Chemistry Exam Questions

• Deduce which of Na+ and Mg2+ is the smaller ion. Explain your answer.

  Magnesium because magnesium has more protons and with same shielding.
• Another student is required to make up 250 cm3 of an aqueous solution that contains a known mass of MHCO3. The student is provided with a sample bottle containing the MHCO3. Describe the method, including apparatus and practical details, that the student should use to prepare the solution

  Weigh the sample bottle containing the solid on a balance, then transfer this to the beaker and reweigh the sample bottle, then record the difference in mass. Then add distilled water and stir until all the solid has dissolved. Transfer this to the volumetric flask with washings.
• Explain how ions are accelerated, detected and have their abundance determined in a time of flight (TOF) mass spectrometer.

  Ions are accelerated by attraction to a negatively charged plate, ions are detected by gaining electrons, abundance is determined by the size of current flowing.
• State two differences between the ‘plum pudding’ model and the model of atomic structure used today.

  The nucleus contains protons and neutrons. Also, electrons are now arranged in energy levels,
• The sample of chromium is analysed in a time of flight (TOF) mass spectrometer.

  Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.

  The ions will be accelerated by an electric field. And, the ions will create a current when they are hit by the detector.
• Describe how the molecules are ionised using electrospray ionisation

  The sample is dissolved in a volatile solvent, then injected through a needle at a high voltage. Each molecule then gains a proton.