Bell metal contains 80% copper. The mass of Bell metal which contains 1.5×10²⁰ atoms of copper is (Cu=64)

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2mg

20mg

40mg

12.8mg

answer is B.

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Detailed Solution

GAW of Cu = 64g

64g contain $large 6.023 times 10^{23}$ atoms

'X'g contain $large 1.5 times 10^{20}$ atoms

$large X = frac{1.5 times 10^{20}}{6.023 times 10^{23}} times 64$

$large X = 16 times 10^{-3}g$

Weight of Cu in bell metal = $large 16 times 10^{-3}g$

Given

$large begin{array}{*{20}{c}} {underline {Bellmetal} } \ {100g} \ {'W'g} end{array};;begin{array}{*{20}{c}} {underline {Cu} } \ {80g} \ {16 times {{10}^{ - 3}}g} end{array}$

$large {W_{Bellmetal}} = frac{{16 times {{10}^{ - 3}} times 100}}{{80}} = 20 times {10^{ - 3}}g$

W_Bellmetal = 20mg

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