[onorton]

> I know how to pronounce 'Kansas', and I was always under the impression that 'Arkansas' is the same with an 'Ar-' prefixed, which is apparently not the case.

The reason why they're pronounced differently is that they have different language origins. Arkansas is from the plural form of the French name of a Native American tribe, so you don't pronounce the '-s' while Kansas comes from the English spelling of a similar(?) tribe, where the '-s' is pronounced. English is notorious for just borrowing words from other languages wholesale but it seems to work most of the time.