Relative formula mass | Exit Quiz | Oak National Academy

What does the relative atomic mass (RAM or Ar) compare the mean mass of an atom to?

• 1/12 of the mass of an oxygen-16 atom

• the mass of an electron

• 1/12 of the mass of a carbon-12 atom

• the mass of a proton

Answer explanation

RAM uses carbon-12 as a reference for comparing the mass of atoms.

Order the following steps to calculate the RFM of a compound correctly. Start withe periodic table.

• add up the total relative atomic masses of all the atoms

• use the periodic table to find the relative atomic masses of each type of atom

• calculate the number of atoms × the relative atomic mass value for each element

• identify the number of each type of atom in the formula

Answer explanation

First, find the atomic masses, then identify how many atoms there are, and finally, add them up.

What is the relative formula mass (RFM or Mr) of water (H₂O)?

• 18

• 20

• 16

• 17

• 19

Answer explanation

The correct answer is 18. Water (H₂O) has an RFM of 18, calculated as (2 × 1) + 16.

Calculate the RFM of carbon dioxide (CO₂)

Answer explanation

The RFM of CO₂ is 44, calculated as 12 + (2 × 16).

To calculate the percentage by mass of an element in a compound, you need:

• the RFM of the compound and the RAM of the element

• the atomic number of the element

• the total number of electrons in the compound

• the number of protons in the element

Answer explanation

The RFM of the compound and the RAM of the element. These values are necessary to calculate the percentage by mass of an element in a compound.

What is the percentage by mass of hydrogen in water (H₂O)? Give your answer to four significant figures.

Answer explanation

The percentage by mass of hydrogen in water is approximately 11.11%, calculated as ((2 / 18) × 100).