The gas constant 8.3145 J/mol*K would really not be the right constant to use, but replacing J with L*kPa would leave you with SI units when you solve for the osmotic pressure, which, I admit, would be more proper than atm as I changed it to. However, .08205746 L atm K-1 mol-1 is correct, and not off by .001 as you suggested. I've always used .08206, but the value I previously stated is the one given on the article for gas constants.
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