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19 October, 20:54

An element A has a triiodide with the formula AI₃ and a trichloride with the formula ACl₃. The triiodide is quantitatively converted to the trichloride when it is heated in a stream of chlorine, according to the reaction.

AI₃ + 3/2 Cl₂ → ACl₃ + 3/2 I₂ If 0.8000 g AI₃ is treated, 0.3776 g ACl₃ is obtained.

Calculate the atomic mass of the element A. Identify the element A.

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  1. 19 October, 21:28
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    The element A is Lanthanum, with a molar mass of 138.9g/mol

    Explanation:

    AI3 + 3/2 Cl2 → 3/2 I2 + ACl3

    Compounds:

    Al ⇒ Molar mass = M

    AI3 ⇒ Molar mass = M + 3*126.9

    ACl3 ⇒Molar mass = M + 3*35.45

    For 1 mole AI3 we have 1 mole ACl3

    (0.800g * AI3) / (M + 3*126,90) g/mole = (0.3776 g * ACl3) / (M + 3*35,45) g/mole

    0.800 * (M + 3*35,45) = 0.3776 (M + 3*126,90)

    0.8M + 85.08 = 0.3776M + 143.75

    0.8M - 0.3776M = 143.75-85.08

    0.4224M = 58.67

    M = 138.9 g/mol

    So A has a Molar mass of 138.9 g/mol ⇒ if we look at the periodic table we can find that Lanthanum (La) has a Molar mass of 138.9g/mol.
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