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The isotope lithium-7 has a mass of 7.0160 atomic mass units, and the isotope lithium-6 has a mass of 6.0151 atomic mass units. given the information that 92.58 percent of all lithium atoms found in nature are lithium-7 and 7.42 percent are lithium-6, calculate the atomic mass of lithium, li (atomic number 3).

Answer :

atomic mass of Li is the weighted average atomic mass with regard to their percentage abundance in nature 
atomic mass of Li - mass of Li-6 x % of Li-6 + mass of Li-7 x % of Li-7
substituting these values in the formula
atomic mass of Li - 6.0151 x 7.42% + 7.0160 x 92.58 %
atomic mass of Li - 0.446 + 6.495 = 6.941 
therefore atomic mass of Li is 6.941

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