It should be no surprise that chalk is actually a mixture ?

It should be no surprise that chalk is actually a mixture and is full of impurities. If your stick of chalk is actually 79.8% gypsum, 12.7% calcium carbonate, and 7.5% other materials, what would be the number of calcium atoms?

Update:

The mass of the chalk is 0.03grams 

1 Answer

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  • 2 months ago
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    Molar mass of CaSO₄•2H₂O (gypsum) = (40.1 + 32.1 + 16.0×6 + 1.0×4) g/mol = 172.2 g/mol

    172.2 g CaSO₄•2H₂O contains 1 mol Ca.

    Molar mass of CaCO₃ (calcium carbonate) = (40.1 + 12.0 + 16.0) g/mol = 100.1 g/mol

    100.1 g CaCO₃ contains 1 mol Ca.

    In CaSO₄•2H₂O:

    (0.03 × 79.8% g CaSO₄•2H₂O) × (1 mol Ca / 172.2 g CaSO₄•2H₂O) × (6.022 × 10²³ Ca atoms / 1 mol Ca)

    = 8.37 × 10¹⁹ Ca atoms

    In CaCO₃:

    (0.03 × 12.7% g CaCO₃) × (1 mol Ca / 100.1 g CaCO₃) × (6.022 × 10²³ Ca atoms / 1 mol Ca)

    = 2.29 × 10¹⁹ Ca atoms

    Assume that the other materials do not contain Ca.

    Totally:

    (8.37 × 10¹⁹ + 2.29 × 10¹⁹) Ca atoms

    = 1.07 × 10²⁰ Ca atoms

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