...lets say the resistor has a resistance of 50 Ohms (I have no idea what the typical LED resistance is)

The resistance of an LED is

non-constant and nonlinear. At low voltages, the resistance is relatively high, and at high voltages, it's very low. (A diode or LED basically "turns off" when the voltage across it is low, and it "turns on" when the voltage across it is high-enough.)

Ohm's Law is a physical law. It's ALWAYS TRUE* and

you CAN calculate the resistance under the particular conditions if you know the EXACT voltage/current characteristics. If the LED is rated 20mA at 2V, that's 2V/0.020 = 100 Ohms.

But, those voltage/current characteristics vary from part-to-part and with temperature. So if you apply 2V, you

*might* get a lot more than 20mA (the resistance might be lower than 100 Ohms) and the LED might burn-up, or you might get a lot less than 20mA, and the LED will be too dim. That's why we use something else (typically a resistor) to control the

*current*, rather than applying a controlled voltage.

* In AC circuits wiht inductors and/or capacitors, there can be phase differences between the current and the voltage. So, if you measure the voltage and current it can

*seem* like Ohm's Law isn't true. But, if you measure voltage and current at any instant in-time, you'll ll find that the law holds true.