![]() The average mass of atoms of that element taking into account the mass and amount of each isotope it contains on a scale where the mass of a C-12 atom is 12.Įxample Question: Work out the relative atomic mass of chlorine which is found to contain 75% of atoms with mass number 35, and 25% of atoms with mass number, 37. So now, we can define Relative atomic mass ( Ar) of an element as: The average mass of chlorine atoms is 35.5 as there are more chlorine atoms with a relative mass of 35 than 37. ![]() In the case of chlorine, 75% of the atoms are Cl-35 with a relative mass of 35 and 25%, Cl-37 with relative mass of 37. Remember: mass number = number of protons + number of neutrons.Ī number of elements such as chlorine, magnesium and copper are made up of atoms of different isotopes. Hence, the relative mass of a single atom is equal to the mass of that atom. So, the relative mass of H is 1, meaning it is 12 times lighter than a C-12 atom. The scale used is C-12 where the mass is defined as being exactly 12. So scientist measure the mass of atoms relative to each other. This number is too small and difficult to use. For example, an atom of C-12 has a mass of 2 X 10-23 g. This means that the bag of sugar is 1,500 times heavier than the cube of sugar.Ītoms have a tiny mass. So if the mass of a bag of sugar is 1,500g more than that of a cube of sugar, we can say that the bag of sugar has a relative mass of 1,500g compared to a cube of sugar. It means it is “considered in relation or in proportion to something else”. What do we mean when we use the word “relative”? Relative atomic mass, relative formula mass / formula mass, relative molecular mass, Avogadro constant, and mole. State the principle of conservation of mass.Ĭalculate the relative atomic mass ( A r ) of chlorine which is found to contain 75% of atoms with mass number 35, and 25% of atoms with mass number, 37.Ĭalculate the relative formula mass ( M r)of the following substances:
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