How can get mass of a compound from a chemical equation?

In order to determine the mass of a compound from a chemical equation, you need to look at the chemical formula for the compound you need to find the mass of. Using the periodic table, add up the atomic mass of each atom in the compound. For example, the molar mass of ethane, C₂H₆, is 12.01+12.01+1.008+1.008+1.008+1.008+1.008+1.008 = 30.068 grams/mole
In order to find the mass of a product in a chemical equation, you need to convert any information you are given into moles. You can use this information to determine how many moles of product are produced using the ratio given by the chemical equation. Then the number of moles of product is converted to mass using the molecular weight of the compound as determined above.

example)
2Na(s) + 2H₂O(l) --> 2NaOH(aq) + H₂(g)
Find the mass of hydrogen gas produced if 23 grams of sodium react with excess water.

Atomic mass of sodium: 22.99 grams/mol
(23grams)/(22.99grams/mol) = 1 mole sodium

The ratio of sodium to hydrogen gas is 2:1 so:

(1 mol Na) * (1 mol H₂)/(2 mol Na) = .5 mol H₂

The atomic mass of hydrogen is 1.008, so the molecular weight of H₂ is 1.008 +1.008, or 2.016 grams/mol.

(.5 mol H₂) * (2.016 grams/mol) = 1.008 grams of hydrogen gas produced.