6. (13 points total) Consider the following thermal decomposition reaction: (NH4

Question

6. (13 points total) Consider the following thermal decomposition reaction: (NH4)2Cr2O7 (s) ) Cr203(s) + N2(g) + 4H20(g) a. (9 points) A student places some solid ammonium dichromate ((NH4)2Cr2O7) in a 1.50 L steel vessel containing 1.00 atm of helium gas at 25 degree C. The student then heats the container at 225 degree C until the ammonium dichromate has decomposed. The total pressure inside of the container at 225 degree C is 2.75 atm. How many moles of ammonium dichromate decomposed? The volume of any solid in the container is negligible and c-an be ignored. You must show all of your work to receive credit.

Explanation / Answer

use:
P*V = n*R*T to find the total number of moles of gas
2.75*1.5 = n*0.0821*(225+273)
n=0.1 mol

Let find the number of moles of H2 added:
P*V=n*R*T
1*1.5 = n1 * 0.0821 * (25+273)
n1=0.06 mol

So,
number of moles of N2+ H2O = 0.1 -0.06 = 0.04 mol

let mol of N2 be x
then moles of H2O = 4x
x+4x = 0.04
5x = 0.04
x=0.008 mol

From reaction,
number of moles of ammoniumdichromate decomposed = moles of N2 formed
    = x
    = 0.008 mol
Answer: 0.008 mol

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