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ICSE Class 9 Chemistry Question 12 of 34

The Language of Chemistry — Question 32

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Question 23

Define the terms – (a) Relative atomic mass (b) Relative molecular mass. State why indirect methods are utilized to determine the absolute mass of an atom. Explain in brief the indirect method used.

Answer

(a) The number of times one atom of an element is heavier than 1⁄12th the mass of an atom of carbon [C12] is known as the Relative Atomic Mass [RAM] of the element.

Mass of one atomRAM=of the element(112) Mass of one atom‾of carbon [C12]\begin{matrix} & \text{Mass of one atom} \\ \text{RAM} = & \text{of the element} \\ & \overline{(\frac{1}{12}) \space \text{Mass of one atom}} \\ & \text{of carbon [C}^{12}] \end{matrix}

(b) The number of times one molecule of the substance is heavier than 1⁄12th the mass of an atom of carbon [C12] is known as Relative Molecular Mass [RMM] of the element.

Mass of one moleculeRMM=of the substance(112) Mass of one atom‾of carbon [C12]\begin{matrix} & \text{Mass of one molecule} \\ \text{RMM} = & \text{of the substance} \\ & \overline{(\frac{1}{12}) \space \text{Mass of one atom}} \\ & \text{of carbon [C}^{12}] \end{matrix}

As atoms are extremely small and very light, hence cannot be weighed directly. So indirect methods are utilized to determine the absolute mass of an atom.

The relative mass of an atom or molecule is hence considered by considering a mass of a light atom and relating the mass of other atoms or molecules to it.