I'm not sure, but I did the problem by taking the mmoles and dividing by molarity to get mL. Since mL and mmol both equal x .001, conversion is not neccesary.
1.68/(6.44 x 10^-2)=26.09 mL
like I said, I'm not sure this is right but it makes sense to me
8.81x10^-2 mol/L = 8.81x10^-2 mmol/ml = 0.0881 mmoles/ml
(x ml)(0.0881 mmoles/ml) = 8.67 mmoles
x = 98.4 ml
94,2 mL