Let mid-point of PQ be S having coordinates (h,k).
Hence, coordinates of P and Q are (2h,0) and (0,2k) respectively.
Also, the equation of PQ using Intercept form is
x2h+y2k=1
Given circle is x2+y2=1
C=(0,0),r=1
Also, PQ is tangent to circle, so distance from centre to PQ is equal to radius
∣∣
∣
∣
∣
∣
∣∣−1√(12h)2+(12k)2∣∣
∣
∣
∣
∣
∣∣=1⇒14h2+14k2=1⇒k2+h2=4h2k2
Hence, the locus is
x2+y2−4x2y2=0