Normally you will find the I2C slave address in datasheet (i.e.: PCA9557) like this:
At first glance you will think the address is 0x30 (0011-0000) (case A2, A1 and A0 be 0). This it is wrong because the least significant bit needs to be eliminated.
Then it will become 0x18 (0001-1000).
Using i2cdetect I could to figure out my fault.