The partial pressure of oxygen in equilibrium with pure liquid lead and pure liquid PbO at 1200 K is 3.83 x 10-9 bar. If SiO2 is added to the liquid PbO such that equilibrium po, for the pure Pb-liquid PbO-SiO2 solution couple is decreased to 9.58 x 10-10 bar, calculate the activity of PbO in the lead silicate melt.