gas consist of 15%CO2-3.5%O2-0.25%SO2 and balance N2. The flow rate of the pre-mixed gas is 80 ml/min. I want to the partial pressure of SO2, O2, S and other gases at temperature of 800 C
First you can find the total pressure from ideal gas law pv=nrt at the temperature you want. After that, since the pressure is directly proportional to n then you can use: pi/ptotal=ni/ntotal (Dalton's Law). From this, you can find the pi which is partial pressure of each component.
Also, you can use the z if you want which is known for each component at certain t and p and apply pv=znrt rather than ideal gas law (depending on your pressure; z approaches 1 at standard conditions).
you can use ideal gas law as suggested by Khaldoon. However, you do not have to worry about the compressibility factor (z) as these gases exhibit ideal gas behaviour within your given temperature range and moderate pressures