When calculating percent error, what does it mean if I get a negative number for mass?
It means that the experimental mass is greater than the accepted mass. However, mass cannot be negative. Refer to the explanation.
It means that the experimental mass is greater than the accepted mass. However, mass is always positive. If you are calculating percent error, the difference between the experimental value and the accepted value is an absolute value. So even if you get a negative number in your calculation, because it is an absolute value, it is positive.
Note: Sometimes the experimental value is called the actual value, and the accepted value is called the theoretical value.
Notice that in the above formulas, absolute value bars are used to indicate that the difference between the two values is positive. Also notice that it doesn't matter which order the two values are in, because the difference is an absolute value.