Ammonia, NH3 / The test for ammonia gas
3 marks · NormalA student measures the volume of a gas collected during a chemical reaction. She suspects it is ammonia. The laboratory temperature is 22°C and the gas is bubbled through 20 cm³ of water. The final pH of the water is found to be 12.2. Pure water has a pH of 7.0.
By how many units did the pH of the water increase? What does this pH reading suggest about the nature of the gas?
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pH increased by 5.2 units. The high pH suggests the gas is alkaline (base), consistent with ammonia.
Also accept: Calculation: 12.2 - 7.0 = 5.2
Common mistakes: Incorrect arithmetic (e.g. 6.2 instead of 5.2); Confusing high pH with acidity
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Marking points
M1 · 1 mark · Correctly subtracts starting pH from final pH (12.2 – 7.0).
A1 · 1 mark · States pH increases by 5.2 units.
B1 · 1 mark · Identifies that a high pH shows the gas is alkaline/base/ammonia.
Worked solution
Initial pH = 7.0; final pH = 12.2
The pH increased by 5.2 units. High pH means the water became alkaline, suggesting the gas dissolved was ammonia (an alkaline gas).