Relative Formula Mass

• What is relative formula mass?

  Sum of relative atomic masses in a compound
• How do you calculate the relative formula mass of MgCl2?

  Add relative atomic masses of Mg and Cl
• What is the relative atomic mass of magnesium (Mg)?

  24
• What is the relative atomic mass of chlorine (Cl)?

  35.5
• How do you find the relative formula mass of MgCl2?

  Calculate 24 + (2 x 35.5)
• What is the relative formula mass of MgCl2?

  95
• How do you find the relative atomic mass from the periodic table?

  It is the same as its mass number
• How many chlorine atoms are in MgCl2?

  Two
• What is the formula to calculate the percentage mass of an element in a compound?

  Ar x number of atoms / Mr x 100
• What is the percentage mass of sodium in sodium carbonate (Na2CO3)?

  43%
• How do you calculate the relative formula mass of Na2CO3?

  Calculate (2 x 23) + 12 + (3 x 16)
• What is the relative atomic mass of sodium (Na)?

  23
• What is the relative atomic mass of carbon (C)?

  12
• What is the relative atomic mass of oxygen (O)?

  16
• How do you find the mass of iron in a mixture containing 20% iron by mass?

  Calculate 50 x 20/100
• What is the mass of iron in 50 g of a mixture with 20% iron?

  10 g
• How do you calculate the percentage mass of iron in iron chloride (FeCl3)?

  Use 56 / (56 + (2 x 35.5)) x 100
• What is the percentage mass of iron in iron chloride (FeCl3)?

  44.09%
• How do you calculate the mass of iron chloride needed to provide 10 g of iron?

  Calculate 10 / 44.09 x 100
• What mass of iron chloride is needed to provide 10 g of iron?

  23 g
• What are the steps to calculate the relative formula mass of a compound?

  1. Look up relative atomic masses on the periodic table.
  2. Add the relative atomic masses of all atoms in the compound.
• What is the process to find the percentage composition by mass of an element in a compound?

  • Use the formula:

  Percentage mass = (Ar x number of atoms) / Mr x 100

  • Substitute the values for Ar and Mr.
• What are the key components needed to solve percentage mass problems in chemistry?

  • Relative atomic mass (Ar) of elements
  • Relative formula mass (Mr) of the compound
  • Mass of the mixture or compound