PV = nRT Edexcel

PV = nRT This should be easy as you have an equation and more or less just have to put the numbers in.

P = pressure: must be in Pascals (Pa). They quite often give the value in kPa, therefore you need to multiply by 1000  Pa.

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If you see the units as Nm-2, these are the same as Pa.

V = volume: must be in m3. Be careful with this as you are very used to cm3 or dm3.

cm3  m3 divide by 1 million dm3  m3 divide by 1000

n = the number of moles R = the gas constant: 8.31 J K-1 mol-1 (should be given in the question) T= temperature: in Kelvin (K). If they give the temperature in degrees celsius, you have to add 273 to convert to K You may see the following phrases used in questions:

STP: standard temperature and pressure. Temp is 0 °C or 273 K. Pressure is 100,000 Pa. RTP: room temperature and pressure. Temp is 25 °C or 293 K. Pressure is 100,000 Pa.

And that’s about it really. It’s just a case of practise as they could ask you to work the volume or the number of moles or the temperature so you will need to be able to rearrange. And be very careful with the units of P and V. You also need to be careful with what exactly you are working out and what you have worked out. Sometimes there will be more than one gas in the products and you need to know if you have the moles of both the gases added together. The 1st and 3rd questions below are examples of this.

Example 1 4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)

Calculate the number of moles of NO produced. The gases occupied a volume of 4.31 m3 at 25 °C and 100kPa. Using equation PV = nRT. You can either put the numbers in first, multiply them then rearrange or rearrange straight away, it’s up to you. I put in the numbers first in this case: P = 100,000 (multiply kPa x 1000), V = 4.31 n is what we want to work out R is always 8.31 T = 298 (25 + 273)

100,000 x 4.31 = n x 8.31 x 298 431,000 = 2476.38 x n n = 431000/2476.38 = 174.04 moles (total moles of gas) You also have to be careful here. We have worked out the total number of moles of gas produced, NOT the moles of NO. From the equation above, we have 10 moles of gas in total produced (4NO and 6H 2O). We are only interested in NO, which makes up 4/10 of the total. Therefore we need to divide by 2.5:

174.04/2.5 = 69.2 moles of NO Example 2 In an experiment, 0.658 moles of CO2 were produced, which occupied 0.0220 m3 at a pressure of 100 kPa. Calculate the temperature of the CO2. This should be quite easy. We have all the numbers that we need, except we need to convert kPa to Pa by x1000. Using:

PV = nRT We can rearrange this equation to give:

T = PV/nR T = 100000 x 0.022/0.658 x 8.31

T = 402 K Example 3 B2O3(s) + 3C(s) + 3Cl2(g)  2BCl3(g) + 3CO(g) The two gases produced occupied a volume of 5000 cm3 at a pressure of 100 kPa and a temperature of 298 K. Calculate the mass of boron oxide that reacted. The emphasis in this question is on the two gases that are produced, so we need to work out the number of moles of these gases. Also, the volume is given in cm3 so we need to divide by 1000 to get m3:

PV = nRT n = PV/RT n = 100000 x 5x10-3/8.31 x 298 n = 0.202 moles of gas produced We have the total gas produced i.e. BCl3 and CO. Now we want to work out the grams of B2O3: We have 5 moles of gas in total which = 0.202 moles. So it is a 1:5 ratio of B2O3:(BCl3 + CO) Therefore we need to divide the moles by 5: 0.202/5 = 0.0404 moles B2O3 So to get the grams we just use number of moles = mass/RFM:

Mass = number of moles x RFM Mass = 0.0404 x 69.6 =2.81 g Example 4 4Fe(NO3)3(s)  2Fe2O3(s) + 12NO2(g) + 3O2(g) Calculate the volume, in m3, of 0.0268 moles of nitrogen dioxide gas at 293 °C and 100 kPa. Remember to add 273 to get the temperature in Kelvin!

PV = nRT V = nRT/P V = 0.0268 x 8.31 x 566/100000

V = 0.00126 m3

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When calculating volume it is usually a small number as the answer is in m3. They may ask for it in cm3 or dm3, so you will need to convert as shown on page 1 of this tutorial.