Chemistry Sig Fig Calculator
Use this free chemistry sig fig calculator to calculate moles from mass and molar mass with the correct number of significant figures. Enter your measured mass and the molar mass to get a properly rounded answer for your lab report.
Enter Values
The measured mass of your substance in grams
The molar mass of the substance from the periodic table
Result
Enter values above and click Calculate to see your result.
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Formula
Divide the measured mass by the molar mass of the substance. Since this is division, the result has the same number of significant figures as the input with the fewest sig figs.
Worked Example
Significant Figures in Chemistry
- Mass measurements are limited by balance precision (e.g. 3 or 4 sig figs for a typical lab balance)
- Molar masses from the periodic table typically have 4 or more sig figs
- The measured mass usually limits the sig figs in your moles calculation
- Always report your final answer with the correct sig figs on lab reports and exams
Beyond moles calculations, sig figs in chemistry apply to dilutions, titrations, stoichiometry, and any calculation involving measured quantities. The same rules (fewest sig figs for multiplication/division, fewest decimal places for addition/subtraction) apply throughout.
You can also calculate changes using our Sig Fig Division Calculator, Sig Fig Multiplication Calculator or Log Sig Fig Calculator.
Frequently Asked Questions
How many sig figs should my answer have in chemistry?
Your answer should have the same number of sig figs as the measurement with the fewest sig figs used in the calculation. For moles = mass / molar mass, it is usually the mass that limits precision.
Do molar masses from the periodic table have unlimited sig figs?
No. Molar masses have a defined number of sig figs based on how precisely the atomic mass is known. However, they typically have 4 or more sig figs, which is often more than your measured mass, so the mass is usually the limiting factor.
How do sig figs work in stoichiometry?
Stoichiometric coefficients (like the "2" in 2H2O) are exact numbers with unlimited sig figs. They never limit your answer. Your measured quantities (mass, volume, concentration) determine the sig figs.
What about sig figs in titration calculations?
Titrations involve multiplication and division (M1V1 = M2V2 or moles = M x V), so use the fewest sig figs rule. The buret reading, pipet volume, and concentration all contribute.
Should I round intermediate steps?
No. Keep extra digits in intermediate calculations and only round the final answer. Rounding too early introduces rounding error that can accumulate through multiple steps.
How do sig figs work with pH in chemistry?
pH uses the special logarithm rule. The sig figs of [H+] become the decimal places in pH. For example, [H+] = 2.5 x 10^-3 (2 sig figs) gives pH = 2.60 (2 decimal places).
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