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For the question regarding the initial reaction rate, I don't quite understand why the answer is D. enter image description here

If I calculated the number of moles from the volume and conc, I would get A=0.4 moles; B=0.2 moles; C=0.2 moles; D=0.1 moles

As original equation is 0.2 moles of NH3, at 25 degree Celsius, I chose C. But why is the answer D, given that it is 0.1 moles not 0.2 moles.

Thanks in advance and sorry for any wrong tags.

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    $\begingroup$ Imagine you have a railroad tank car full of $\ce{HNO3}$; would you expect it to react with the same amount of magnesium any faster? It is concentration, not amount, that is important. $\endgroup$ – Ivan Neretin Jan 19 '16 at 20:10
  • $\begingroup$ @IvanNeretin, can you explain why I can't use the moles to figure it out? $\endgroup$ – CCC Jan 19 '16 at 21:24
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    $\begingroup$ No, I can't, if my previous comment makes no sense to you. $\endgroup$ – Ivan Neretin Jan 19 '16 at 21:32
  • $\begingroup$ @IvanNeretin, I don't understand, because concentration is moles per unit volume, and I am also calculating moles in that volume I don't understand how the two are different. $\endgroup$ – CCC Jan 20 '16 at 11:45
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The magnesium is present in form of a solid powder. Therefore, the reaction of magnesium with acid can only occur at the solid–liquid interface between magnesium and the aqueous nitric acid solution. Accordingly, the initial reaction rate depends on the surface area of the solid–liquid interface. For the experiments described in the question, the same mass of magnesium powder is used; therefore, the available surface area of the magnesium powder is approximately the same, too. Hence, as long as the added volume $V$ of the nitric acid solution is sufficient to cover the entire available surface of the magnesium powder, the initial reaction rate does not depend on the added volume of the solution. (Assuming a reasonable geometry of the reaction vessel, a volume of $V=100\ \mathrm{ml}$ should be more than enough to cover a mass of $m=5.0\ \mathrm g$ of magnesium powder.)

Furthermore, the reaction rate depends on the concentration $c$ of the acid at the solid–liquid interface. Therefore, the initial reaction rate depends on the concentration of the added nitric acid solution.

And finally, the reaction rate also depends on temperature $T$. Therefore, the initial reaction rate depends on the temperature of the reactants, i.e. the magnesium powder and the added nitric acid solution.

Hence, in order to obtain the same initial reaction rate, the concentration of the nitric acid solution $(c=1.0\ \mathrm{mol\ l^{-1}})$ and the temperature $(T=25\ \mathrm{^\circ C})$ should be the same during both experiments.

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