WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

Study point

Worked example A sample of sulfur dioxide occupied a volume of 12.0dm 3 at 10.0°C and a pressure of 105kPa. Calculate the amount, in moles, of sulfur dioxide in this sample. (The gas constant R = 8.31JK −1  mol −1 ) pV  =  nRT Therefore n =  pV RT SI units must be used: p  = 105000Pa (105 × 1000) V  = 0.012m 3 (12/1000) T  = 283K (10 + 273) n  = 105000 × 0.012 8.31 × 283 n  = 0.536mol We can also use the ideal gas equation to calculate the volume a gas would occupy at temperatures and pressures other than those at which it was actually measured. If a gas has a volume V 1 measured at pressure p 1 and temperature T 1 , then: p 1 V 1 T 1  =  nR If the same gas’s volume V 2 is now measured at a new pressure p 2 and temperature T 2 , then: p 2 V 2 T 2  =  nR Therefore p 1 V 1 T 1  =  p 2 V 2 T 2 Worked example A sample of gas collected at 30.0°C and 1.01 × 10 5  Nm −2 had a volume of 40.0cm 3 . What is the volume of the gas at standard temperature and pressure (stp)? (At stp, pressure = 1.01 × 10 5  Nm −2 and temperature = 273K) p 1 V 1 T 1  =  p 2 V 2 T 2 p 1  = 1.01 × 10 5  Nm −2 P 2  = 1.01 × 10 5  Nm −2 V 1  = 40.0cm 3 V 2  = ? T 1  = 30°C = 303K T 2  = 273K (Temperature must be in kelvins) 1.01 × 10 5 × 40 303  = 1.01 × 10 5 × V 2 273 V 2  = 1.01 × 10 5 × 40 × 273 1.01 × 10 5 × 303 V 2  = 36.0cm 3

In calculations involving p 1 V 1 T 1 = p 2 V 2 T 2 remember to change temperature to kelvin.

10 Knowledge check State the units that must be used for pressure, volume and temperature in the ideal gas equation.

11 Knowledge check Calculate the volume, in dm 3 , occupied by 0.10mol of carbon dioxide at 1.01 × 10 5  Nm –2 and 110 °C. (Gas constant R = 8.31 JK –1  mol –1 )

12 Knowledge check Calculate the volume of a gas at stp if it occupies 200cm 3 at 50 °C and 2atm. (Conditions at stp are 273K and 1atm)

There are three important ideas describing the behaviour of gases: Boyle’s law, Charles’ law and Avogadro’s principle. DRAFT 40