Convert mM to M.
0.45cancel"mM O"_2""xx(1"M O"_2"")/(1000cancel"mM O"_2"")="0.00045 M O"_2"
"0.00045 M O"_2"="0.00045 mol/L O"_2"
Convert 300 mL to liters.
300.0cancel"mL solution"xx(1"L")/(1000cancel"mL")="0.3000 L solution"
Determine moles of "O"_2" dissolved in "0.3000 L" of solution.
0.3000cancel"L solution"xx(0.00045"mol O"_2)/(1cancel"L solution")="0.000135 mol O"_2"
"STP"="273.15 K" and "100 kPa"
Use the Ideal Gas Law
PV=nRT, where P="pressure", V="volume", n="moles", R="gas constant", and T="Kelvin temperature".
Given/Known
P="100 kPa"
n="0.000135 mol O"_2"
R="8.3144598 L kPa K"^(-1) "mol"^(-1)"
T="273.15 K"
Unknown
volume, V
Solution
Rearrange the equation to isolate V and solve.
V=(nRT)/P
V=(0.000135cancel"mol O"_2xx8.3144598"L" cancel("kPa") cancel("K"^(-1)) cancel("mol"^(-1))xx273.15cancel"K")/(100cancel"kPa")="0.00307 L O"_2"
Convert liters to milliliters.
0.00307cancel"L O"_2xx(1000"mL")/(1cancel"L")="3.1 mL O"_2" (rounded to two significant figures)