If the S.V.P of water vapour was 13.5mmHg at 33ºC and 7.3mmHg at 7ºC. Find the percentage relative of the air…

Analyze the question

We need the relative humidity of air. We're given:

• SVP at air temperature (33°C) = 13.5 mmHg
• SVP at dew point (7°C) = 7.3 mmHg

Concept

At the dew point, the water vapour present in the air just saturates the air. So the actual vapour pressure of the air equals the SVP at the dew point temperature.

Relative humidity = (SVP at dew point ÷ SVP at air temperature) × 100%

Solution

Relative humidity = (7.3 / 13.5) × 100%

7.3 ÷ 13.5 = 0.5407

So the relative humidity ≈ 54.07%, which is about 54%.

Answer: Option C (54%)

Tip: In relative humidity questions, always remember "dew point SVP on top, air temperature SVP at the bottom." The answer must be less than 100%, so if you get a number above 100, you have flipped the fraction. Also, do not confuse this with the mass-based formula; when SVP values are given, use the simple ratio.