Gas law problem

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[Hikari]

Sorry, this is the last problem I'm going to ask for help on today! ><

Calculate the volume of nitrogen dioxide produced at 750.8 torr and 301.4 K by the reaction of 7.45 cm^3 Cu (d= 8.96 g/cm^3) with 224.6 mL nitric acid (d= 1.42 g/cm^3; 68.0% HNO3 by mass):

Cu (s) + 4HNO₃ (aq) --> Cu(NO₃)₂ (aq) + 2NO₂ (g) + 2H₂O (l)

I found Cu to be the limiting reagent, so I computed it's mass, converted it to mols, used mol ratios to find mols of nitrogen dioxide, then used ideal gas law to find volume of NO₂, but my answer is incorrect. Where might I have gone wrong?

[Otis]

Are you certain that Cu is the limiting reagent?

How many mol of Cu and HNO₃ did you determine?

Did you take into consideration that the HNO₃ solution is only 68% HNO3?

Otis

[Hikari]

Wait, so the HNO3 on the reactant side isn't all HNO3? I wasn't really sure why that mass percent was given.

 

[Otis]

So instead of 318.9 g of HNO₃, only 68 g of 100 g of solution is HNO₃.

Otis

[Hikari]

Okay, I got the right answer now. Thank you very much!