Chemists place the object on a scale or balance and measure the force, and then call it "mass" (when they should call it weight).
Force = mass x acceleration
On earth the accelaration due to gravity is essentially constant so 1 gram of mass is said to be equal to 1 gram of weight everywhere on earth. However 1 gram of mass on a balance or scale on the moon would have a weight of about 1/6 of a gram.
