Before setting the device in the master mode, make sure you have another device which is in slave mode. I have HC-06 which is always in the slave mode.
connect the first pin of the PCF8574 to the VSS of the LCD, 2nd pin to the VCC and so on. The input pins of the PCF8574 i.e. SCL and SDA should be connected with the SCL and SDA of the microcontroller.
First start (S) is transmitted by microcontroller. Next uc sends address of the device, on receiving address, device sends the A (ack) signal back to uc.
Next uc sends data bits (04, 00, 24, 20). After receiving each data byte, device sends the A(ack) to uc.
At last uc sends P( stop) signal to indicate the end of communication.
ADC is used by most of the sensors for eg- temperature sensor, force sensor, IR sensor, potentiometer, and many more. All these sensors send the data to the microcontroller in analogue form, which then converts it into digital values.