X, a gas, has a mass of 0.270g and is present in a gas syringe with a volume of 105.0cm^3 at 97C and 100kPa. Calculate the Mr of X. (5 marks)
This is a classic A-level question which uses the formula:PV=nRT First, this equation (which needs to be learnt, it is not given in the exam papers) must be rearranged to:n=PV/RTWhat all these letters in the formula mean will be discussed with the student as we work through the problem. P,V and T are all given in the question but must be converted to the units that are needed to use the above formula:P=100kPa =100,000PaV=105cm3 =0.000105m3T=97C =370KThese conditions, and the way a student can change to these units will be discussed as we work through the problem. Then the new values must be substituted into the formula to give:n=100,000x0.000105/8.31x370n=0.00341moles - units! Then a mass relation calculation is used to work out the Mr, what the question has asked for:Mole=mass/RFM therefore, Mr= Mass/mole = 0.27/0.00341 = 79.1 - The answer This is just one example of the maths needed for A-level chemistry and in my experience this is what most students struggled with. Other examples of questions include titration calculations and questions regarding energetics (Hess cycles and Born-Habor cycles.
