Question

A miniature volcano can be made in the laboratory with ammonium dichromate. When ignited, it decomposes in a fiery display. (NH4)2Cr2O7(s) → N2(g) + 4 H2O(g) + Cr2O3(s) If 0.70 g of ammonium dichromate is used, and the gases from this reaction are trapped in a 13.3 L flask at 28 °C, what is the total pressure of the gas in the flask? What are the partial pressures of N2 and H2O?

Answer 1

(NH4)2Cr2O7(s) ---> N2(g) + 4 H2O(g) + Cr2O3(s)

1 mole (NH4)2Cr2O7 = 1 mole N2 = 4 mole H2O = 1 mol Cr2O3

no of mole of (NH4)2Cr2O7 = w/mwt = 0.7/252.07 = 0.00278 mol

no of mole of N2(g) = 0.00278 mol

no of mole of H2O(g) = 4*0.00278 = 0.01112 mol

total no of mol of gas mixer = 0.00278 + 0.01112 = 0.0139 mol

total pressure of gas (Ptotal) = nRT/V

   = 0.0139*0.0821*(28+273.15)/13.3

= 0.026 atm

partial pressure of N2 gas = Xn2*Ptotal

                           = (0.00278/0.0139)*0.026

                           = 0.0052 atm

partial pressure of H2O gas = XH2O*Ptotal

                            = (0.01112/0.0139)*0.026

                            = 0.0208 atm