How Many Digits Should We Use in Formula or Molar Mass Calculations?

摘要

Teachers are often asked by their students:"How many digits should I use when calculating the formula or molar mass of a substance?"A focus on the formal calculation of uncertainty may discourage students before they have had chance to learn the chemistry.Uncertainty calculations,although essential,can be intimidating and discouraging,but to ignore them is a serious error.The objective of this article is to streamline the calculations of uncertainties in the formula or molar masses of compounds.Results of uncertainty calculations are treated as estimations as they are seldom (if ever)exact quantities.Three rules of increasing complexity are proposed.These rules probably overestimate the uncertainty so there is little if any risk that the true values are missed.