What is the relation between reactive power and power factor?

Reactive Power (kVAR) produces no productive work. Because the inductive electrical equipment employing magnetic fields requires this Reactive Power, which produces no productive work, the Total Power (kVA) provided by the generating source (your utility) must always be greater than the Productive Power (kW).

• The ratio of Productive Power (kW) to Total Power (kVA) is called the Power Factor (PF = kW / kVA). It is a measure of the systems electrical efficiency in an alternating current circuit and is represented as a % or a decimal.

For the relationship between active power and reactive power, basing on the common power factor angle of a system represented by the symbol “ϴ” common to both power we can say that their relationship can be referred to the following formulas: (Same on single phase or three phase)
Active power (Resistive) P = Volts (V) x Current (I) x Cosine ϴ
Reactive power (Inductive or Capacitive) P = Volts (V) x Current (I) x Sine ϴ

Power factor (P.f) = cosϕ
Reactive power (Q) = VIsinϕ
So you need to first find ϕϕ using the power factor and then use it to find sinϕ to calculate reactive power.