4.00 g metal carbonate, MCO3 reacts with acid to liberate a gas that occupies 0.5878 dm3 at 25˚C and 2.0 x 105 Pa. Identify the group 2 metal, M. Info: R = 8.314 J K-1 mol-1

First it is important to recognise that this is an ideal gas equation question – the exam boards love these so try to get used to spotting if you have the necessary quantities. This is a multi-step problem, don’t panic, take it a step at a time. From your GCSE knowledge you should know that when acids react with metal carbonates they react to give CO₂. Writing the equation shows us that 1 mole of metal carbonate gives one mole of CO₂ gas, so by finding the number of moles of gas, we can also know the number of moles of MCO₃. Rearranging the ideal gas equation, pV=nRT can allow us to calculate the number of moles. Exam questions love to try and trick you by giving you quantities in the incorrect units. For the ideal gas equation temperature should be in K, volume in m³ and pressure in Pa. Therefore in this instance, we must convert the temperature by adding 273 and the volume from dm³ into m³. There is a simple way of doing this which I will show on the board. Plugging all of this into the calculator will give you the number of moles as 0.0475 moles. Using the equation Mr = mass/moles, an Mr of 84 g mol^-1 can be calculated. Taking off 60 for CO₃ leaves us with M having an Mr of 24 g mol^-1. A quick look at the periodic table should show you that this is magnesium!