consider Cu(s)+4HNO3(aq)-->Cu(NO3)2(aq)+2NO2(g)+2H20(l)

Question

a. If Cu (s) is my limiting reagent, how many molecules of NO2 (g) will be produced from the reactant of 20346 g Cu with concentrated nitric acid?

 

b. If I conduct this experiment at 25 degrees Celsius and at 749 torr, what volume of NO2 gas can I produce?

 

 

Answer 1

a. Assuming Cu is limiting, then Cu is what determines how much product is made. We need to go from our starting grams of Cu to MOLECULES of NO₂. We do this by stoichiometric relationships.

 

1. Convert grams copper to moles:

20346 g of Cu x (1 mol Cu)/(63.55 g Cu) = 320.16 mol Cu

 

2. Convert moles copper to moles NO₂ using the balanced equation:

320.16 moles Cu x (2 mols NO₂)/(1 mol Cu) = 640.32 mol NO₂

 

3. Convert moles to molecules using Avogadro's number:

640.32 moles NO₂ x (6.022 x 10²³ molecules)/(1 mole) = 3.8560 x 10²⁶ molecules

 

 

b. Given temperature, pressure, and information from part a, we can use the ideal gas formula to find volume.

PV=nRT; We are solving for V and have P, T, and R. We will use the answer from part a. as "n." NOTE: Temperature must be in Kelvin (°C + 273.15) and Pressure must be in atmospheres (1 atm=760 torr) Therefore:

 

PV=nRT ⇒ V=nRT/P

V= (640.32 mol NO₂ x 0.0821 L⋅atm/K⋅mol x 298.15 K)/(0.9855 atm) = 1.5904 x 10⁴ L NO₂

 

 

 

Answer 2

April, I'll point you in the right direction on this one, since it's your third question today (that's close to spamming!)

In chemistry, you must be agile at converting among several units of measure for substances. The one that's convenient to measure directly in the lab is mass (frequently in grams). The other two are moles, and number of atoms (or molecules, depending on the problem). Thinking about a certain number of atoms is convenient, but measuring out such small amounts in order to do a lab experiment is impractical (though possible, with rather advanced, expensive techniques). Also, looking at a reaction as written above (for atoms and/or molecules) doesn't immediately tell us how much copper reacts with a given amount of nitric acid, and so on. Therefore, there's that third unit, the mole, which unites the other two. It's related to number of molecules, because you take Avogadro's number of molecules to make one mole (Avogadro's number is very big, but you memorize it once, and then use it happily ever after). And you figure out how massive one mole of a substance is, by adding up the atomic masses of all the atoms in one molecule of it, and then changing the UNITS MEASURE from DALTONS to GRAMS. Lastly, you calculate ingredients for reactions because balanced reactions, though written as if for molecules, can be just as correctly thought of as written for moles of materials.

Thus, in your problem above, first convert from g copper to moles copper (divide by the atomic weight of copper); next scale up (or down, as the case might require) by the [ratio of the coefficient of your desired product molecule to the coefficient of your starting molecule (or atom, here)] -- i.e. multiply by 2/1 --; next, realize that you now have moles of NO2, so to get to number of molecules multiply by [a large number, referenced in the paragraph above].

For part b., backtrack to the nmuber of moles of NO2 you obtained as an intermediate step in (a.), and use the ideal gas law -- you know, pV=nRT, making sure that R is in the units you input to the equation for the other variables. Remember that temperature is only in degrees K!!!!

This should give you a value for NO2 volume; just eyeballing it looks to me around 6700 L, but you should calculate exactly and the round to the precision (usually, number of significant digits) that is proper.

-- S. de Riel

Answer 3

20346 g Cu * 1mole/63.546 = 320 moles of copper.

 

From the equation, each mole of copper produces 2 moles of NO2 gas so you get 640 moles of NO2 gas.

 

1 mole = 6.023 *10²³ molecules  so we have 640*6.022 *10²³ = 3.85 * 10²⁶ molecules.

 

You can solve for the number of volume using the ideal gas law PV = nRT.   Make sure you use the value of R that corresponds to your pressure units and make sure you convert all temperatures to Kelvin before using the equation.  1 torr = 1 mmHg.