Tutorial Questions



  1. What is the molar mass of dinitrogen pentoxide?

  2. The molar mass of dinitrogen oxide is 44.02 gmol-1. What is the weight in milligrams of a sample of 4.00 x 1020dinitrogen oxide molecules?

  3. How many oxygen atoms are there in 1.0 mmole of ozone (O3)?

  4. A solution was prepared by dissolving 5.00g of NaCl (molar mass = 58.5 gmol-1) in 100mL of 1.50M NaCl solution. What is the molarity of the final solution if the addition of the extra solid causes no volume change?

  5. Let us suppose that a reaction takes place, and the products have lower energy than the reactants. Which of the following is true?

    (i) energy is released

    (ii) energy is absorbed

    (iii) the energy of the system increases

    (iv) the thermal motion of the atoms in the surroundings decreases

    (v) the reaction is endothermic.

  6. Which is the correct definition of the joule, the derived SI unit of energy?

    (i) 1 kg m-2 .s-2

    (ii) 1 kg m 2.s-2.

    (iii) 1 L atm

    (iv) 1 torr atm

    (v) 1 L atm s-1

  7. a) Two compounds having the structure C6H5CH=N(OH) are known (one of melting point 33°C, the other of melting point 128°C). Draw them and explain the nature of the isomerism. NB. C6H5 is the phenyl substituent, a benzene ring with one linkage to it. It simply helps to make the compounds easy to isolate.

    b) Explain how the compound C6H5-S(=O)CH3 is chiral (the sulphur atom is linked to two carbon atoms and one oxygen atom). Draw each enantiomer and the mirror plane which relates them.

  8. Draw all the compounds of molecular formula C3H6O, putting in any lone pairs. Draw resonance hybrids for those compounds which have them.

  9. The following question is more difficult. Ethane-1,2-diol, HOCH2CH2OH, reacts with periodic acid (hydrogen periodate, HIO4) in 1:1 mole ratio to give methanal (theoretically 2 mol per mol ethanediol). An intermediate that has a five membered ring has been postulated.

    i) Write a balanced equation for the reaction.

    ii) Work out the oxidation numbers of the species containing iodine. What are their shapes?

    iii) Draw the intermediate and try to provide a mechanism (with arrows) for its fragmentation

    iv) How might you confirm the pathway?

  10. A substance has a enthalpy of sublimation of 100.0 kJ mol-1, and an enthalpy of melting of 4.0 kJ mol-1. What is the enthalpy of vaporisation?

  11. If 100J of heat is removed from a mixture of 100g of ice and 100g of water, then what happens?

    (i) the temperature rises

    (ii) some of the water freezes

    (iii) the temperature decreases

    (iv) some of the ice melts

    (v) some of the water vaporises.

  12. The standard molar enthalpy of formation of H2O2 is -187.8 kJ mol-1, whilst that of HO(l) is --285.8 kJ mol-1. Calculate the enthalpy of reaction for

    2H2O2(l) =====> 2H2O(l) + O2(g)

  13. 1 mol of CaCO3 was heated to 700 degrees Celsius, when it decomposed. The experiment was carried out in a container closed by a piston resting on the sample, and restrained throughout by the atmosphere. How much work was done during the decomposition? Repeat the analysis when the experiment was carried out in an open vessel with no piston. Hint: you need the gas constant R = 8.31 JK-1mol -1.

  14. Tris is an organic buffer which is commonly used in biochemical experiments. In its acidic form (HB) it has a pKof 8.08. Calculate the pH of 1.00 L of solution containing 0.050 mol of B and 0.100 mol of HB.

  15. Valine is an amino acid which acts like a diprotic acid (HA) in its acid form, with pK= 2.29, and pK= 9.72. Calculate two pH values for which valine acts like a buffer. Calculate also the pH of 1.00 L of solution containing 1.00 mol of HA, and 1.00 mol of HA.