Calculate the mass of:
(i) an atom of sodium
(ii) in a single molecule of water.

Solution

left parenthesis straight i right parenthesis space 6.023 space straight x space 10 to the power of 23 space atoms space of space sodium space equals space Gram space atomic space mass space of space sodium space equals space 23 straight g

therefore space 1 space atom space of space sodium space equals space fraction numerator 23 over denominator 6.023 space straight x space 10 to the power of 23 end fraction space equals space 3.82 space cross times space 10 to the power of negative 23 end exponent straight g

(ii) Gram molecular mass of H₂O = 18 g
Now 6.023 x 10²³molecules of water have mass = 18 g.

therefore space space space 1 space molecule space of space water space will space have space mass space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space equals fraction numerator 18 over denominator 6.023 space cross times space 10 to the power of 23 end fraction space equals space 2.99 space cross times space 10 to the power of negative 23 end exponent straight g

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