Enter the values of three known quantities of the chemical substance. Make sure to use the same units on both sides of the sign of equality but enter numbers only. This is not a unit converter! Use decimal point, no commas or blanks as in this example 3.50e-2

When you hit the Calculate button the value of the unknown quantity of the chemical substance will be displayed with as many significant digits as in the smallest count of significant digits in the values entered. 

A double click in the M - textbox will open a window with a prompt for the chemical formula associated with M. The Molar Mass Calculator on this site calculates the molar mass and the result is rounded to four signifikant digits.