Typically, datasheets will specify the device's "Slave Address" as a 7-bit number.
In the case of the MPR121, the datasheet says 5A (with the ADDR pin tied to ground).

In order to use that address in the control byte of the I2C commands, you have to shift it left 1-bit to make room for the Read/Write bit.

So the control byte should be ... $B4.