) =$$24.3 +$$71.0 =$$95.3$
• How do you calculate the relative molecular mass of ethanoic acid (CH₃COOH)?

  • Find the relative atomic masses:
  • Carbon (C) = 12.0
  • Hydrogen (H) = 1.0
  • Oxygen (O) = 16.0
  • Calculate:

  $M =$$(2 \times 12.0) +$$(4 \times 1.0) +$$(2 \times 16.0) =$$12.0 +$$3.0 +$$16.0 +$$16.0 +$$1.0 =$$60.0$
• How do you find the relative formula mass of calcium hydroxide (Ca(OH)₂)?

  • Find the relative atomic mass of calcium (Ca) = 40.1
  • Find the relative atomic masses of oxygen (O) and hydrogen (H):
  • O = 16.0, H = 1.0
  • Calculate:

  $M =$$40.1 +$$[(16.0 + 1.0) \times 2] =$$40.1 +$$34.0 =$$74.1$
• What does the small number after the bracket in the formula Ca(OH)₂ indicate?

  It indicates that there are two of everything inside the brackets
• What is the relationship between relative molecular mass and relative formula mass for simple covalent compounds?

  They are usually referred to as the same thing
• How does the calculation of relative atomic mass differ for elements with multiple isotopes compared to those with a single isotope?

  For multiple isotopes, it is an average based on abundance, while for a single isotope, it equals the mass number