Question

how many molecules of water are present in a drop of water which has a mass of 50 milligram

Answer 1

No. of moles of water molecules = 0.05 /18 = 0.002 mole
So no. of water molecules in 0.002 mole = 0.002 x Na (Na=6.022x10^23)

Answer 2

Hope u like my process
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=> 1 mg = 0.001 g

=> 50 mg = 0.05 g

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=> 18 g of water contains 6.023 × 10^(23) molecules

=> 1g of water contains

$\bf \frac{6.023 \times {10}^{23} }{18} \: \: molecules$


=> 0.05 g of water contains

$= \bf\frac{6.023 \times {1 0 }^{23} }{18} \times 0.05 \\ \\ = \bf \: \underline{1.673 \times {10}^{21} } \: \: molecues$
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