Calculate Relative Mass
2 guided lessons · 18 checks · 1. Principles of Chemistry → Chemical Formulae, Equations, Calculations
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Relative Atomic Mass and Relative Formula Mass
Every element carries a relative atomic mass, Ar, printed on the Periodic Table as the larger of its two numbers. A compound has no atomic mass of its own, so it borrows one: the relative formula mass, Mr, totals the atomic masses of all the atoms its formula contains. Add the Ar value of each atom present, and the total is the Mr you can take into any later working.
What you know already
The Periodic Table gave you a relative atomic mass for every element - the larger of the two numbers beside each symbol. This note builds compounds from that starting point: the relative formula mass totals the atomic masses of every atom a formula contains, brackets included. It is the number every reacting-mass working later in this subtopic rests on, so the adding has to be exact.
Key points of understanding
- The relative atomic mass (Ar) of an element is the larger of the two numbers beside its symbol on the Periodic Table.
- The relative formula mass (Mr) of a substance is the total of the atomic masses of every atom its formula contains.
- Mr is also called the relative molecular mass when the substance is a simple molecule.
- Conservation of mass means the total relative formula mass of the reactants equals that of the products in a balanced equation.
- Ar values come from the mass number and the relative abundances of an element's isotopes.
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1.Relative Atomic Mass and Relative Formula Mass
Every element carries a relative atomic mass, Ar, printed on the Periodic Table as the larger of its two numbers. A compound has no atomic mass of its own, so it borrows one: the relative formula mass, Mr, totals the atomic masses of all the atoms its formula contains. Add the Ar value of each atom present, and the total is the Mr you can take into any later working.
Key points
- The relative atomic mass (Ar) of an element is the larger of the two numbers beside its symbol on the Periodic Table.
- The relative formula mass (Mr) of a substance is the total of the atomic masses of every atom its formula contains.
- Mr is also called the relative molecular mass when the substance is a simple molecule.
- Conservation of mass means the total relative formula mass of the reactants equals that of the products in a balanced equation.
- Ar values come from the mass number and the relative abundances of an element's isotopes.
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