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1 mole of H₂ reacts with 0.5 mole of O₂ to form 1 mole of H₂O.
At STP, the volume is directly proportional to moles.

Since 1 mole of H₂ reacts with 0.5 mole of O₂, 20 ml of H₂ reacts with 10ml of O₂ and form 10ml of H₂O.

volume of 1 mole H₂O = 22.4 l = 22400 ml
number of moles in 10ml H₂O =  \frac{10}{22400}= \frac{1}{2240}\ moles
number of molecules in 1 mole = 6.022 \times 10^{23}

number of molecules of water in 10ml =  \frac{1}{2240} \times 6.022 \times 10^{23}=2.688 \times 10^{20}\ molecules

So 2.688×10²° molecules of water are produced.

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