Hi, to calculate the pressure, you just need to use the ideal gas law, PV = nRT. You have the volume and temperature (don't forget to convert to Kelvin), and for n, the moles of gas, you know from the coefficients in the reaction that you get a total of three moles of gas molecules for every one mole of NH4NO3 that decomposes. So, calculate the moles of NH4NO3 in 3.05 g and multiply by three to get the total moles of N2O (g) and H2O (g). It doesn't matter that this is a mixture of two different gases, the ideal gas law says all gases behave the same, so you just need the total moles of all of the gases present to calculate the total gas pressure.
You can also calculate the partial pressures of N2O and H2O using the moles of each of these individually in the ideal gas law. Then you would add the two partial pressures together to get the total pressure, the same answer you would get from just using the total moles of gases as above.
Steve
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