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

 1.3 Chemical calculations

Volumes of gases For reactions involving gases, it is more usual to consider the volumes of reactants and products rather than their masses. This provides a way of calculating the amount of gas present. At standard temperature and pressure (stp), 0°C and 1atm, one mole of any gas occupies 22.4dm 3 . This is known as the gas molar volume , v m . At room temperature and pressure (rtp), 25 ˚C and 1 atm, one mole of gas occupies 24.5 dm 3 . The steps in calculations involving molar volume are similar to the ones for reacting masses. For example, what volume of hydrogen is produced, at 25 ˚C and 1 atm, where 3.00g of zinc reacts with excess hydrochloric acid? (1 mole of hydrogen occupies 24.5dm 3 at 25 ˚C and 1 atm) Zn + 2HCl →  ZnCl 2 + H 2 Step 1 Change the mass of zinc into moles Amount of moles of zinc = 3.00 65.4  = 0.0459 Step 2 The mole ratio from the equation is 1Zn : 1H 2 therefore 0.0459mol Zn gives 0.0459mol H 2 Step 3 Change the moles into volume of gas Volume of hydrogen = 0.0459 × 24.5 = 1.12dm 3 The ideal gas equation In the real world, most reactions are not carried out at room temperature and pressure. The number of moles in a certain volume of gas at any temperature and pressure can be calculated using the ideal gas equation: pV = nRT where p is the pressure measured in Pa (pascals) V is the volume measured in m 3 n is the number of moles R is the gas constant and has the value of 8.31 JK –1 mol –1 T is the temperature measured in K (kelvins)

Study point To calculate amount of moles, n , of a gas from volume, use the equation v = n × v m Remember that the unit for v m is dm 3 . Key term Molar volume, v m , is the volume per mole of a gas. (In the exam, it will be given on the data sheet.)

Exam tip

Only use molar volume if the temperature is 0 ˚C or 25 ˚C and the pressure is 1 atm.

Knowledge check Calculate the volume of carbon dioxide produced when 3.40g of calcium carbonate is heated and decomposes according to the equation CaCO 3 (s) →  CaO(s) + CO 2 (g). (Assume that 1mol of gas occupies 24.0dm 3 under the experimental conditions.)

9

Exam tip It is vital to use the correct units when using the ideal gas equation, i.e. m 3 , Pa and K. Volumes are normally measured in cm 3 or dm 3 . Since 1 m 3 = 1000 dm 3 and 1 dm 3 = 1000 cm 3 to change from m 3 to cm 3 multiply by 10 6 to change from cm 3 to m 3 divide by 10 6 or multiply by 10 –6 Pressures are often measured in atmospheres 1 atm = 1.01 × 10 5 Pa and 1000 Pa = 1 kPa, therefore 1 atm = 101 kPa Temperatures are often measured in °C To change from ˚C to K add 273 so 25 ˚C = 298 K

39 DRAFT