The formula for osmotic pressure (#Pi#) is

#color(blue)(bar(ul(|color(white)(a/a)Pi = MRTcolor(white)(a/a)|)))" "#

where

#M# = molarity of the solution

#R# = the Ideal Gas Constant

#T# = the temperature of the solution

We can rearrange the above equation to get

#M= Pi/(RT)#

In your problem,

#Pi = 1.5 color(red)(cancel(color(black)("Torr"))) × "1 atm"/(760 color(red)(cancel(color(black)("Torr")))) = "0.001 97 atm"#

#R = "0.082 06 L·atm·K"^"-1""mol"^"-1"#

#T = "(25 + 273.15) K" = "298.15 K"#

∴ #M = ("0.001 97" color(red)(cancel(color(black)("atm"))))/("0.082 06 L" color(red)(cancel(color(black)("atm·K")))·"mol"^"-1" × 298.15 color(red)(cancel(color(black)("K")))) = 8.07 × 10^"-5" color(white)(l)"mol/L"#

#"Moles of myoglobin" = "0.005 00" color(red)(cancel(color(black)("L"))) × 8.07 × 10^"-5"color(white)(l) "mol"·color(red)(cancel(color(black)("L"^"-1"))) = 4.03 × 10^"-7"color(white)(l) "mol"#

#"Molar mass" = "mass"/"moles" = (3.50 ×10^"-3" color(white)(l)"g")/(4.03 × 10^"-7" "mol") = "8700 g/mol"#

**Note:** I must be having a brain burp here. This is about half the recognized value for human, horse, and sperm whale myoglobin. Where is my mistake?