If the API gravity of a crude oil is 40°, what is the corresponding specific gravity (relative density) of that oil?

• A. 0.92
• B. 0.75
• C. 1.0
• D. 0.825

Answer: (D)

Understanding how API gravity converts to specific gravity is the key here. API gravity is defined by the relation API = (141.5 / SG) − 131.5, where SG is the oil’s specific gravity relative to water at standard temperature. To find SG from API, rearrange to SG = 141.5 / (API + 131.5).

Plugging in API = 40 gives SG = 141.5 / (40 + 131.5) = 141.5 / 171.5 ≈ 0.825. So the corresponding specific gravity is about 0.825. Since SG is less than 1, the oil is lighter than water, which is consistent with an API of 40.

The other numbers would imply API values quite different from 40 (for example, SG around 0.92 would yield a lower API, SG around 0.75 would yield a higher API, and SG of 1.0 would correspond to API roughly 10), so they don’t fit the given API.