Identify which of the following spectra would be

seen by the space telescope through path A.

When borane gas (BH₃(g)) is bubbled through water, the solution it forms is acidic. However this acidity does not come from hydrogens on the BH₃ molecule. 

Explain this phenomenon in no more than 3 sentences. (Hint: refer to the different theories of acids and bases learnt this term) [3 marks]

• Buffers are added to foods and beverages to modify flavours and preserve them.
• Cola drinks include phosphoric acid and carbonic acid (from dissolved CO₂) that lower the pH of the drink and produce buffer systems.
• The primary buffer system that determines the pH of cola drinks is the phosphoric acid:dihydrogen phosphate (H₃PO₄/H₂PO₄^-; pK _a H₃PO₄ = 2.16) buffer system.

1. The results show that the drinks Coca Cola and Pepsi Cola have very similar pH values (2.7 and 2.5 respectively)., what does this imply about the composition of their buffer systems? [1 mark]

2. For the Coca Cola drink 5.19 mL of 0.05 M NaOH was required to raise the pH by 1.0. However, for the Pepsi Cola drink 6.35 mL of 0.05 M NaOH was required to raise the pH by 1.0. Explain this observation, accounting for your response to the question above and the fact that both cola drinks have the same buffer system. [3 marks]

In your own words explain how a substance can exist as a vapour  and has a vapour pressure despite being at a temperature that is below the substance's boiling point.  [5 marks]

Your answer should refer to the theory covered in the course and be 4-5 sentences in length.

Explain why buffer potential is maximised at the pH that matches the pKa of the weak acid of the buffer. What does this have to do with buffer capacity?

The side chains of amino acids are responsible for giving proteins their secondary and tertiary structure (they control the way the proteins fold). The two amino acids above are largely responsible for producing the protein structures known as α-helices. However, these structures are very sensitive to the pH of the solution in which the protein is suspended. Explain why these structures are particularly sensitive to pH.

With reference to the diagram below and using the theory covered in this course, describe how soap works to remove dirt from human skin.

Of the two gases, nitrogen (N₂) and carbon dioxide (CO₂), which one do you expect would deviate the most from ideal gas behaviour at low temperatures? Justify your answer. 

By referring to the concept of electronegativity and bonding types, why is sodium chloride a solid at 25 °C and atmospheric pressure, whereas nitrogen (N₂) is a gas?

The current “petrochemical economy” is based on the combustion of fossil fuels, of which liquid octane is a typical example:

C₈H₁₈(l) + 12.5 O₂(g)  →  8 CO₂(g) + 9 H₂O (l)

The so-called “hydrogen economy” is based on H₂(g) produced from water by solar energy. The gas is then burned as fuel:

2H₂(g) + O₂(g)  →  2H₂O(l)

1. A major disadvantage of hydrogen as a fuel is that it is a gas, and therefore harder to store. Calculate the volume of H₂ gas at 25 °C and 1 atm required to produce the same amount of heat as 1.00 L of liquid octane. (The density of liquid octane is 0.67 kg L^–1)

   

2. Rather than simply burning H₂(g), it can be used to produce an electric current using a fuel cell (pictured above). based on the diagram above, determine the half equations for this cell and thus the expected E° of the cell (note: A standard reduction table is provided in formula sheet). What would you expect to happen to the E of the cell if the pressure of O₂ was increased?

Type your answers to both parts a and b in the box below