? Still waiting for OP to respond so we can find out whether he typoed 'to find ph' instead of 'to find pOH'.
if it's to find pH I can see where you're coming from, but my calculations are only.giving me 3.5 when I find pOH, and if it's to find pOH I don't understand how proticness of an unknown...
Are you sure you didn't mean pOH of the rainwater = 3.5 and not pH of rainwater is 3.5?
Assuming ^ is true:
-log10 [ (0.0095 * 0.0083)/0.25 ] = 3.501138311 = 3.5 =pOH
Otherwise if it's to find the pH then:
14+ log10 [ (0.0095 * 0.0083)/0.25 ] = 10.498886169 = pH