Do you know what Mᵣ stands for?
Relative Atomic Mass and Relative Formula Mass
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What is an isotope?CorrectIncorrect
What is the symbol for relative atomic mass?CorrectIncorrect
What is the symbol for relative formula mass (also known as relative molecular mass)?CorrectIncorrect
Chlorine has two stable isotopes, ³⁵Cl (75.77%) and ³⁷Cl (24.23%). Calculate the Aᵣ of chlorine to 2 decimal places.CorrectIncorrect
Hydrogen has two stable isotopes, ¹H (abundance – 99.972%) and ²H (abundance – 0.001%). Calculate the Aᵣ of hydrogen.CorrectIncorrect
Lead has 4 stable isotopes. ²⁰⁴Pb (abundance – 1.4%), ²⁰⁵Pb (abundance – 24.1%), ²⁰⁷Pb (abundance – 22.1%) and ²⁰⁸Pb (abundance – 52.4%). Calculate the Aᵣ of lead. Answer to 2 decimal places.CorrectIncorrect
Which of the following descriptions matches the Aᵣ of an element?CorrectIncorrect
Which of the following descriptions matches the Mᵣ of a compound?CorrectIncorrect
Calculate the relative formula mass of carbon monoxide (CO). Carbon Aᵣ = 12, Oxygen Aᵣ = 16.CorrectIncorrect
Calculate the relative formula mass of water. Hydrogen Aᵣ = 1, Oxygen Aᵣ = 16.CorrectIncorrect
Calculate the relative formula mass of copper oxide (CuO). Copper Aᵣ = 63.5, Oxygen Aᵣ = 16.CorrectIncorrect
Relative atomic mass used to have a different name. What was that name?CorrectIncorrect
What is Relative Atomic Mass?
Relative atomic mass (Aᵣ) is the mean mass of all of the isotopes of an element. There are many different isotopes of each element and the masses of those which are stable (do not decay over time) can be averaged to find this number.
To calculate Aᵣ you should multiply the mass of each isotope by its relative abundance and add the results together.
Example Aᵣ calculation:
Lithium has two stable isotopes, ⁶Li (abundance – 1.9%) and ⁷Li (abundance – 98.1%).
(6 x 0.019) + (7 x 0.981) = Aᵣ 6.981
What is Relative Formula Mass?
Relative formula mass (Mᵣ) is the sum of the relative atomic masses (Aᵣ) shown in the molecular formula.
Mᵣ is used to determine multiple other important values in chemistry, such as the empirical formula if given the masses of the elements.
Example Mᵣ calculation:
Question: Find the relative formula mass of CO₂.
Add together the Aᵣ of each element, multiplied by the number of its atoms in the compound:
Carbon Aᵣ = 12. Oxygen Aᵣ = 16
12 + (2 x 16) = Mᵣ 44
For more practice Mᵣ and Aᵣ calculations visit this page from the Oregon Institute of Technology
We hope you have enjoyed this quiz on relative atomic and relative formula masses.