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Kinetics P2 Questions

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QS: 

The rate expression for this reaction is rate = k [N2O]2 and the rate constant is
0.244 dm
3 mol–1 s–1 at 750 °C.

 A sample of N2O of concentration 0.200 mol dm–3 is allowed to decompose. Calculate the rate when 10 % of the N2O has reacted.

ANS: 

0.9 × 0.200 = 0.180 (mol dm–3);

rate = (0.244 × (0.180)2 =) 7.91 × 10–3 mol dm–3 s–1

 

Why do we have to times 0.9 to 0.200? I thought since 10% has reacted, 0.1 should be times instead of 0.9.

Please help me clarify this question.

Thanks alot

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The concentrations used in rate calculation are how much are present. If 10% has been consumed then the concentration of N2O remaining is 90% of starting concentration.

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