1. how does a balanced chemical equation show that mass is conserved?

it has the same coefficients on both sides of the equation.
it has the same element symbols on both sides of the equation.
it has the same number of atoms of each element on both sides of the equation.
it has the same number of molecular formulas on both sides of the equation.

It has the same number of atoms of each element on both sides of the equation.

Explanation:

Law of conservation of mass:

According to the law of conservation mass, mass can neither be created nor destroyed in a chemical equation.

Explanation:

This law was given by french chemist  Antoine Lavoisier in 1789. According to this law mass of reactant and mass of product must be equal, because masses are not created or destroyed in a chemical reaction.

For example:

In given photosynthesis reaction:

6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂

there are six carbon atoms, eighteen oxygen atoms and twelve hydrogen atoms on the both side of equation so this reaction followed the law of conservation of mass.