The formula of nitric acid is HNO₃.

In nitric acid, the observed colour is red due to its acidic nature. In ammonia solution, the colour observed is blue, indicating the presence of hydroxide ions (OH⁻).

The correct row is D, which shows the pH of ammonia solution at the start as 10 and the pH after addition of excess nitric acid as 2.

To calculate the percentage by mass of oxygen:

1. Determine the total mass of oxygen in the compound:

   • There are 3 oxygen atoms in NH₄NO₃.
   • Oxygen mass = 3 × 16 = 48.

2. Calculate the relative formula mass of NH₄NO₃:

   • H: 4 × 1 = 4
   • N: 2 × 14 = 28
   • Total = 4 + 28 + 48 = 80.

3. Calculate the percentage: rac{mass \, of \, oxygen}{total \, mass} \times 100 = \frac{48}{80} \times 100 = 60\%

Therefore, the percentage by mass of oxygen in ammonium nitrate is 60%.

Method to Investigate Temperature Changes

1. Use a suitable container: Use an insulated beaker or test tube.
2. Weigh ammonium nitrate: Use a balance to measure different masses of ammonium nitrate (e.g., 1g, 5g, 10g).
3. Add water: Measure a fixed volume of water (e.g., 100 ml) and place it in the container.
4. Record initial temperature: Use a thermometer to take the initial water temperature.
5. Stir the solution: Add the measured ammonium nitrate to the water and stir it to dissolve completely.
6. Measure final temperature: Record the final temperature after the ammonium nitrate has fully dissolved.
7. Calculate temperature change: Determine the temperature difference between the initial and final temperatures for each mass of ammonium nitrate.
8. Repeat: Perform the experiment multiple times for accuracy and to ensure consistent measurements.