Relative formula mass

30 Mar Relative formula mass

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For today’s Chemistry tuition, we’ll be learning about relative formula mass. Atoms have very little mass. Instead of using their actual masses in kilograms, we use their relative atomic masses. Relative atomic mass is given the symbol Ar. The Ar of an element is the mean mass of its atoms compared to 1/12th the mass of a carbon-12 atom. Relative formula mass is given the symbol Mr. If the substance consists of molecules, the term relative molecular mass can be used instead.

Relative atomic mass takes up a huge weightage in the topic of Moles and for one to understand the topic better, one should definitely understand the relative atomic mass to ace in the topic. The relative atomic mass of an element shows its mass compared with the mass of atoms of other elements. The relative atomic mass of carbon is 12, while the relative atomic mass of magnesium is 24. This means that each magnesium atom is twice the mass of a carbon atom

Aside from relative atomic mass, relative atomic masses can be used to find the relative formula mass of a compound too. To find the relative formula mass (Mr) of a compound, you add together the relative atomic mass values (Ar values) for all the atoms in its formula. The relative formula mass of a substance, shown in grams, is called one mole of that substance. So one mole of carbon monoxide has a mass of 28 g, and one mole of sodium oxide has a mass of 62 g. To calculate Mr for a substance, firstly, work out how many atoms of each element there are in the chemical formula. Then, add together the Ar values for all the atoms of each element present.

For example, calculate the relative formula mass of carbon dioxide.
(Relative atomic masses: C=12.0, O=16.0)
Mr of CO2 = 12.0 + 16.0 + 16.0 = 44.0
It could also be calculated this way:
Mr of CO2 = (1×12.0) + (2×16.0) = 12.0 + 32.0 = 44.0

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