The percentage error is the error of the measured value to the true value. To find he percent error, the equation is as follows:
Percent error = |Measured Value - True Value|/True Value * 100
The || is needed to get the absolute value of the difference. Substituting the values,
Percent error = |(10.085 g/10 mL) - 0.9975 g/mL|/0.9975 g/mL * 100
Percent error = 1.1%