Mass in a reaction

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​The first thing you need to know is that what ever mass you get at the beginning of a reaction you will have the same mass at the end of the reaction (products). Unlike in nuclear reactions, chemical reaction atoms are neither created or destroyed. All what is happening is the reshuffling of these atoms from one compound or element to another.

Example 

Sodium + chlorine => Sodium chloride      
​12g    +      8g     =>       20g

PRECIPITATION reactions
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A precipitation reaction is one of the best examples of the conservation of mass. A precipitate is an insoluble solid mass formed in a chemical reaction within a solution. At the beginning of the reaction you will measure the mass and then finally after the reaction. You will notice that the mass will stay the same even if a solid has been formed, it's easy to make the misconception that the mass will increase.