Under an O2(g) pressure of 1.00 atm, 30.9 cm3 of O2(g)at 25C dissolves in 1.00 L H2O at 25C. Assuming that the soln volume remains at 1.00L, what is the Molarity of a saturated O2(aq) soln at 25C when partial pressure O2=1 atm?

Question

DRBOB222 said to: Convert 30.9 cc O2 at 25C to moles. Then M = moles/L.

I did that conversion (22.4 moles O2/L)(0.0309 L)=0.69216 moles O2. ),69216 moles O2/1.00L =0.69216 M. However, textbook answer says 1.26 x 10-3 M...i am now confused. Help! Thank you.

Answer 1

You didn't convert to moles correctly. The molar mass of O2 is 22.4 L/mole ONLY at STP and 25C is not STP.
Use PV = nRT, solve for n, then n/1L = M and I get 0.00126M. (Note: IF you could use 22.4, you should have divided 0.0309/22.4 = 0.00138 but that would be at 273K (zero C and not 25C)