Some Basic Concepts Of Chemistry

Question

Naphthalene (moth balls) contains 93·71% carbon and 6·29% hydrogen. If its molar mass is 128 g mol^–1, calculate its molecular formula.

Solution

(i) To calculate the empirical formula:

(ii) To calculate the molecular formula:
Empirical formual mass

space space left parenthesis straight C subscript 5 straight H subscript 4 right parenthesis space equals space 5 cross times 12 plus 4 cross times 1 space equals space 64 straight g space mol to the power of negative 1 end exponent

Molar space mass space equals space 128 space straight g space mol to the power of negative 1 end exponent

[given]

space straight n equals fraction numerator Molar space mass space of space napthalene over denominator Empirical space formula space mass end fraction equals fraction numerator 128 space straight g space mol to the power of negative 1 end exponent over denominator 64 space straight g space mol to the power of negative 1 end exponent end fraction space equals 2

therefore

Molecular formula of napthalene =

left parenthesis Empirical space formula right parenthesis subscript straight n

equals left parenthesis straight C subscript 5 straight H subscript 4 right parenthesis subscript 2 equals space straight C subscript 10 straight H subscript 8

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