Answer and explanation
To predict the product, you need to first have or know your reactans, for example, if you have magnesium (Mg) + oxygen (O2), the product should include both reactants. Oxidation numbers also play a role of the final charge of the product, magnesium has a charge of 2+ and O has a charge of 2-, therefore the product will be MgO
Example 2
Na2CO3 + HCl
When sodium carbonate (Na2CO3) reacts with HCl, Na+, O^-2 and CO2 dissociates in the solution, H+ and Cl- also dissociates, what happens is that the oppositely charges ions attract and form compounds, Na is positive and Cl is negative, they will combine to form NaCl, the hydrogen has a positive charge (+1), it will combine with excess oxygen (O^-2) forming water (H2O) and the CO2 will not combine with anything since it is not an ion
Final equation:
Na2CO3 + HCl = NaCl + H2O + CO2
Now we can balance the equation:
Na2CO3 + 2HCl = 2NaCl + H2O + CO2