The relative atomic mass of a naturally occurring lithium consisting of 90% Li and 10% Li is

The relative atomic mass of an element is the average mass of all the isotopes of that element, taking into account their natural abundance. Lithium has two naturally occurring isotopes: lithium-6 and lithium-7. The first isotope, lithium-6, makes up 90% of naturally occurring lithium, while the second isotope, lithium-7, makes up the remaining 10%. To find the relative atomic mass of this mixture of isotopes, we need to take the weighted average of their atomic masses, where the weights are their natural abundances. The atomic mass of lithium-6 is 6 atomic mass units (amu), while the atomic mass of lithium-7 is 7 amu. Using the formula for weighted average, we get: (0.9 x 6 amu) + (0.1 x 7 amu) = 5.4 amu + 0.7 amu = 6.1 amu Therefore, the relative atomic mass of naturally occurring lithium consisting of 90% Li-6 and 10% Li-7 is 6.1 amu, which is closest to option (C) 6.2.