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10 May, 03:28

Washing soda is a hydrate of sodium carbonate. Elemental analysis of a sample of washing soda gave 4.20% C and 7.05% H. What is the formula for washing soda?

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  1. 10 May, 03:34
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    Na₂CO₃ · 10H₂O

    Explanation:

    The formula for sodium carbonate hydrate is:

    Na₂CO₃ · xH₂O

    The unknown "x" is the number of water molecules contained in the hydrate.

    To find "x" we have to use the hydrogen percentage in the sample, 7.05 % H.

    First we calculate the molecular weight of Na₂CO₃ · xH₂O:

    molecular weight of Na₂CO₃ · xH₂O = 23 * 2 + 12 + 16 * 3 + 18x

    molecular weight of Na₂CO₃ · xH₂O = 106 + 18x g/mole

    Now we devise the fallowing reasoning tanking in account 1 mole of Na₂CO₃ · xH₂O:

    if in 106 + 18x grams of Na₂CO₃ · xH₂O we have 2x grams of hydrogen

    then in 100 grams of Na₂CO₃ · xH₂O we have 7.05 grams of hydrogen

    106 + 18x = (100 * 2x) / 7.05

    106 + 18x = 28.4x

    106 = 28.4x - 18x

    106 = 10.4x

    x = 106 / 10.4

    x = 10.2 ≈ 10

    The formula for the washing soda is Na₂CO₃ · 10H₂O.
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